Capacitor in Launch Controller Circuit?
Chuck Pierce
I just finished building a relay controller which shorts a 9.6v Ni-MH
RC battery across the initiator. After finishing it up, I had a
thought that I wanted to run past you HPR pros. It's probably not a
good idea since I haven't read or heard of anyone else doing it, but I
thought a capacitor could be added into the circuit to provide a
little more umph through the initiator. It would have to be upstream
of the relay contacts and could charge between shots. When the relay
opens for a launch, the capacitor could dump its charge into the
initiator along with the current coming from the RC battery. What
bothers me about this setup is that I not sure that there would be
sufficient drop in voltage at the capacitor to discharge it. I'm an
aero engineer that knows enough about electronics to be REALLY
dangerous, and this hypothesis clearly falls into the danger category,
which is why I wanted to solicit some comments before I ran down to
the electronics store and ended up jolting the begeebers out of
myself.
Thanks,
Chuck
NAR 78629
Gene Costanza
have to assume you mean). Regardless of whatever arrangement you choose,
you ALWAYS need new or freshly charged HEALTHY batteries. A capacitive
discharge circuit is WAY more complicated to build into an existing
controller than the whole shebang needs to be. It also won't necessarily
get an ignitor doin' its biz that much quicker either; if you don't
balance the capacitive discharge circuit's output impedance with the
ignitor, you won't get maximum power transfer either.
--
METRA BOD TRA #07017 L2 NAR #74503 L2
******************************************************************
There is no armour against Fate;
Death lays his icy hand on kings.
Andrew Harpin
from your car's ignition system. You'll want to turn this on electronically
mind as if you do it with the switch you'll burn the switch out before you
get a spark across the ignitor
Andy
"Gene Costanza" <lev...@nyc.rr.com> wrote in message
news:3A3D2FFF...@nyc.rr.com...
Oliver
firing the ejection charge?
Oliver
TRA: 08312
Stop losing your rockets!
Check out my rocket locators at http://biznet.maximizer.com/rockets
Check out my R/C rocket gliders at:
http://prime-fe1.lvcablemodem.com/enjoy
"Chuck Pierce" <cpi...@knology.net> wrote in message
news:371q3t4egqna3ogh6...@4ax.com...
Gene Costanza
batteries and that they're a capacitive discharge circuit.
--
TRA #07017 L2
NAR #74503 L2
METRA BOD
Sent via Deja.com
http://www.deja.com/
Jim Z in VT
the current output.
Oliver <rockets...@attglobal.net> wrote in message
news:t3qllke...@corp.supernews.com...
Ray Drouillard
battery is high enough to limit the current going to the igniter. Whether
it is necessary or successful depends on a lot of factors, such as the
resistance profile of the igniter (the resistance will go up or down as the
igniter gets hot), the current needed for successful ignition, the time
needed, the resistance of the wires, the ability of the switch to handle the
current, etc.
If the voltage at the point where you would place the capacitor (during
attempted firing) isn't significantly lower than the open circuit voltage of
the battery, the capacitor won't help you at all.
If you really wanted, you could charge up a large capacitor (like a farad or
so) using your car's battery and use it by itself to ignite the rocket.
Ray Drouillard
"Chuck Pierce" <cpi...@knology.net> wrote in message
news:371q3t4egqna3ogh6...@4ax.com...
Doug Sams
> A large capacitor will definitely help if the internal resistance of
> the battery is high enough to limit the current going to the igniter.
> Whether it is necessary or successful depends on a lot of factors,
> such as the resistance profile of the igniter (the resistance will go
> up or down as the igniter gets hot), the current needed for successful
> ignition, the time needed, the resistance of the wires, the ability of
> the switch to handle the current, etc.
>
> If the voltage at the point where you would place the capacitor
> (during attempted firing) isn't significantly lower than the open
> circuit voltage of the battery, the capacitor won't help you at all.
>
> If you really wanted, you could charge up a large capacitor (like a
> farad or so) using your car's battery and use it by itself to ignite
> the rocket.
This is a good analysis, but when the numbers are crunched, it fizzles.
I, too, have been wondering about using a capacitor for air-starting
and/or ejection charges. My preliminary calculations concur with another
poster's input that the additional circuitry was more trouble than it's
worth.
Here's my analysis:
A "super-cap" (Double Layer Capacitor) is unsuitable. For one, its
voltage is marginal. These devices are usually rated at 5.5V, which by
itself is marginal, but a typical application circuit involves
performing an "analog OR" using diodes. Most likely, the circuits around
the super-cap will be at a higher voltage thereby keeping the
super-cap's diode turned off and the super-cap will never contribute
current to the load. The other problem with the super-cap is its output
resistance - on the order of 30 ohms - way too high to have a practical
application in ignitor (high current) circuits. (Super-caps are great in
battery-backup applications where the loads are in the 1 micro-amp
range.)
(BTW: The "analog-OR" gate is two diodes whose cathodes (low side) are
connected together. They work much like check valves. Whichever one has
its anode (high side) at the higher voltage will drive current while the
other will be off. In an ignitor application, the capacitor will drive
the ignitor thru its diode until the cap voltage falls to the battery
level whereupon the cap's diode will turn off and the battery's diode
will turn on. The battery will then drive the load (until the ignitor
burns thru and open-circuits.)
The alternative to the super-cap is the ordinary electrolytic capacitor.
These are available with reasonable capacitances (~10,000 uF), good
working voltages (16V and up), and with low series (internal; output)
resistance (0.1 ohm).
My theoretical circuit is as follows: A 9.6V NiMH battery drives a
Schottky diode. In parallel, a 1000uF cap also drives a Schottky diode.
The cap is charged (via a separate terminal) to 14V (~ car battery
level). The two diodes drive the ignitor. The ignitor is grounded thru
a FET. The electronics (altimeter, etc) will turn on the FET to close
the circuit to the ignitor.
The cap has an internal resistance of 0.1 ohms. The NiMH battery has an
internal resistance of 1.4 ohms. The series resistance of the FET, when
on, is 0.1 ohms. These resistance values are derived from the datasheet
specs of the various devices. (The NiMH resistance seems high, but its
value has no impact on the capacitor analysis, and in fact, if it is
lower, will only make the battery better.)
Initially, when the FET first turns on, the resistance seen by the cap
is 1.2 ohms. The 14V is dropped by 0.3V thru the Schottky. (The
resistance is Rcap+Rign+Rfet=0.1+1.0+0.1=1.2ohms.) The initial current
then is 13.7/1.2 = 11.41 amps. Sounds good, right? NO!
The final current, after the cap discharges sufficiently low, is
calculated as follows: The NiMH loses 0.3V thru its Schottky. The net
resistance is Rbat+Rign+Rfet=1.4+1+.1=2.5ohms. The current is
9.3/2.5=3.72 amps.
This alone should be enough to fire the ignitor. The additional current
from the cap might be useful, but further analysis reveals this: The cap
is on for a whopping 450 micro-seconds - less than 1/2000 of a second.
This ain't enough time to heat up a nano-turd, much less an ignitor.
So, for all the additional circuitry and mass, the cap don't contribute
squat. It's all done by the NiMH battery.
I will say this, though: That 9.6V battery has got to weigh a bunch. It
would be nice to have something as small and light as an electrolytic
cap which can deliver the current of that battery without the weight.
Doug Sams
Oliver
news:91lnm2$4vo$1...@nnrp1.deja.com...
> Ray Drouillard wrote:
> So, for all the additional circuitry and mass, the cap don't contribute
> squat. It's all done by the NiMH battery.
--
Thanks
Chris Lewis
A capacitor is of value when the battery has a relatively high internal
resistance. This is of interest when you're striving for minimum weight
in the electronics, and you're using batteries that can't generally deliver
high currents (like a 9V transistor radio battery). Eg: triggering
ejection charges with flight avionics.
High internal resistance is _not_ something that Ni-NH RC (or NiCads or
automotive) batteries suffer from, so the capacitor would be more trouble
than it's worth.
--
Chris Lewis,
For more information on spam, see http://spam.abuse.net/spam
It's not just anyone who gets a Starship Cruiser class named after them.
Chris Lewis
> "Doug Sams" <doug_m...@my-deja.com> wrote in message
> news:91lnm2$4vo$1...@nnrp1.deja.com...
> > Ray Drouillard wrote:
> > So, for all the additional circuitry and mass, the cap don't contribute
> > squat. It's all done by the NiMH battery.
> So why do some altimeters use a large capacitor?
Small batteries, high source resistance. Ie: the internal resistance of
the battery is the same or higher than the igniter. You won't encounter
this _except_ in flight ejection electronics.
If you needed a cap in a launcher, you'd be better off getting a better
battery.
Sams Family
> So, for all the additional circuitry and mass, the cap don't contribute
> squat. It's all done by the NiMH battery.
Oliver wrote:
> So why do some altimeters use a large capacitor?
Oliver,
There can be two reasons. I should have better qualified my statement. I was
basing my analysis on a 1 ohm ignitor ala Estes. Other ignitors may require
more or less current, for longer or shorter durations. (My Estes ignitors
seem to fire in about 1 second after the switch is pressed.) As I understand
it, ignitors such as electric matches require very little energy (voltage *
current * time) to ignite, so perhaps with the right value of capacitance
charged to the right voltage, a capacitor might be useful.
OTOH, the cap could become too (physically) large to be impractical, or the
charging voltage may be inconvenient. (That 14V from the car battery is quite
convenient.)
I've seen schematics which show a cap used to assist firing an ignitor, but
no capacitance value was given.
The other reason for having a large cap *could be* this: When power is
applied to an ignitor, its low impedance can overload the battery and (due to
the battery's internal resistance) pull the system voltage way down. This is
the same effect as when my wife plugs in the iron and the lights go dim :)
When the voltage is pulled this low, the electronics in the rocket controller
(altimeter, accelerometer, etc) may go off in the weeds and therefore fail.
However, these electronics typically draw on the order of 10s of milliamps,
so a capacitor *could* be used to power them for a short time until the
battery's load is shed - ie, the ignitor burns thru. This scenario would
require that the electronics be isolated from the load via a diode. The cap
is charged by the battery, but the diode prevents the ignitor from drawing
current from the cap. Without seeing the schematics, I can only guess that
this might be how it is used.
--
As has been posted, the beauty of the cap is its very low internal
resistance, but the analysis indicates that caps with this good low R quality
cannot store enough charge to provide current for a sufficient length of
time. In my previous example, if we used a 10,000uF cap, the cap would
provide current to the load for 4.5msecs instead of 450usecs. I doubt most
ignitors will benefit from this. And a 10,000uF cap is about the volume of
roll of quarters, so now it's getting large.
What would be really nice is to have some data on the various ignitor types
(like we get for capacitors, transistors, ICs, etc) so that we can see the
voltage/current/time properties for them. This would help us more closely
determine the minimal battery/capacitance/charge storage requirements. If I
had 10 or 20 of each ignitor type, I could run some experiments on them to
see what each one takes to pop. Determining DC requirements is relatively
straightforward, but using a cap is trickier. Since the voltage from the cap
will decay, characterizing the firing behavior is rather involved. Basically,
here's what can happen: If you're using an Estes type ignitor, and you apply
too much voltage (and thereby too much current), the nichrome may burn thru
too fast without igniting the pyrogen, and thereby not light the motor or
ejection charge. Using a cap to do this is more involved. How fast the
nichrome heats up is a function of the cap's (pre-discharged) voltage, its
capacitance, and the series resistance in the loop, so characterizing this
would be non-trivial.
I understand that ejection charges usually use electric matches, and that
these require little current, so I can see how a low mass, low volume circuit
could be used. I also understand electric matches are used in airstarting, so
it sounds like they are the ultimate solution for keeping the electronics
simple. However, acquiring that personal experience still lies ahead for me.
I do believe caps can somehow be used to fire ignitors. But until I am
convinced it can be done with the same reliability as batteries, I plan to
use batteries.
Doug Sams
Bill Westfield
voltage is marginal. These devices are usually rated at 5.5V, which by
itself is marginal, but a typical application circuit involves
performing an "analog OR" using diodes. Most likely, the circuits around
the super-cap will be at a higher voltage thereby keeping the
super-cap's diode turned off and the super-cap will never contribute
current to the load. The other problem with the super-cap is its output
resistance - on the order of 30 ohms - way too high to have a practical
application in ignitor (high current) circuits.
Not anymore.
Amoung others. Super-caps of this new style are now finding uses INCREASING
the peak current capability of battery systems...
BillW
Sams Family
> A "super-cap" (Double Layer Capacitor) is unsuitable.
Bill Westfield wrote:
> Not anymore.
>
> http://www.powerstor.com
>
> Amoung others. Super-caps of this new style are now finding uses INCREASING
> the peak current capability of battery systems...
Bill,
Thanks for the URL. I was getting pumped until I saw the working voltage was
2.75V :( Although that may still light an electric match. And I'm going to show
them to our applications engineers at work. We may want to use them in some
applications examples for our NVRAMs and real-time clocks.
I guess one could cascade them in series to increase the working voltage...
BTW, I wish the electric match gurus out there would chime in here and talk
about the VI characteristics of those devices.
Doug Sams
G. Clark
A capacitor is very likely unnecessary as NiMH batteries have low series
resistance to begin with.
However, you will find the life of your battery as well as the length of
charge greatly extended if you use a sufficient capacitance to boost the
high current draw period as the ignitor is shorted. Batteries have their
worst performance under extreme load conditions.
So you will probably benefit greatly by putting a 3.3 Farad, double layer,
symmetric capacitor in parallel with your battery. You will probably need
to series these caps as their usual voltage rating is only 2.5 volts.
These caps are pretty expensive but you will probably make up their cost
in terms of extended NiMH battery life.
I know NEC and Panasonic both make these types of caps. Other than that, I
do not have a very good source for these. Digikey sells some 3.3 Farad
caps, but I believe they are asymmetric which have significantly higher
series resistances and are more suited to memory backup applications than
high current applications.
Glenn Clark
Martin Maney
> What would be really nice is to have some data on the various ignitor types
> (like we get for capacitors, transistors, ICs, etc) so that we can see the
> voltage/current/time properties for them. This would help us more closely
> determine the minimal battery/capacitance/charge storage requirements. If I
I've been wondering, as I somewhat belatedly read this thread, when someone
would mention Robert Briody's recent paper that presents some interesting
measurements on a variety of common ignitors. It's found on the "products"
page one click removed from http://www.uniquerocketry.com/, and while the
survey is far from complete it provides interesting food for thought.
> I do believe caps can somehow be used to fire ignitors. But until I am
> convinced it can be done with the same reliability as batteries, I plan to
> use batteries.
This past year Cally and I have fairly often borrowed the use of a pad at
NIRA's monthly launches; at first because we didn't have our own, and later
because many of the launches were pretty busy, and it was easier and less
cluttered than setting up Yet Another Pad. Pretty often, we've used the pad
and launch system owned by the club president, Rick Gaff. The launch system
is comprised of two ordinary alkaline 9V batteries, a big honking capacitor
(100,000uF or more, I forget), and an elegantly minimal control box. The
same pair of cheap 9V batteries have been firing off all manner of ignitors
from Estes to crapperheads with excellent reliability for, IIRC, about two
years now.
Firing ignitors doesn't require a very large amount of energy as these
things go. What they do need is for that energy to be delivered fairly
quickly. Rick's launch system and Robert's data, taken together, have made
me contemplate the gross inefficency of the typical ignitor control system.
I haven't finished pondering this stuff yet, but I think there's
considerable room for improvement in igniter trigger efficency. I'm afraid
it's not likely to result in anything soon enough for Adam's hoped-for
club-level R&D contest next spring. :-(
--
There is overwhelming evidence that the higher the level of self-esteem,
the more likely one will be to treat others with respect, kindness, and
generosity. -- Nathaniel Branden
Kurt Kesler
gac...@pop.dnvr.uswest.net says...
more) are wired in parallel with the battery to handle demand peaks.
Check with a local stereo shop.
--
Kurt Kesler
Oliver Arend
NiMH are rechargeable, you shouldn't worry too much about lifespan if you
treat them correctly. They'll last longer than the hugest battery available
(including recharging them ;-).
Oliver
TRA #08311
RMV Göttingen #001
Joe Pfeiffer
> This past year Cally and I have fairly often borrowed the use of a pad at
> NIRA's monthly launches; at first because we didn't have our own, and later
> because many of the launches were pretty busy, and it was easier and less
> cluttered than setting up Yet Another Pad. Pretty often, we've used the pad
> and launch system owned by the club president, Rick Gaff. The launch system
> is comprised of two ordinary alkaline 9V batteries, a big honking capacitor
> (100,000uF or more, I forget), and an elegantly minimal control box. The
> same pair of cheap 9V batteries have been firing off all manner of ignitors
> from Estes to crapperheads with excellent reliability for, IIRC, about two
> years now.
Note that the 1-3F capacitors other people have mentioned are 10-30x
this size.
--
Joseph J. Pfeiffer, Jr., Ph.D. Phone -- (505) 646-1605
Department of Computer Science FAX -- (505) 646-1002
New Mexico State University http://www.cs.nmsu.edu/~pfeiffer
VL 2000 Homepage: http://www.cs.orst.edu/~burnett/vl2000/
Martin Maney
> Note that the 1-3F capacitors other people have mentioned are 10-30x
> this size.
Yeah, I don't really believe in those yet. Although the product page about
those Aerogel thingies was moderately convincing. :-)
I'm not quite clear: are folks actually making full-Farad size capacitors
with working voltages suitable for more than CMOS backup? At a price mere
mortals can afford, that is, and not filling the bottom of a 19" rack
chassis like the storage bank in old Eclipse minicomputers...
Sheesh. Get out of the biz for a few years and they go and change things.
Aside from semiconductors, that is, that are supposed to be in constant flux.
--
The work was fun partly because they didn't have to do it,
and partly because they were all doing it. Regardless of
what you may have been told, that is the way the Pyramids
were built. -- Panshin, _The Thurb Revolution_
Joe Pfeiffer
>
> I'm not quite clear: are folks actually making full-Farad size capacitors
> with working voltages suitable for more than CMOS backup? At a price mere
> mortals can afford, that is, and not filling the bottom of a 19" rack
> chassis like the storage bank in old Eclipse minicomputers...
>
> Sheesh. Get out of the biz for a few years and they go and change things.
> Aside from semiconductors, that is, that are supposed to be in constant flux.
I think I'll have to see one to actually believe it... I have a mental
image of a launch controller that's permanently mounted to a
fifth-wheel trailer!
Kurt Kesler
says...
> Martin Maney <ma...@pobox.com> writes:
>
> I'm not quite clear: are folks actually making full-Farad size capacitors
> with working voltages suitable for more than CMOS backup? At a price mere
> mortals can afford, that is, and not filling the bottom of a 19" rack
> chassis like the storage bank in old Eclipse minicomputers...
>
> Sheesh. Get out of the biz for a few years and they go and change things.
> Aside from semiconductors, that is, that are supposed to be in constant flux.
> I think I'll have to see one to actually believe it... I have a mental
> image of a launch controller that's permanently mounted to a
> fifth-wheel trailer!
>
Seeing is believing, I guess, although I certainly have no reason to
deceive. Then again, maybe it is the typical RMR "I know more than you"
mentality. Anyway...
Average size of the 1 farad ~14v caps used in audio applications is about
a foot long with a diameter about that of a ball bat. They cost about
100 bucks and have been around about 10 years.
http://www.eau.com/sw/intecaps/ and http://www.eau.com/sw/capbanks/
Do a search if you need to see more.
--
Kurt Kesler
Wayne Johnson
disassembled. We wired up all the capacitors we could find, about 2 farads
worth. Filled up most of a desktop.
We charged it to about 30 volts and then dropped a 1/4" screwdriver across the
contacts. It vaporized the screwdriver. Me thinks 2-3 farads a bit overkill for
an initiator/ignighter.
The formula for calculating the amount of energy dumped out of a capacitor is
t=rc, there t is time in seconds for the voltage to drop 1/3 of the way, r is the
resistance of the ignitor & wire, and c is the capacitance.
Now if it takes 3 seconds to fire the ignitor, and we want to make sure the
voltage drops only a volt or two, and the resistance of the ignitor is only an
ohm, it should be pretty easy to figure the needed farads.
BTW, a farad size poly capacitor has a pair of plates about the size of a
football field.
Martin Maney
> Me thinks 2-3 farads a bit overkill for an initiator/ignighter.
Yep. Unless maybe you're trying to fire off clustered AP motors using
Copperheads. :-)
> The formula for calculating the amount of energy dumped out of a capacitor is
> t=rc, there t is time in seconds for the voltage to drop 1/3 of the way, r is the
> resistance of the ignitor & wire, and c is the capacitance.
Uhm... no. I'm not which part of what you thought you were saying is
mangled here, but this Just Ain't Right. The energy stored in a capacitor
is proportional to the square of the voltage... just a hint, there.
> Now if it takes 3 seconds to fire the ignitor, and we want to make sure the
> voltage drops only a volt or two, and the resistance of the ignitor is only an
> ohm, it should be pretty easy to figure the needed farads.
For constant current - a reasonable approximation here - the delta V is
equal to the current divided by the capacitance, all multipled by the time
interval. Say 1 ohm at 12 volts initially, and allowing it to drop 1V per
second you'll need all of 12 farads!
This *is* way overkill - I've seen crapperheads fired from a capacitor that
was not more than 1/4 of a farad, maybe only 1/10. What's off is the
assumption that it takes more than a *small* fraction of a second to fire
off an igniter. Go look at the data Briody presents in that paper I
mentioned a little while back. IIRC, even Copperheads lit off in 1/4 of a
second, maybe less - but the highest current draw isn't always from the
actual ignition phase. Briody attributes the rise post ignition to the
conductive plasma produced by the pyrogen, and while that is probably
present, inthe case of the copperheads I'd bet that big fat spike is the
copper ribbons shorting out. This would be a strong contraindication for
using those things for clusters in a simple parallel circuit!
--
Self-pity is like sitting there and peeing your pants:
at first, it's warm and comfy, but pretty soon it gets cold
and then it starts to stink. - anonymous, as is traditional
Alan Jones
>Wayne Johnson <wd...@yahoo.com> wrote:
>> Me thinks 2-3 farads a bit overkill for an initiator/ignighter.
>
>Yep. Unless maybe you're trying to fire off clustered AP motors using
>Copperheads. :-)
>
>> The formula for calculating the amount of energy dumped out of a capacitor is
>> t=rc, there t is time in seconds for the voltage to drop 1/3 of the way, r is the
>> resistance of the ignitor & wire, and c is the capacitance.
>
>Uhm... no. I'm not which part of what you thought you were saying is
>mangled here, but this Just Ain't Right. The energy stored in a capacitor
>is proportional to the square of the voltage... just a hint, there.
V**2? Lets see, A 120 microF Capacitor from a Kodak Max camera
charged to 350V (from a 1.5V battery). How does that compare to the
xF Capacitors charged to 9V. What do we really need to sucessfully
fire a single igniter? Energy, power, Impulse, current?
Alan Jones
Martin Maney
> On 29 Dec 2000 22:21:09 -0600, Martin Maney <ma...@pobox.com> wrote:
>>Wayne Johnson <wd...@yahoo.com> wrote:
>>> The formula for calculating the amount of energy dumped out of a capacitor is
>>> t=rc, there t is time in seconds for the voltage to drop 1/3 of the way, r is the
>>> resistance of the ignitor & wire, and c is the capacitance.
>>
>>Uhm... no. I'm not which part of what you thought you were saying is
>>mangled here, but this Just Ain't Right. The energy stored in a capacitor
>>is proportional to the square of the voltage... just a hint, there.
On re-reading, I think I see what Wayne was trying to say. Assuming a fixed
voltage, the energy stored will be proportional to the duration of the
discharge (since the fixed voltage fixes the initial current). For that
case, the energy stored is simply proportional to the capacitance.
> V**2?
Yep. The stored energy is voltage * charge, and the charge stored is
proportional to the voltage to which the capacitor has been charged. Of
course, there are issues with getting that energy to transfer efficently
into the load that make it a little more interesting than just "use a
smaller capacitance charged to a higher voltage".
> Lets see, A 120 microF Capacitor from a Kodak Max camera
> charged to 350V (from a 1.5V battery). How does that compare to the
> xF Capacitors charged to 9V. What do we really need to sucessfully
> fire a single igniter? Energy, power, Impulse, current?
You need to heat the resistive element. For a given ignitor, ideally you
need a certain current * time product. This makes direct application of a
really high voltage charge problematical for a couple reasons.
(added later: another reason is that the results will deviate from that
idealized current * time product when you get too far away from the nominal
driving conditions. Obvious extremes are fusing at a very high current, and
never getting warm enough due to heat dissipating into the environment at
very low current.)
One physical constraint is that higher working voltage electrolytics have
lower capacitances and higher internal resistances than a the reference
low-voltage capacitor. In most cases the V**2 factor gives the higher
voltage capacitor a modest advanatage iff you can transfer the stored energy
to the load with comparable efficency. Simply connecting the capacitor to
the ignitor, even through a lossless switch, won't work. The problem is
that the greater energy is in a form that that is suitable for a much higher
resistance load than the ignitor - in fact, a 12V source is probably about
optimum, or maybe even above, for most ignitors (this is a seat'o'pants
opinion).
The energy stored in that 350V cacpacitor may be double that in a comparable
capacitor at 12V, but the charge stored will be lower; hence the current *
time product that it can deliver to a load is lower. It's an impedance
matching problem, in effect, and a possible solution is a switching
converter. This could be a pretty simple series current-controlled switch,
but I don't know if this would overall offer any net advantage over using a
conventional low voltage. I have been thinking about current-controlled
switching controls as an aid to consistent cluster ignition and/or for their
greater efficency - I hate to lug heavy batteries around! :-)
--
If there were a verb meaning "to believe falsely", it would not have
any significant first person, present indicative. -- Wittgenstein
Dave Lyle
>
>
> > V**2?
>
> Yep. The stored energy is voltage * charge, and the charge stored is
Twice the voltage, 4 times the energy.
Dave
G. Clark
NiMH are more durable than NiCd batteries but less durable than a sealed lead
acid battery, though. The problem with NiMH is that they have tricky charging
parameters. You can look up info on the Panasonic web site for more details.
The catch is this, NiMH cells work by generating hydrogen and storing it a
metal hydride form. Larger cells even have a valve separating the redox cell
and the hydrogen storage. Small cells use the hydride storage as a battery
electrode, so cell seals and ions placed in the hydride part of the cell all
degrade with time and can degrade seriosly in cases of overcharging,
overloading, or over-temp charging and operation.
I am going on memory here, but I beleive Panasonic rates their NiMH cells at 50
cycles before significant performance below specification.
So the point is, NiMH cells are wonderful things, but they certainly do NOT
have unlimited life in terms of charge and recharge cycles. Putting a bank of
high capacity capacitors in parallel with rechargable cells will definately
increase their life both in terms of number of charge cycles as well as life of
an individual charge.
All this is probably off the original question asked. There is no need for
capacitors from the standpoint of supplying a load (unless the load is huge or
the temperature of the battery is extremely low, like say, in Alaska or maybe
dark space). Capacitors are only an additional feature to increase the number
of launches per charge on the battery and to increase the number of charges
before the batteries need to be replaced.
Compared to the cost of Rockets, even NiMH cells are cheap, so the short answer
to the original question is that no capacitors are required.
G. Clark
actually two types of double layer caps out. The symetrical caps, have no
polarity and very, very low series resistances. You can use these things for
spot welding if you want.
The second type are asymetrical, have polarity and are really designed for
low current applications like memory backup and such. The asymetrical caps
are much more available, though, so it seems like they are the only type
around. I know NEC makes the symetrical caps.
As for the voltage rating, you can place the caps in series to increase the
voltage rating but this also divides the capacitance rating. Also, with such
large caps, some balancing resistors are probably a good idea to keep
variations in capacitance from causing one cap in a string from going over
rating and blowing up. ( maybe a new ejection system here ).
I am not sure why a diode OR would be required, but a single diode and a
current limit resistor might be desireable to keep in-rush current below a
reasonable level when a fully discharged cap is getting charged from the
battery. As was pointed out, the space gap (forward bias) of the cap will
decrease the voltage available to the cap by approx 0.6v for a silicon diode
or 0.3v for a germanium diode.
Other uses for large capacitors (not necessarily double-layer) are for
keeping electronics running while an ignitor is drawing load from the main
power source. I built a small Electric Vehicle where I had to place a large
capacitor and a diode in the microcontroller power circuit to keep it
running properly while the starting current of the motors dropped battery
voltage. This may actually be the reason that caps appear in altimeters,
etc.
Glenn
http://www.protean-logic.com
Doug Sams
My original goal was to try to use a cap to assist in airstarting and/or
ejection deployment. The concerns are weight, volume and cost. The
appeal of a cap is its low ESR, low weight, and (perceived) low cost. I
would like to dabble with electronics in LMR type applications, perhaps
as low as D-power. (I was thinking of an air-started D sustainer and of
parallel staging.)
I haven't gained experience with e-matches yet, so I was looking for a
way to airstart an Estes Solar ignitor (or equivalent). Furthermore, I
am not sure how suitable an e-match is for airstarting. I know they are
commonly used in ejection charges. (The assumption here is that a more
sustained ignitor burn is required to ignite a (BP) motor than an
ejection charge. But, ICBW.)
The problems with using batteries are that many of them have too much
output resistance (ESR) and are relatively heavy.
My perspective was to use the cap in parallel with a battery. The cap
would give the ignitor an extra kick up front while the battery would
provide sustaining current until the ignitor opened. (This current vs
time curve might have the same general shape as a thrust curve.)
One problem with caps is getting sufficient charge storage capacity -
Farads. But since they can often be charged to higher voltage levels, we
can put a fair amount of charge in them by elevating the cap voltage
well above the battery voltage. Hence the need for at least one diode to
prevent the cap from discharging into the battery. The other diode was
to keep from diverting the battery-to-ignitor current into the cap once
the cap had discharged.
My preliminary analysis indicated that a very large capacitance was
required in order to get more than a few milliseconds of current from
the cap.
When it was all said and done, we had an expensive super-cap which means
me might as well just get a better battery :(
--
If I ever get around to it, I plan to try some bench experiments to see
if I could get a reasonable cap (vol, weight, $$) to reliably ignite a
BP motor, perhaps using e-matches. I have looked at several firing
circuit drawings I've collected from the web, including some from
altimeters, and none of them strike me as being robust enough to handle
solar ignitors.
(For the HPR guys, the weight and volume issues go away, so the solution
gets much simpler.)
da...@execpc.com
Doug Sams <doug_m...@my-deja.com> wrote:
> Glenn,
>
> My original goal was to try to use a cap to assist in airstarting
and/or
> ejection deployment.
I've got one of the flash units out of a disposable camera. Just for
grins, I did some playing with it.
The unit is powered by a 1.5v AA battery. The cap is a 160 uF 330 volt
unit, and in operation it charges to a measured 303 volts.
I opened the ground lead of the Xenon flash tube, and inserted a 1.5 ohm
non-inductive resistor. Put a 'scope across the resistor, turned on the
flash unit, and set it off.
The voltage across the resistor exceeded 150 volt, or an amazing 100
amps peak current. Only problem is the current had decayed to near 0 in
about 10 ms.
How that fits in with the energy requirement of various ignitors and
ematches, I have no idea.
Dave
Jim Z in VT
Doug Sams wrote:
>
> My original goal was to try to use a cap to assist in airstarting and/or
>
> I haven't gained experience with e-matches yet, so I was looking for a
> way to airstart an Estes Solar ignitor (or equivalent).
Hi Doug. I know from experience that you can light Estes igniters with
9v NiCad batteries (which are actually 7.2v....don't ask me why they market
them as 9v). A pair of them light four Solar igniters fine in my 5 motor Big
Daddy, and I'd think one battery would work well for one or two igniters. (I
dip my airstart igniters in IgniterMan pyrogen for a little added
assurance).
--
Jim Z in Vermont
JimZ...@adelphia.NoThanks.net
(to reply by e-mail, remove the polite decline from my address)
Sams Family
> Hi Doug. I know from experience that you can light Estes igniters with 9v
> NiCad batteries (which are actually 7.2v....don't ask me why they market them
> as 9v). A pair of them light four Solar igniters fine in my 5 motor Big
> Daddy, and I'd think one battery would work well for one or two igniters. (I
> dip my airstart igniters in IgniterMan pyrogen for a little added assurance).
Jim,
Yeah, I figured that battery would be the one which would do it, but my
engineer's mind wants mass quantities of safety margin. I suppose the ultimate
device is a fully charged Sears Diehard packed into a AA cell :)
I used some spec sheet numbers from Panasonic's web site for their 9/7.2V unit,
and the internal resistance wasn't too bad, so I figured no matter what I may
experiment with, that battery would still be the best answer, and of course, I
knew you'd done it which was reassuring.
In fact, I was envisioning the capacitor being used in conjunction with that
battery, but (in the absence of my own empirical data) I think all the analysis
reveals that the only bona fide improvement to that NiCad is to add another
NiCad and ditch the cap :)
This exercise reminds me of the old test engineering adage that "one test is
worth a thousand expert opinions" :)
Doug
Jim Yanik
Well,hook up a few Estes igniters to it,and see if they fire. I suspect
that they will. I'll bet you could fry a .25 or .125w 10 ohm carbon comp
resistor.
Jim Yanik,NRA member
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Alan Jones
My preference would be to only fly the cap, ingiter, and "swiitch",
but I don't know how I'd want to trigger it yet. Someone else
suggested putting the igniter in series with the flash tube,
suggesting that the rest of the baggage should be carried anyway.
That flash tube might even serve as the igniter, with pyrogen added.
With a high voltage system we have massive amps delivered across a 1
Ohm or so igniter. The resistive element will be vaporized. The
question is if the metal vapor wil have enough heat enrgy to ignite
the pyrogen, or just blow the igniter apart without ignition. It
should work fine even with clusters, but I don't know enough about
ignijters and very high voltage & currents. It seems like a waste to
have to drop the voltage down, before sending it through the igniter.
Or maybe we should use a 20 Ohm ignitor?
Alan Jones
G. Clark
Well, several interesting ideas floating around here that are a little
strange to me. So, maybe a little discussion can clear things up.
The first interesting issue is that of voltage required to fire an ignitor.
Measure the resistance of an ignitor. It is probably something like .01 ohm
or less. (I'll measure the resistance of some of the Estes ones I have after
I dig them up.) This means an ideal voltage source of just one volt will
push 100 amps through the ignitor. So why the concern over voltages on power
sources....... The non ideal nature of the voltage source. If a battery has
just one ohm of internal resistance, then the maximum current it can deliver
is equal to its voltage. A carbon zink 9 volt battery usually has about an
ohm or two of series resistance. (See manufacturers specifications for
details) So, the ignitor can only get about 4 or 5 amps from them when the
battery is completely healthy and fully charged. Throw in an ohm or two for
lead resistance or poor connections between battery and the ignitor and you
see the source for possible problems. Most rechargable cells have much lower
internal resistance, so they are a much better candidate for this sort of
high instantaneous load application.
The next interesting issue being discussed has to do with capacitors and
their relative performance with batteries. There is NO question that
batteries have higher energy densities. However, this comparison makes NO
sense unless you intend to completely discharge the battery to fire one
ignitor. The capacitor only needs to have sufficient energy to meet the
needs of the ignitor until it burns up. Conventional electrolytic or
tantilum capacitors EASILY have that much energy storage and low enough
series equivelent resistances to fire ignitors when charged at even low
voltages. Their physical size is probably too large for sufficient
capacitance, however. So this leads to the new double layer caps.
I took an AL series double layer cap, 3.3 Farad with a rated 0.3 ohm series
resistance, Panasonic number EEC-A0EL335 or Digikey number P6964-ND, charged
it to 2.5 volts and connected various fuses to it. This cap is approximately
23mm long and 12.5mm in diameter. A 2.5 volt charge draining through a .3
ohm load (assumed zero resistance for the fuse) should deliver a peak
current of 8.33 amps and have an instantaneous current expressed by the
following formula:
i(t)=Vinit/R * exp( t/( R * C ) )
where i(t) is the current, Vinit is the initial charge on the cap, t is time
in seconds, R is series resistance in ohms and C is capacitance in farads.
So, at 100 ms after the circuit is connected,
I = 2.5/0.3 * exp( -0.1 / ( 0.3 * 3.3 )) = 7.53 amps.
after 1 second the current will be 3.04 amps.
I found that I could blow a 3 amp fast blow very consistantly. However, it
would not blow a 5 amp slow blow fuse. I'll dig up some ignitors and perform
some more experiments to see what I can see with the devices I have around
and report back.
Glenn
grassy
is really not necessary since you are effectively providing a stepped up
voltage in a sense at a compromised current. A good 9v Nicad offers plenty
of current. A 2 - 3000 UF cap will help it out loads.
Grassy
<da...@execpc.com> wrote in message news:93inb5$kvc$1...@nnrp1.deja.com...
da...@execpc.com
"grassy" <1...@1.com> wrote:
> I doubt 10ms at any current would fire a match or igniter. The camera
idea
> is really not necessary since you are effectively providing a stepped
up
> voltage in a sense at a compromised current. A good 9v Nicad offers
plenty
> of current. A 2 - 3000 UF cap will help it out loads.
Nothing beats a good battery.
The idea with the cap is to get the most energy storage into the
smallest and lightest package. Energy storage is a function of the
capacitance and the square of the voltage. A 160 uF cap charged to 300
volts can dump 7.2 joules of energy. A 5333 uF cap charged to 9 volts
(same C x V product) can dump only .22 joules.
The problem would be controlling the way the higher voltage cap is
discharged to allow a standard ignitor to 'use' that energy efficiently.
Discharging the cap through a pulse forming network comes to mind, but
at the impedance levels involved (1 - 10 ohm output) the inductors in
such a network would problably be faily large and heavy - not what we
want.
grassy
You go into RC time constants below. The peak current of the circuit is
8.3333 amps, assuming .3 load and a perfect cap charged to 2.5v. Next you
talk about 1 second time and so on. If you start with a one second Time
constant, the current in this circuit assuming your cap has a 2.5 volt
charge would decrease by 67% leaving you with approximately 2.74 amps at the
one second interval.
A 1 farad capacitor is an obscene size and would not fit in a rocket. A 3.3
farad capacitor would be worse and heavier.
For typical applications, the .3 ohms is fine but most people use 2200uf and
maybe a little higher with a 9volt Nicad so:
.3ohms * .0022farads = 660us or .000660 seconds which basically means after
just .0006 seconds the cap will have 33% of its charge left. The ideal
current in the circuit would be 2.5volts / .3 ohms = 8.333 ohms but most
people use 9 volt nicads so lets figure 9.5 or so / .3 ohms which yields
31.666 amps. At one time constant of .00066 seconds, you would have about
10 amps in the circuit. After 1.2 ms or so you would have roughly 1.5amps
then less than amp and so on. In less than a second you would have 0 amps.
Because the cap appears in parallel with the load, you really can not look
at it as a cap supplying the circuit because the Nicad is there for the
entire time, until the igniter burns out to supply current where the cap
bolsters that current.
Figuring 9.5volts for a healthy Nicad /.3 resistance you have a current of
31.6666 amps which would never happen due to internal resistances. The cap
when that circuit connects dumps that much more current and appears as a
parallel supply as indicated above.
In simple terms, a decent Nicad 9 volt with a 2200uf works well for most
apps.
Grassy
G. Clark <gac...@pop.dnvr.uswest.net> wrote in message
news:3A5D4E51...@pop.dnvr.uswest.net...
grassy
I have always used 2200 uf which has worked while I have used one 9 volt to
power my board and the other to fire the igniter. Nothing like a brown out
to send your rocket home faster than you planned. The math I think confuses
a lot of people so I would say that they ought to use the 9 volt battery
with a cap of somewhere in the 2200 ballpark.
grassy
<da...@execpc.com> wrote in message news:93kpn6$cek$1...@nnrp1.deja.com...
Leonard Fehskens
>If you start with a one second Time
>constant, the current in this circuit assuming your cap has a 2.5 volt
>charge would decrease by 67%
1 - 1/e = .632
Is that relevant? Does it move the CG aft?
len.
Dave Lyle
Yes, assuming the cap is forward of the CG. As the cap discharges, it
loses energy. As we all know, energy and mass are related by the famous
formula E = MC^2. So as the energy is lost, the mass decreases moving
the CG aft.
Dave
Kurt Kesler
> A 1 farad capacitor is an obscene size and would not fit in a rocket.
Now that would depend on the rocket, would it not? ;-)
--
Kurt Kesler
G. Clark
Actually I am holding in my hand a 3.3 Farad cap that is 12.5mm by 25mm..... You
must build very, very small rockets.
As for RC constants, RC time constants are a more crude form of evaluating
current flow or charge in an RC circuit. The formula I gave you is the basis for
the derivation of the RC time constant rule of thumb.
Anyway, The discussion here is not meant to discourage the use of batteries.
Rather, to explore alternate methods. The relatively new Double Layer Caps,
which utilize the surface area a carbon latice for the plates have huge
capacitance in small area and mass. These caps have been available for about 5
years, but are just now getting cheap enough to play with. The 3.3 farad 0.3 ESR
cap I have in my hand costs about $6.00 in quantity one. You can buy a 10 Farad
0.1 ESR cap for about $13.00 (US) qyt 1.
So, I am in agreement about batteries being both the easiest as well as the
tried and field tested method. However, technology has CHANGED and farad range
caps are now very small and well worth considering in special situations like
in-flight power sources.
I am primarily active in the fields of electronics and microcontrollers, but
those of you who want to try electronics in rockets and are willing to try new
things SHOULD consider the double layer cap technology. It has lots of promise
for short term high load power sources. These things are smaller than batteries,
much lighter, fully charge in seconds, are relatively temperature independent,
and have low ESR for their physical size.
To give you an example, I used one of these double layer caps (the 3.3 farad one
I mentioned) hooked it up to an inexpensive DC gear motor, a diode and a high
brightness LED. By spinning the DC motor for a few seconds, I could place
sufficient charge on the CAP to light the LED for about 5 minutes. This
"mechanical flashlight" would never have been possible with either batteries or
older style electrolytic caps.
So, there is the info, do what you want with it. Ill report back when I actually
have some experience firing engines with capacitor powered ignitors.
Glenn
grassy
I challenge you to build a rocket that could carry a one farad cap. If you
had any idea how big that would be you would not say what you are saying.
As long as we're on the topic, how about 3.3 farads....
Kurt Kesler <ne...@keslers.removetosend.net> wrote in message
news:C5CA83DA5BF6D1DD.EFAB4207...@lp.airnews.net...
Robert Galejs
>
> Kurt,
>
> I challenge you to build a rocket that could carry a one farad cap. If you
> had any idea how big that would be you would not say what you are saying.
> As long as we're on the topic, how about 3.3 farads....
>
(AL series gold) with a 12.5 mm diameter and 23 mm length. I bet I
could fly that on an A8-3.
- Robert Galejs
Kurt Kesler
from under the seat in my car, drop it in my L3 bird, and send it to
~7000 feet. How much would you like to wager? (hopefully enough to pay
for the reload)
A 1 farad, 14v cap is a little bigger than a Pepsi can. Here is one link
http://www.eau.com/sw/intecaps/
and here is one installed in a car
http://www.netwizards.net/~thndr50/snd1.jpg (black thing between the
amps)
My original comment was tounge-in-cheek, BTW, as in "I bet a Sat V could
probably lift it, along with my car, the neighbors kids, etc." I do,
however, think that I know what I am talking about.
--
Kurt Kesler
Mark S.D.
>Kurt,
>
>I challenge you to build a rocket that could carry a one farad cap. If you
>had any idea how big that would be you would not say what you are saying.
>As long as we're on the topic, how about 3.3 farads....
>
How big would a 1 farad capacitor be?
keep in mind, he didn't say it was in the range of rockets he could build, but
there are some very big rockets out there.
Mark
Mark's Pyrotechnics and Rocketry Page
Kurt Kesler
soci...@aol.com says...
> Then come to Orangeburg this weekend. I will pull the 1 farad ~14v cap
> from under the seat in my car, drop it in my L3 bird, and send it to
> ~7000 feet. How much would you like to wager? (hopefully enough to pay
> for the reload)
> A 1 farad, 14v cap is a little bigger than a Pepsi can. Here is one link
> http://www.eau.com/sw/intecaps/
> and here is one installed in a car
> http://www.netwizards.net/~thndr50/snd1.jpg (black thing between the
> amps)
> My original comment was tounge-in-cheek, BTW, as in "I bet a Sat V could
> probably lift it, along with my car, the neighbors kids, etc." I do,
> however, think that I know what I am talking about.
<Not for you, Mark, but just in general>
I do not understand all the doubt cast upon what I am saying. I have
shown web links, etc. and if that is not good enough then I will put a
~14v 1 farad capacitor in a doubters hands if need be.
--
Kurt Kesler
grassy
specs. .0000033farads = 3.3 uf
Robert Galejs <gal...@ll.mit.edu> wrote in message
news:3A5F2CC6...@ll.mit.edu...
grassy
I have not used the newer caps so yes they would work but 1 farad? Isn't it
a bit overkill? Yes it would work but then you could use an arc welder to
launch a rocket too but obvious overkill.
The Time constant I used only as an alternative. I went about my
calculations a little different but it works as long as you are working with
measured values. I have used time constants accurately with measured
values. It was something I took notice of with your calculations but once
again, wouldn't you consider it a bit overkill for this stuff?
Grassy
G. Clark <gac...@pop.dnvr.uswest.net> wrote in message
news:3A5F27F4...@pop.dnvr.uswest.net...
grassy
Kurt Kesler <ne...@keslers.removetosend.net> wrote in message
news:DFCB24CE526BAF47.56BFD0A4...@lp.airnews.net...
grassy
farad cap at one point would be your seat not go under it.
One of the points I was making is that it seems a bit of overkill to use a 1
farad cap though.
Grassy
grassy <1...@1.com> wrote in message news:3a5f6...@news3.buffnet.net...
David Weinshenker
> I haven't gained experience with e-matches yet,
> so I was looking for a way to airstart an Estes
> Solar ignitor (or equivalent). Furthermore, I
> am not sure how suitable an e-match is for
> airstarting. I know they are commonly used in
> ejection charges. (The assumption here is that
> a more sustained ignitor burn is required to
> ignite a (BP) motor than an ejection charge.
> But, ICBW.)
An electric match should be able to ignite a BP
motor easily IF you can fit it into the nozzle!
(Might work with a D12 but the ones I've seen are
a bit big to fit in most of the B or C motors.)
A BlackSky "HiRMi" (it's the same as one of the
DaveyFire types) will easily fire on a 9 volt
battery, with rapid ignition of the flammable
composition, and sufficient energy output to
easily ignite the surface of a BP grain.
(The current ratings are "no fire" .4 amps and
"all fire" 1.0 amps, which a 9 volt battery
can easily supply.)
If you want to ignite composite motors, one
technique that I've seen recommended is to use
an electric match dipped in one of the commercial
igniter compositions (Igniterman, Firestar, etc.)
which gives the same easy ignition and a more
vigorous burn.
-dave w
grassy
saying that I thought 1 farad was a little like killing a fly with a sledge
hammer.
Grassy
Kurt Kesler <ne...@keslers.removetosend.net> wrote in message
news:37866411CEFAB339.36DCC517...@lp.airnews.net...
Christopher Cox
> > >constant, the current in this circuit assuming your cap has a 2.5 volt
> > >charge would decrease by 67%
> >
> > 1 - 1/e = .632
> >
> > Is that relevant? Does it move the CG aft?
>
> Yes, assuming the cap is forward of the CG. As the cap discharges, it
> loses energy. As we all know, energy and mass are related by the famous
> formula E = MC^2. So as the energy is lost, the mass decreases moving
> the CG aft.
LOL!!!!!
I can't wait for some one to solve for M now!
Bob Kaplow
> How big would a 1 farad capacitor be?
> keep in mind, he didn't say it was in the range of rockets he could build, but
> there are some very big rockets out there.
There's two flavors of big honken caps on the market today. One has a
relatively higher internal resistance, low voltage rating, and is used for
things like CMOS battery backup in computers. They are small enough to fly
in questes stuff with payload sections. But are relatively useless for
launch systems and hte like.
Then there are the power supply caps used in computer and audio
applications. They are more like the size of a can of pop, and maybe a bit
heavier. Still easilly lofted by most HPR rockets. These are the kind of
caps we're talking about for launch systems. Refinements onthe electrolytic
caps from old computer power supplies.
Then there is the VAX 11/780 power supply in my basement! Ray Halm would have
no trouble sending this into the sky, but many of us might find it
difficult.
Bob Kaplow NAR # 18L TRA # "Ctrl-Alt-Del"
Kaplow Klips & Baffle: http://www.nira.chicago.il.us/Leading_Edge/MayJun00.pdf
NIRA: http://www.nira.chicago.il.us NAR: http://www.nar.org
hern...@fill.zzn.com an...@eircom.net postm...@127.0.0.1 FTNC...@aol.com
lisa...@ntlworld.com to...@cartoonbank.com joel....@welshexec.com u...@ftc.gov
homewo...@alloymail.com Lisa.C...@attunity.com k...@net2000.com.au
David...@trendmicro.com web...@trendmicro.org Newsl...@editor.com
Bob Kaplow
> I hate to blow your bubble but that is probably in micor farads. Check the
> specs. .0000033farads = 3.3 uf
Nope. He's talking about real multi farad caps. But not ones useful for
rocket launch system applications.
Kurt Kesler
> I am not doubting you, I went to the links and learned something new. I was
> saying that I thought 1 farad was a little like killing a fly with a sledge
> hammer.
>
> Grassy
>
Thank you. Several people have questioned my credibility about
the existence of these over the past couple of weeks.
Re: usage...I quite agree with you. Someone asked about availability of
large capacitors and since I was aware of some I responded. I have no
idea why they would require that large a capacitor (unless you have some
big 12v amps in your rocket or launch controller ;-)
--
Kurt Kesler
5...4...3...2...1...boom boom boom
Sams Family
> An electric match should be able to ignite a BP
> motor easily IF you can fit it into the nozzle!
> (Might work with a D12 but the ones I've seen are
> a bit big to fit in most of the B or C motors.)
>
> A BlackSky "HiRMi" (it's the same as one of the
> DaveyFire types) will easily fire on a 9 volt
> battery, with rapid ignition of the flammable
> composition, and sufficient energy output to
> easily ignite the surface of a BP grain.
> (The current ratings are "no fire" .4 amps and
> "all fire" 1.0 amps, which a 9 volt battery
> can easily supply.)
>
> If you want to ignite composite motors, one
> technique that I've seen recommended is to use
> an electric match dipped in one of the commercial
> igniter compositions (Igniterman, Firestar, etc.)
> which gives the same easy ignition and a more
> vigorous burn.
>
> -dave w
Thanks, Dave. Great post. This one's going into the
save file.
Doug
John Grassi
Bob Kaplow <kapl...@eisner.decus.org.mars> wrote in message
news:wDYMPD...@eisner.decus.org...
John Grassi
Bob Kaplow <kapl...@eisner.decus.org.mars> wrote in message
news:wDYMPD...@eisner.decus.org...
Steve S.
They have Panasonic caps up to 10 F. They are gold, double layer and a size
that could be launched in a model rocket. Science staggers ahead. Ten years
ago you would expect a one farad cap to fit in a room. Now you can carry it
in your shirt pocket.
So what's this about a challenge?
"grassy" <1...@1.com> wrote in message news:3a5f1...@news3.buffnet.net...
Glen Gardner
ago would it?
Dave Lyle wrote in message <3a56a6f8$0$36804$272e...@news.execpc.com>...
>Martin Maney wrote:
>>
>>
>> > V**2?
>>
>> Yep. The stored energy is voltage * charge, and the charge stored is
>Energy stored in a capacitor in Joules (watt-seconds) is = (C * V^2)/2
>
>Twice the voltage, 4 times the energy.
>
>Dave
Dave Lyle
>
> This wouldn't be the same Dave Lyle that worked at Heath Company many years
> ago would it?
No, but I sure built my share of HeathKits years ago.
Dave
Z
What do you mean by 5 motor big daddy?
z2...@aol.com
Leonard Fehskens
>
>I'm sorry but I'm new at this.
>
>What do you mean by 5 motor big daddy?
This was a conceptual dragster design by Don "Big Daddy" Garlits that
use 5 supercharged V-8 hemis, one on each wheel and one to power an
extra turbocharger. It was never built...
len.
Jim Z in VT
Leonard Fehskens wrote:
Z at z2...@aol.com wrote:
> >What do you mean by 5 motor big daddy?
>
> This was a conceptual dragster design by Don "Big Daddy" Garlits that
> use 5 supercharged V-8 hemis, one on each wheel and one to power an
> extra turbocharger. It was never built...
:-) excellent!
But the 5 motor Big Daddy in question here is my modified Estes Big
Daddy kit (a.k.a. the "Big Bad Voodoo Daddy), flying on a cluster of 5
motors: a 24mm core (usually an F24-7) surrounded by four 18mm outboards
(usually C6-0's) that are airstarted just after launch by an ignition
circuit I built into the nosecone. Flys to about 1100'. Not the baddest Big
Daddy around, for sure, but a way cool flight nonetheless.
--
Jim Z in Vermont
JimZ...@adelphia.NoThanks.net
(to reply by e-mail, remove the polite decline from my address)
Mark BEST
have a thin
bridgewire encased in pyrotechnic material.
Minimum "All-Fire" current is typically 750ma
Recommended fire current is 1 amp
Typical resistance of ignitor is 2.5 ohms which means a voltage of 2.5 volts is required
to successfully
Ignite the device.
I have carried out some tests and have found ignition to occur within 6 microseconds of
applying the recommended current of 1 amp. The bridgewire remained conductive for at
least 12 microseconds. This is an important consideration when using fuses in series
(for fireworks applications).
There is little value in using a capacitor for a low voltage system. These are of more
value when a high
voltage discharge system is employed (especially suitable for long distance ignition).
Some of the 'wind
up' type exploders used in the explosives industry generate 1600 Volts and charge a 6
microfarad
capacitor. It doesn't take much effort to charge a “Beethoven Exploder” to firing
condition - 20 to 30 'winds' of the handle - but it delivers a healthy jolt!
"Energy" is more critical when capacitors are the sole energy supply. But there are
other factors - A 6
microfarad capacitor charged to 1600 volt (7.68 Joules of energy) will successfully
ignite a fuse 2km away
on zip wire (26 awg) but similar energy (7.2 Joules) in a 0.1 Farad capacitor charged
to 12 Volts will not
ignite the same circuit - It is unable to provide the current of 1 amp as the circuit
resistance would be in the
order of 40 ohms (ie the current would be 12/40 = 0.3 amp). If you're close enough, with
reasonable
cable a 12 volt battery should be fine for most applications.
Don't overlook the usefulness of car 'remote' alkaline batteries which are smaller than
AAA and are 12 volt. Two of these in series makes a compact , cheap power source which
can supply around 3 amps in a 'short' condition (i.e. max current) they can be used many
times as the ‘energy’ used to ignite the ‘fuse’ is minimal ( I believe it is in the
order of 8 milliJoules for explosive detonators). The construction is similar to
detonators and many igniters in the fireworks industry exhibit the same basic
characteristics.
Hope this is of some use or at least sparks some interest in the rocket ignitors.
PS energy in a capacitor is Half the product of the capacitance and the square of the
voltage (Vsquare x Cap / 2) ie Disposable Photo flash = 285 Volts across a 160
microfarad cap = 6.498 Joules
Alan Jones wrote:
> On 29 Dec 2000 22:21:09 -0600, Martin Maney <ma...@pobox.com> wrote:
>
> >Wayne Johnson <wd...@yahoo.com> wrote:
> >> Me thinks 2-3 farads a bit overkill for an initiator/ignighter.
> >
> >Yep. Unless maybe you're trying to fire off clustered AP motors using
> >Copperheads. :-)
> >
> >> The formula for calculating the amount of energy dumped out of a capacitor is
> >> t=rc, there t is time in seconds for the voltage to drop 1/3 of the way, r is the
> >> resistance of the ignitor & wire, and c is the capacitance.
> >
> >Uhm... no. I'm not which part of what you thought you were saying is
> >mangled here, but this Just Ain't Right. The energy stored in a capacitor
> >is proportional to the square of the voltage... just a hint, there.
>
> V**2? Lets see, A 120 microF Capacitor from a Kodak Max camera
> charged to 350V (from a 1.5V battery). How does that compare to the
> xF Capacitors charged to 9V. What do we really need to sucessfully
> fire a single igniter? Energy, power, Impulse, current?
>
> Alan Jones
M Dennett
Mark BEST <mark...@ozemail.com.au> wrote in message
news:3A895B90...@ozemail.com.au...
> I am new to this newsgroup but here's something I know about fireworks
igniters - which
> have a thin
> bridgewire encased in pyrotechnic material.
>
> Minimum "All-Fire" current is typically 750ma
> Recommended fire current is 1 amp
> Typical resistance of ignitor is 2.5 ohms which means a voltage of 2.5
volts is required
> to successfully
> Ignite the device.
Mark,
The specs vary from product to product. An electric match I use commonly on
fireworks shows is the Daveyfire N28B, which has a nominal bridgewire
resistance of 1.6 ohms and all-fire current of 0.37 Amps. Typical response
is 2ms at 1 Amp. The "28" means 28 micron bridgewire diameter.
There are other versions, such as the "mini" fusehead M28F which uses the
same bridgewire and thus has the same resistance, but the composition is
different and the all-fire current is higher as a result of the increased
activation energy of the pyrogen.
I would caution anyone not to assume that specs are similar to another
product when proper specs are not provided with an igniter product. I have
seen two rockets pile in because the electronics couldn't provide sufficient
output for the electric match on the recovery system, though they would test
out fine with a known entity such as the N28B. In an effort to make
electronics packages smaller, lighter etc., hopefully mfgs will make
allowances for a range of electrical requirements. As an extreme example,
though you won't find these extremes in the rocket or pyro market, Daveyfire
offers bridgewires in the N series from 15 micron (0.16 Amp all-fire) to 155
micron (12 Amp all-fire).
- Mike D.
Mark BEST
Thanks for the reply!
I thought my post might have been buried too deep in a boring topic!!
My experience to date has provided little variety in Australia regarding
electric matches. I am aware of some of the daveyfire products and have some of
their data sheets but not the interseting data you have provided. Specs also are
a little rare which is whay I have performed tests - which I found to be very
interesting and rewarding.
I plucked values from the ether that would be reasonable for most applications,
but realise that a careless test/continuity current could be near the 0.16 amp
you mention.
Have you heard of the SCB - Semiconductor Bridge igniter. Similar devices, I
believe, are used by 'real' rocket people to fire the charges that seperate
rocket stages. One article stated 20usec pulse of 30A = 0.03 mJ. Low energy but
high current required to achieve the necessary effect.
As always, I recommend that everyone be familar with the products they use and
if possible safely test the components that are critical to the
flight/safety/performance. I also should recommend that if multiple igniters
are to be used in one circuit (e.g. 3 igniters in series) that all igniters are
of the same type.
Regards,
MB