Hot wire cutter question, power supply
Jon Anderson
Probably not more than 12-16" span, max. Found a simple setup on
Instructables using a 24v 2a transformer and a wall light dimmer.
Well, I've got a 12v 4a DC power supply, seems to me that this ought to
be enough, but thought I'd ask.
Thanks,
Jon
Bill Noble
how are you going to control the voltage on the power supply - you don't
want the wire glowing white hot, you want it just warm enough to do the
job. Many 12V power supplies are regulated, so it would take a bit of
futzing around to make the supply adjustable
Winston
If you plug your 12v supply into an autotransformer (Variac) and read
it's DC output voltage (across a 1.2K ohm 1/2 W resistor) with various
input voltages, you will find out if your supply has an internal
regulator or is unregulated.
If it is unregulated, Bobs Your Uncle. Just run the supply at the
appropriate power level using your autotransformer.
If it is internally regulated, you get to insert an external
variable regulator between the supply and your cutter or you get
to replace it with a 12 V variable bench supply.
--Winston
Richard
That's something I usually cobble together when needed.
Safety wire, a board and some nails are all it takes.
(ok, and a battery charger)
Nail some nails in the board, zig-zag some wire between them (wrap
around the nail, of course).
That's the voltage regulator.
Stretch the cutter wire tight (in a frame - probably more of that board.
Clip the charger to one end of the cutter, the other end to the last
nail (farthest from the cutter) and turn it on.
Move the clip to a closer nail to make it hotter.
The right temperature will make crisp bacon sizzle when the wire touches
the foam.
It doesn't smell as tasty though...
Richard
James Waldby
As Bill Noble and Winston have noted, you could find out if the
power supply (PS) includes a voltage regulator. Then you could
find out if it's a linear or switching regulator. A 12V 4A linear
PS would usually include a 60Hz transformer weighing 3 or 4 pounds.
If you wanted to de-regulate, you could make an output connection
after the rectifiers and filter caps but before the regulator.
Some 60Hz transformer VA vs size vs weight stuff:
<http://amfone.net/Amforum/index.php?action=printpage;topic=16537.0>
If the supply is light weight it probably has a switching regulator
and may be unsuitable for de-regulating.
An alternative to changing voltage is changing wire gauge or length.
<http://www.heatersplus.com/nichrome.htm> has four worked examples
of computing wire gauge, length, voltage, and amperage to get
desired temperatures etc, about 1/3 thru the page.
For example, that 24V 2A PS would need thin wire, say #28 or #30,
to avoid excessive current while still heating up enough. I have
a 21" long hot wire cutter with #24 nichrome (about 0.020 diameter)
powered by a battery charger. Draw is 1.5 - 2A at 6V setting and
3.5 - 4A at 12V, which from above link would be about 500F and 900F
respectively. It works well at 12V except that the section of wire
that is cutting cools off too much, which slows down cutting speed.
A better setup would be #21 wire with a 6 or 7 amp 20V PS -- it would
run a few hundred degrees hotter (to cut much faster), would have
more metal in the cut (hence more heat capacity, to cool off more
slowly in the cutting zone), and could be subject to higher spring
tension without breaking (thus bowing less, giving a straighter cut).
Another nichrome wire properties chart:
<http://www.interfacebus.com/properties-of-nichrome-wire.html>
--
jiw
Wild_Bill
lamp dimmer module on the primary 120VAC side of a step-down transformer.
An actual AC-DC power supply isn't required, but could be used if the design
is a simple AC/DC adapter without extra circuitry.
The dimmer module performs the task of a variable temperature control for
the heated wire.
A similar device is a triac circuit that will vary the duty cycle of the
120VAC input to the transformer primary. With only a triac, possibly a small
heatsink, variable resistor/pot, capacitor and inductor, the circuit is
really compact and simple. This same circuit has been used in a variety of
heating devices and soldering iron variable temp controls.
The most important part of the wiring will be the safety ground. If the
safety ground is ignored or done improperly, a fatally hazardous condition
may exist.
The input line needs to be properly fused, and have a reliable, easy to
reach power switch.
A good quality step-down transformer "should" provide adequate line
isolation.
A variac (autotransformer) does not provide isolation, and the output will
be a direct, fatally hazardous connection to the utility power in the
building.
The transformer frame and one secondary winding connection should definitely
be safety earth ground connections. These connections will guard the user
from any transformer leakage issues.
Standard insulating and connection practices, including a grounded metal
enclosure for the electrical connections, will result in a safe device.
If any of the connection or grounding info is unclear, seek qualified,
knowledgeable and experienced help from someone that can check your assembly
methods before using a DIY home-built line voltage operated device.
Secondly, you may find it very helpful to look up info regarding
vapors/fumes from heated synthetic materials.
Some plastics, probably more than the plastics industry would want us to
know, will give off toxic fumes when heated or burned.. maybe even small
amounts of sarin, for example.
I generally don't even use plastics in a microwave oven, but not because I'm
particularly paranoid about it, but I've seen the damage that MW heating
does to plastics.
--
WB
.........
"Jon Anderson" <jande...@comcast.net> wrote in message
news:Us4Bo.364382$Yn5.1...@en-nntp-14.dc1.easynews.com...
Steve Lusardi
There are some good suggestions here, but I would use a heavier wire for thermal mass reasons and a PWM (Pulse Width Modulation),
DC, PM speed control. They are inexpensive and do not use high voltage, so is much safer. The real driver for me would be the
trigger operated pot to instantly control the wire heat when in use to compensate for the cooling effect on the wire by the
substrate.
Steve
"Jon Anderson" <jande...@comcast.net> wrote in message news:Us4Bo.364382$Yn5.1...@en-nntp-14.dc1.easynews.com...
Doug White
news:ib39pb$lkk$03$1...@news.t-online.com:
There's a ton of info here:
http://www.ar15.com/forums/topic.html?b=3&f=4&t=292695
A lot of photos of folks work, but also a lot of different hardware set
ups to consider.
Doug White
Sunworshipper
I had to make one of those to make long triangles to insulate the
inside corners of the pump house, that reminds me, got to set that up
for winter today.
I went to a hobby shop and bought a beat up old train transformer with
the big variable switch on top for $10 IIRC. Then I found some of
those old ceramic rings light bulbs use to be screwed into and hooked
a spring to each one and strung stainless steel aircraft safety wire
between the two insulators. I guess one on a spring would work also,
can't recall why I did that. Anyhow worked great.
I even set up a 10' long table and a fence that I could move to either
side of the wire to make "crown molding" out of 4"X 4' X 8' foam.
Thinking of setting it back up soon to make lost foam molds for
aluminum.
SW
Ned Simmons
As an alternative, a stick/TIG welder is an adjustable constant
current supply. I've used mine to power a quick and dirty hot wire
cutter. The welder's minimum current may require using a wire heavier
than the typical cutter. Mine will go down to about 5A.
--
Ned Simmons
Bill Noble
> The hot-wire foam cutter that I looked at several years ago used a wall
> box lamp dimmer module on the primary 120VAC side of a step-down
> transformer.
>
snip---------
danger - copying the above into your own design is likely to be both
risky and unsatisfactory - please consider using a proper transformer
with isolated primary and secondary circuits
Jon Anderson
That in fact, is precisely the version I saw on Instructables....
I appreciate all the great input. I should have given a bit more detail.
I'm quite certain the power supply I have is a linear. I plan to control
input voltage with a variac.
This is nothing special or long term, it's part of my project to line my
Vidmar drawers with foam and cut pockets for micrometers and other
tools. I found the perfect foam, lining the cases used to ship a medical
device one of my customers manufactures. They gave me the contact info
for the company that does the custom foam inserts, and I ended up with 6
sheets of nice dense black Polyurethane foam, 1/2" thick, cut to fit my
drawers exactly (25" square, where most foam I found came in 24" widths)
After cutting a pocket, I am going to hot-wire about a 3/16 thick slice
off the piece I just removed, and put it back in the cutout.
Thanks everyone, great info as always!
Jon
Wild_Bill
that's the way you interpreted it, Bill.
I described the dimmer going to the primary side of a step-down transformer.
I think most people that know transformers, understand that a step-down type
consists of a primary and a secondary winding.
It's not easy describing electrical circuits in a way for everyone to
understand them.. that's why I always try to remember to include the seek
help statement.
In the DIY article I saw, there was a step-down transformer utilized, and
numerous comments about how some dimmers won't last long when attempting to
drive (the primary winding of) a transformer, instead of driving a resistive
load of incandescent lamp.
--
WB
.........
"Bill Noble" <Nob...@nowhere.invalid> wrote in message
news:ib3r0a$qg8$2...@news.eternal-september.org...
Terry
<jande...@comcast.net> wrote:
Jon,
I started with a goodly length of nichrome (dunno what gauge, it was a
heating element I picked up) and a 12V battery charger. Decreased the
length of nichrome til it was just right for that output.
--
Best -- Terry
Rich Grise
>
> In the DIY article I saw, there was a step-down transformer utilized, and
> numerous comments about how some dimmers won't last long when attempting
> to drive (the primary winding of) a transformer, instead of driving a
> resistive load of incandescent lamp.
>
reluctant to use SCR control on the primary of a transformer. I'm "only" a
tech, so I can't quote numbers; it's just sort of a gut feeling that the
inductive reactance could cause a phase shift and upset the firing sequence
of the SCRs.
Of course, I'd be happy to hear of anyone else's experience with this kind
of setup.
Thanks,
Rich
Winston
> Wild_Bill wrote:
>>
>> In the DIY article I saw, there was a step-down transformer utilized, and
>> numerous comments about how some dimmers won't last long when attempting
>> to drive (the primary winding of) a transformer, instead of driving a
>> resistive load of incandescent lamp.
>>
> I'm an electronics guy by experience and training, and I'd be terribly
> reluctant to use SCR control on the primary of a transformer. I'm "only" a
> tech, so I can't quote numbers; it's just sort of a gut feeling that the
> inductive reactance could cause a phase shift and upset the firing sequence
> of the SCRs.
You are right, Rich. (And thanks for your post because
it caused me to solve a problem I didn't even know I was
about to encounter!) :)
http://www.st.com/stonline/books/pdf/docs/3566.pdf
See the first five pages for an accessible analysis of the
hazards of driving inductive loads with 'SCR' type devices.
See pages '6/16' and '7/16' for a clever triac circuit ST
developed to drive inductive loads without the flaws of
the earlier circuits.
Quoting them:
"This circuit has been developed by the STMicroelectronics
applications laboratory and used with success for a wide
range of equipment."
See the text at the bottom of page
'6/16' for a circuit explanation. See Figure 9 on page
'8/16' for component values.
--Winston
Bill Noble
it depends on whether the SCR (triac really) is chosen to drive an
inductive load or only a resistive load - the motor speed control in a
variable speed drill is sized and rated for an inductive load, a lamp
dimmer, more particularly an older lamp dimmer, is very much not sized
that way
Gunner Asch
Ive got a bunch of brand new rolls of hi-temp wire, if anybody wants any
Gunner
--
"Confiscating wealth from those who have earned it, inherited it,
or got lucky is never going to help 'the poor.' Poverty isn't
caused by some people having more money than others, just as obesity
isn't caused by McDonald's serving super-sized orders of French fries
Poverty, like obesity, is caused by the life choices that dictate
results." - John Tucci,
Gunner Asch
OOOH!! Excellent idea! I never thought about doing that.
Very nicely Done!
Crom but I like reading the smart guys here!!
Winston
Er. That's page '9/16' for component values.
>
> --Winston
Rich Grise
> Rich Grise wrote:
>> Wild_Bill wrote:
>>>
>>> In the DIY article I saw, there was a step-down transformer utilized,
>>> and numerous comments about how some dimmers won't last long when
>>> attempting to drive (the primary winding of) a transformer, instead of
>>> driving a resistive load of incandescent lamp.
>>>
>> I'm an electronics guy by experience and training, and I'd be terribly
>> reluctant to use SCR control on the primary of a transformer. I'm "only"
>> a tech, so I can't quote numbers; it's just sort of a gut feeling that
>> the inductive reactance could cause a phase shift and upset the firing
>> sequence of the SCRs.
>
> You are right, Rich. (And thanks for your post because
> it caused me to solve a problem I didn't even know I was
> about to encounter!) :)
>
> http://www.st.com/stonline/books/pdf/docs/3566.pdf
>
Thanks!
Rich
Ken Moffett
> polyurethane foam
DO NOT CUT POLYURETHANE FOAM WITH A HOT WIRE !!!
The fumes are very toxic... unlike polystyrene...including
hydrogen cynide (think gas chamber).
Check the MSDS's.
Ken
Jon Anderson
> The fumes are very toxic... unlike polystyrene...including
> hydrogen cynide (think gas chamber).
>
> Check the MSDS's.
A BIG Thank You! I'd planned to do it outdoors, but I don't like messing
with anything toxic I don't -have to-. I have an alternative method to
slice down the foam, and I think I'll go that route...
Jon
Joseph Gwinn
Rich Grise <ri...@example.net.invalid> wrote:
Your instincts are correct - most thyrister dimmers don't like to work
through a transformer. I don't recall the exact reason, but I don't
think it was the primary inductance. (They *really* don't like driving
a big capacitor through a rectifier bridge either.)
But it doesn't matter. Lutron and the other dimmer manufacturers offer
dimmers intended to work through a transformer. Look for dimmers
intended to drive low voltage lamps, such as task lights under kitchen
cabinets.
If you want to know the reason transformers require special handling,
read the patents listed on the dimmer or its box or instructions.
Joe Gwinn
Rich Grise
closed-cell PE (pipe insulation) and neoprene, like they use on roll bars.
I used a bread knife. It went through them like butter. :-)
Cheers!
Rich
Martin Eastburn
that allows you to dial in the voltage you want or need.
If a DC supply, then the output has be controlled - likely with a
series power resistor that can be adjusted.
Martin
Lloyd E. Sponenburgh
news:h4mBo.372942$mN7.1...@en-nntp-08.dc1.easynews.com:
> If a transformer input - then use a vari-ac or adjustable transformer
> that allows you to dial in the voltage you want or need.
>
> If a DC supply, then the output has be controlled - likely with a
> series power resistor that can be adjusted.
>
> Martin
There are some sophisticated control methods that will allow you to keep
the heat of the wire constant under load. Modern hot wire foam cutters
generally use them.
We buy some 50K little foam disks every year -- slabs are "sawn" from
blocks of foam, and the disks cut by CNC equipment similar to a CNC
router. I have not explored it myself (not having the need), but the
vendor told me that they keep the wire temperature constant during all
phases of traversing and cutting.
LLoyd
Martin Eastburn
and do as you say. That wasn't what was being tried on the kitchen
table. That is a basic process by someone that isn't into electronics.
Martin
Larry Jaques
I use an electric carving knife (Proctor Silex, $8.88 at BigLots) for
furniture cushion foam, if something like that will work for you.
--
If we attend continually and promptly to the little that
we can do, we shall ere long be surprised to find how
little remains that we cannot do. -- Samuel Butler
Michael A. Terrell
Jon Anderson wrote:
>
> On 11/6/2010 2:04 PM, Ken Moffett wrote:
>
> ? hydrogen cynide (think gas chamber).
> ?
> ? Check the MSDS's.
>
> A BIG Thank You! I'd planned to do it outdoors, but I don't like messing
> with anything toxic I don't -have to-. I have an alternative method to
> slice down the foam, and I think I'll go that route...
Meat slicer with a rotating knife? :)
--
Politicians should only get paid if the budget is balanced, and there is
enough left over to pay them.
cl...@snyder.on.ca
<Nob...@nowhere.invalid> wrote:
>On 11/5/2010 10:04 PM, Jon Anderson wrote:
>> Want to make a very simple hot wire cutter to cut polyurethane foam.
>> Probably not more than 12-16" span, max. Found a simple setup on
>> Instructables using a 24v 2a transformer and a wall light dimmer.
>> Well, I've got a 12v 4a DC power supply, seems to me that this ought to
>> be enough, but thought I'd ask.
>>
>> Thanks,
>>
>> Jon
>
>how are you going to control the voltage on the power supply - you don't
>want the wire glowing white hot, you want it just warm enough to do the
>job. Many 12V power supplies are regulated, so it would take a bit of
>futzing around to make the supply adjustable
GENERALLY an AC transformer is a better choice - you know there is
no fancy regulation, and no rectifiers to pop.
A lamp dimmer can work for controll, but some do not like inductive
loads like transformers. A simple powerstat / variac controlling the
transformer works great - and you can use any transformer with
adequate current output because you have full control of the voltage.
A variac alone will do the job, but DO NOT be tempted to go that route
as it has no isolation - the cutter will be "live" in relation to
ground. You NEED an isolation transformer of some type in the circuit.
cl...@snyder.on.ca
The circuit he is looking at HAS an isolating step down transformer -
and is generally a safe design - IF his dimmer will handle an
inductive load.
Using the dimmer alone does not provide either fine enough control OR
isolation.
cl...@snyder.on.ca
I have used lamp dimmers on transformers in the past with mixed
results. Generally a 600 watt or better dimmer handles a 50-100 watt
transformer load reasonably well. The control at the low end, IIRC, is
far from linear and predictable.
cl...@snyder.on.ca
Variacs are MUCH more to be preferred (reccomended) for the job.
Bill Noble
at the risk of being hostile, "nonsense" - variacs are NOT to be
preferred, unless you have no clue about electronics and are unwilling
to purchase a suitable thyrsitor or SCR controller. The variac is
larger, more prone to physical dammage, they wear out, and they are not
a regulating supply, a simple triac controller can hold constant
voltage at the output and avoid the inductive transients by switching at
zero crossing, or by using higher rated parts. Your statements are
wrong, the controllers that do this job are available for a few dollars
(go to harbor freight and buy a "router speed control" when it's on
sale) and have been common for a few decades.
A variac does have its uses, particularly if distortions of the AC
waveform are undesireable, but this is not one of them
Jon Anderson
> Electric carving knife? I had a project once where I had to cut some
> closed-cell PE (pipe insulation) and neoprene, like they use on roll bars.
Not far off, I have a blade that will fix a hacksaw, but looks more like
a bread slicer blade. Maybe it is... Anyway, I'll just screw it to a
piece of scrap plastic with shims to set height. A little side to side
wiggling of the foam as I advance it into the blade should do nicely.
I was looking at the hot wire as I just thought it would be cool to make
and use one.
Jon
Jon Anderson
> Meat slicer with a rotating knife? :)
Those things scare me! lol...
Jon
Michael A. Terrell
>
> ?Wild_Bill wrote:
> ??
> ?? In the DIY article I saw, there was a step-down transformer utilized, and
> ?? numerous comments about how some dimmers won't last long when attempting
> ?? to drive (the primary winding of) a transformer, instead of driving a
> ?? resistive load of incandescent lamp.
> ??
> ?I'm an electronics guy by experience and training, and I'd be terribly
> ?reluctant to use SCR control on the primary of a transformer. I'm "only" a
> ?tech, so I can't quote numbers; it's just sort of a gut feeling that the
> ?inductive reactance could cause a phase shift and upset the firing sequence
> ?of the SCRs.
> ?
> ?Of course, I'd be happy to hear of anyone else's experience with this kind
> ?of setup.
> ?
> ?Thanks,
> ?Rich
> I have used lamp dimmers on transformers in the past with mixed
> results. Generally a 600 watt or better dimmer handles a 50-100 watt
> transformer load reasonably well. The control at the low end, IIRC, is
> far from linear and predictable.
The transformer core can saturate from the DC bias caused by cheap
dimmers.
Michael A. Terrell
Jon Anderson wrote:
>
> On 11/6/2010 5:59 PM, Michael A. Terrell wrote:
>
>
> Those things scare me! lol...
I bought one, but haven't used it yet. No room on the kitchen
counter until the remodeling is done. :)
Gunner Asch
<jande...@comcast.net> wrote:
A cheap electric carving knife from Goodwill works quite well for poly
foams
Winston
> On 11/6/2010 9:00 PM, cl...@snyder.on.ca wrote:
>> On Sat, 06 Nov 2010 13:44:23 -0700, Rich Grise
>> <ri...@example.net.invalid> wrote:
>>
>>> Winston wrote:
>
>>>>
>>> Aw, gawrsh, 'tweren't nothin'. ;-)
>>>
>>> Thanks!
>>> Rich
>> Variacs are MUCH more to be preferred (reccomended) for the job.
>
> at the risk of being hostile, "nonsense" - variacs are NOT to be
> preferred, unless you have no clue about electronics and are unwilling
> to purchase a suitable thyrsitor or SCR controller.
Bill, help me understand please.
I have an autotransformer in good condition; sized
to easily drive my simple unregulated power supply.
The contact brush is nearly new and I have yet to
detect any contact noise or from it.
My Powerstat *is* much larger than a triac controller
and I agree that it does not self-regulate, but in this
application and with the stability of power that I
have available, I am not concerned about line voltage
variations or load current variations because I can
keep an eye on the performance of the hot knife and
tweak power into it as necessary.
I agree that it is a Lexus solution to a Corolla
problem, but the Powerstat is paid for, it is installed
and it works fine. Let me emphasize that it's lack
of isolation does not concern me because my downstream
power supply has a fully isolating transformer.
Would you agree that it is perfectly reasonable,
thrifty, safe, correct and honorable to use it to power
that transformer power supply, especially if it means
that I can work on my project instead of wait for a
triac controller to arrive in the mail?
Is there one reason that it would be either
unreasonable, wasteful, unsafe, incorrect,
dishonorable or somehow wrong for me to do so?
I ask the literal question without a trace of sarcasm
or snark.
What say you?
--Winston
Joseph Gwinn
Winston <Win...@BigBrother.net> wrote:
> Rich Grise wrote:
> > Wild_Bill wrote:
> >>
> >> In the DIY article I saw, there was a step-down transformer utilized, and
> >> numerous comments about how some dimmers won't last long when attempting
> >> to drive (the primary winding of) a transformer, instead of driving a
> >> resistive load of incandescent lamp.
> >>
> > I'm an electronics guy by experience and training, and I'd be terribly
> > reluctant to use SCR control on the primary of a transformer. I'm "only" a
> > tech, so I can't quote numbers; it's just sort of a gut feeling that the
> > inductive reactance could cause a phase shift and upset the firing sequence
> > of the SCRs.
>
> You are right, Rich. (And thanks for your post because
> it caused me to solve a problem I didn't even know I was
> about to encounter!) :)
>
> http://www.st.com/stonline/books/pdf/docs/3566.pdf
>
> See the first five pages for an accessible analysis of the
> hazards of driving inductive loads with 'SCR' type devices.
>
> See pages '6/16' and '7/16' for a clever triac circuit ST
> developed to drive inductive loads without the flaws of
> the earlier circuits.
>
> Quoting them:
> "This circuit has been developed by the STMicroelectronics
> applications laboratory and used with success for a wide
> range of equipment."
>
>
> See the text at the bottom of page
> '6/16' for a circuit explanation. See Figure 9 on page
> '8/16' for component values.
But I need to point one thing out: A transformer feeding a resistive
load like a hot wire is not an inductive load unless the transformer in
question has extremely large leakage inductance (transformers in
tombstone welders would qualify).
The presented load is in fact largely resistive, and I think the point
made by another poster about DC unbalance saturating the transformer
core is a key issue. If one is in fact driving an inductor, one can
have both problems at once; the problems are independent.
In any event, Lutron et al make semiconductor dimmers intended to drive
resistive loads through step-down transformers.
Joe Gwinn
Bill Noble
There is absolutely nothing wrong with using what you have. I was
objecting to the blanket statement about a solution being better - from
a technology point of view, the statement was false. From a practical
point of view, if you have all the pieces to make a slightly less
wonderful solution, but it is an adequate solution, then of course you
can do it.
Here is the pro/con of your solution, whcih I assume is a variac driving
an unregulated DC power supply:
pro
you have the parts
it is easy to assemble
it will be reliable enough for your (non production line) use
con
heavier, more bulky
more likely to get damaged if power supply gets knocked off the table
if you had to buy parts, it would be more costly (but you don't)
no ability to regulate current/temperature of the wire automatically
so, for YOU, given that you have the parts, use what you have.
Bill Noble
Joe - not correct because of the way triacs work - if driven by a pure
sine wave, it is true (or nearly so). But if you turn on say 90 deg
after zero crossing you have a huge current spike - that is due to the
inductive nature of the transformer and the fact that there is no flux
in the core at turn on. Work out the math if you don't believe me, or
try it with a transformer and a battery and a scope - measure the
current pulse at turn on with say 12V applied to the 12V winding of a
transformer - you are looking for the first 1 ms of current
Winston
(...)
> But I need to point one thing out: A transformer feeding a resistive
> load like a hot wire is not an inductive load unless the transformer in
> question has extremely large leakage inductance (transformers in
> tombstone welders would qualify).
This is a puzzlement for me, Joe.
Just now I modeled a 1:1 transformer in SPICE
and loaded the output with a selection of
different resistances.
While driving the input with 60 Hz, I noticed
that the primary voltage peaked before the
primary current did at about ~89 degrees
separation with the secondary loaded at 1
ohm through 100K ohms.
That all looks inductively reactive to me.
What did I do wrong? Are you talking about
saturated core mode?
Thanks!
--Winston
Winston
(...)
> so, for YOU, given that you have the parts, use what you have.
Thanks for the clarification.
My Powerstat is bolted to my workbench. If it falls
onto the floor, I have much more pressing issues on my
hands than just damage to that one tool. :)
--Winston
Joseph Gwinn
Winston <Win...@BigBrother.net> wrote:
> Joseph Gwinn wrote:
> (...)
>
> > But I need to point one thing out: A transformer feeding a resistive
> > load like a hot wire is not an inductive load unless the transformer in
> > question has extremely large leakage inductance (transformers in
> > tombstone welders would qualify).
>
> This is a puzzlement for me, Joe.
>
> Just now I modeled a 1:1 transformer in SPICE
> and loaded the output with a selection of
> different resistances.
>
> While driving the input with 60 Hz, I noticed
> that the primary voltage peaked before the
> primary current did at about ~89 degrees
> separation with the secondary loaded at 1
> ohm through 100K ohms.
>
> That all looks inductively reactive to me.
Then there is problem with the spice model. More below.
> What did I do wrong? Are you talking about
> saturated core mode?
In saturated core mode, the primary inductance of the drive winding
drops to a very low value, because saturated iron has roughly the
permeability of air, and one will see huge current surges when the core
saturates and the inductive reactance drops to near zero. This is not
what I think you are seeing.
Never mind computer modeling systems like spice. Let's talk physics.
As seen from the terminals, the voltages of an ideal single-phase
one-path transformer are all in phase (or inverted in sign). The
relation between voltage and current will depend on what is connected to
the transformer. If all loads are resistive, then currents will be in
phase with voltages. If loads are in aggregate reactive, then voltage
and current will get out of phase.
Another way to think about this is to note that no matter how many
windings there are, there is exactly one core, and the net flux through
this core links all windings equally.
Practical transformers very well approximate ideal transformers so long
as:
The inductive reactance of the windings exceeds the impedance of what's
connected to those windings by a factor of at least five or ten.
The stray inductance is insignificant.
The core remains out of saturation.
Eddy currents are reduced to insignificance by lamination of the core or
by use of a ferrite core.
The frequency isn't so high that stray capacitance has a significant
effect.
This is the short form. Transformer design can be a career. But for
all their imperfections, transformers work pretty well.
So, the spice model seems to be giving non-physical results, but
computer models are known for such things, and one must always validate
such models.
War story: Many years ago, I was interested in the physics of xenon
flash lamps. The physical model is pretty simple, a charged capacitor
discharging into an arc, which arc heated the xenon to incandescence,
the resulting light and heat radiation carrying the energy away. This
yields a set of coupled ordinary differential equations that one solves
numerically, time step by time step. I got it all working, and all was
well. Then I changed the duration of the time steps, and more energy
came out as light and heat than was stored in the capacitor. Oops.
Violated the conservation of energy.
Turns out I had made two mathematical mistakes, which mistakes cancelled
one another only for the original step duration.
Joe Gwinn
Joseph Gwinn
Bill Noble <Nob...@nowhere.invalid> wrote:
> On 11/7/2010 8:56 AM, Joseph Gwinn wrote:
> > In article<ib49n...@news6.newsguy.com>,
[snip]
> >
> > But I need to point one thing out: A transformer feeding a resistive
> > load like a hot wire is not an inductive load unless the transformer in
> > question has extremely large leakage inductance (transformers in
> > tombstone welders would qualify).
> >
> > The presented load is in fact largely resistive, and I think the point
> > made by another poster about DC unbalance saturating the transformer
> > core is a key issue. If one is in fact driving an inductor, one can
> > have both problems at once; the problems are independent.
> >
> > In any event, Lutron et al make semiconductor dimmers intended to drive
> > resistive loads through step-down transformers.
> >
> > Joe Gwinn
>
>
> Joe - not correct because of the way triacs work - if driven by a pure
> sine wave, it is true (or nearly so). But if you turn on say 90 deg
> after zero crossing you have a huge current spike - that is due to the
> inductive nature of the transformer and the fact that there is no flux
> in the core at turn on.
It's true that the triac turns on abruptly, in a microsecond or two. At
90 degrees, this imposes a full voltage step on the transformer input.
But, wouldn't inductance slow the spike down, whatever the state of the
core? This is a classic homework problem. The current rises linearly
from zero, and the higher the inductance the slower the rate of rise.
> Work out the math if you don't believe me, or
> try it with a transformer and a battery and a scope - measure the
> current pulse at turn on with say 12V applied to the 12V winding of a
> transformer - you are looking for the first 1 ms of current
This sounds like we would have significant DC current through the
transformer winding, which could saturate the core. A saturated core
will certainly cause a current spike. Maybe I don't understand the
proposed test circuit.
I recall that Lutron had patents in this area, and these patents
provided a summary of the issues to be solved, so I did some searching.
A good discussion appears in US patent 4,876,498. A later version of
the same patent is 4,954,768. Leviton reacted with their own solution,
in patent 7,482,758. And 5,477,111 speaks directly to control of
inductive loads like motors. (Go to <http://www.pat2pdf.org> to get
copies.)
Anyway, the big issue in these patents is DC causing saturation, and (in
two-wire circuits) inductance-caused errors in knowing when to trigger
the triacs.
Joe Gwinn
cl...@snyder.on.ca
I was not talking about specialized thyrister controllers - I was
talking about "lamp dimmers" - and I am not electronically
illiterate. The one advantage I have found with variacs is they are
linear in response, and rcontrol smoothly from virtually ZERO to full
(or even higher than full) voltage.
MOST thyrister controllers - even specialized ones for motor control,
have a somewhat limitted control range. Many will not control to below
approxemately 5%.
That is not an issue in this application since with a stepdown
isolating transformer in the mix the control range would be quite
narrow anyway.
Yes, specialized controllers will do the job, and do it well - but
last time I priced a 600 watt magnetically rated Lutron it was over
$69, and I've picked up numerous used and surplus Variacs, from 0.5 to
2.5 kva for under $20 each.
Most of the amateur built (in the USA they call then experimental)
plane builders I know who have used hot wire cutters have used
Variacs. Myself, I'm building with ChroMo steel and 6061T6 Al.
cl...@snyder.on.ca
Like I said - far from linear and predictable control.
I favour Variacs and have about 5 different sizes - as well as a
couple good-sized isolation transformers- very usefull for
troubleshooting old radio equipment etc - prevents getting shocks from
"live" chassis, and combined allows ramping up power to see what
happens instead of just plugging it in and letting all the "magic
smoke" escape at once.
Have a "buck-boost" transformer as well that can add or subtact 6 or
12 volts from line voltafe - but I like the variac better for that
too. (4 terminal, not 3)
Michael A. Terrell
You can use a handful of old filament transformers & switches to get
a lot more choices from a Buck/Boost configuration. 240V variacs are
usually cheap, but have to be derated becasue you are using them at half
voltage.
Winston
(...)
>> That all looks inductively reactive to me.
>
> Then there is problem with the spice model. More below.
Joe, I did the experiment in the real world just now.
You are right. With a resistive load on the secondary
I saw undetectable phase shift between voltage and
current on the primary. With an inductively reactive
load on the secondary, I saw the expected significant
lag in the current peak in relation to the voltage peak
on the primary.
That will be what I learned today and I thank you.
(...)
> War story: Many years ago, I was interested in the physics of xenon
> flash lamps. The physical model is pretty simple, a charged capacitor
> discharging into an arc, which arc heated the xenon to incandescence,
> the resulting light and heat radiation carrying the energy away. This
> yields a set of coupled ordinary differential equations that one solves
> numerically, time step by time step. I got it all working, and all was
> well. Then I changed the duration of the time steps, and more energy
> came out as light and heat than was stored in the capacitor. Oops.
> Violated the conservation of energy.
>
> Turns out I had made two mathematical mistakes, which mistakes cancelled
> one another only for the original step duration.
Wow!
That one must have had you scratching your head for a while!
Thanks again
--Winston
Joseph Gwinn
Winston <Win...@BigBrother.net> wrote:
It did. If only I could patent it, I could be rich and famous.
Boundless power source. I would have settled for rich.
More seriousy, models can be dangerous.
Joe Gwinn
cl...@snyder.on.ca
You would, ideally, use a reverse phase dimmer/controller. They turn
the current OFF at a particular phase angle, instead of on. Some of
the high end dimmers for magnetic transformer low voltage lighting are
reverse phase dimmers. They are NOT cheap. They do, however, reduce
the EMI/RFI emissions significantly and almost totally eliminate the
filament hum common to standard Thyrister dimmers (which Variacs also
totally eliminate , which is why they are still used in many studio
lighting systems) Darn electronic theatre lighting systems are pretty
noisy - I work next to one several hours a day.
cl...@snyder.on.ca
<mike.t...@earthlink.net> wrote:
My old buck-boost is one I built from a heavy fillament transformer
many many years ago, that has followed me around like a well trained
puppy.
As for the 240 volt variacs, they are good for the nameplate AMPERAGE
on 120, which reduces the power (va or wattage) rating significantly.
And I see my fat-finger syndrome spelt voltage wrong again. Thought I
better correct it before someone else jumped on it.
Winston
(...)
> If only I could patent it, I could be rich and famous.
> Boundless power source. I would have settled for rich.
Luckily, we both settled for "devilishly handsome".
> More seriousy, models can be dangerous.
Yup.
--Winston
Michael A. Terrell
Yes, but small ones can be dirt cheap. Most people seem to think
they can handle double the current when one is used at half the
nameplate voltage, and can't be convinced otherwise till smoke pours out
of their recently departed variac. :(
> And I see my fat-finger syndrome spelt voltage wrong again. Thought I
> better correct it before someone else jumped on it.
Joseph Gwinn
cl...@snyder.on.ca wrote:
> lighting systems). Darn electronic theatre lighting systems are pretty
> noisy - I work next to one several hours a day.
Reduce filament noise and EMI? That certainly makes sense. There are
many patents on reverse-phase dimmers as well, with reduced noise and
EMI given as advantages.
As for variacs and EMI, I have a war story: In the 1970s, I lived in
Baltimore. A friend of mine was an electronics tech at Johns Hopkins
University Hospital. The hospital did much basic research, and the
electrophysiology department had a screened room (big walk-in box made
of copper window screen fabric and having a metal door), within which
they attempted to record nerve potentials and pulses, but were greatly
hindered by electrical noise despite the screenroom.
My friend was asked to solve this problem. It developed that the
overhead lights, which shone through to copper screen fabric, were
fluorescent. Ouch. No wonder. Replaced the fluorescent lamps with
incandescent lamps controlled by a wall-mounted variac dimmer. The
noise level inside the screenroom dropped to insignificance (relative to
the inherent noise of the instruments). One could also dim the lights,
making the instrument displays easier to see. The doctors were very
pleased.
I would guess that the EMI from the fluorescent lamps was enough to get
through the power-line EMI filters where power enters the screenroom to
supply the instruments. The good doctors probably also left the door
open, but the powerline path was probably by far the most important, as
only noise below a megahertz or so can affect electrophysiology
experiments.
Joe Gwinn
cl...@snyder.on.ca
<joeg...@comcast.net> wrote:
The emi from the flourescent bulbs was not just going back the power
lines. The radiation from the bulbs themselves was significant, and
anything short of a full faraday cage wasn't going to catch it all.
The copper screen was not sufficient (either not full enough coverage,
or not grounded, etc)
Doug White
news:joegwinn-C6B307...@news.giganews.com:
What's worse is a model in the hands of a freshly minted graduate who has
no common sense. Designs for things like high gain omnidirectional
antennas with 150% efficiency is what you get. They tend to get more
excited than suspicious, which is very depressing. They've been taught
that anything that comes out of an expensive computer program MUST be the
truth, and they have no horse sense to tell them otherwise.
I spend a lot of my time at work raining on kid's parades...
Doug White
Gerald Miller
> The emi from the flourescent bulbs was not just going back the power
>lines. The radiation from the bulbs themselves was significant, and
>anything short of a full faraday cage wasn't going to catch it all.
>The copper screen was not sufficient (either not full enough coverage,
>or not grounded, etc)
I'll go along with that - I have problems with a wireless mouse 24"
below a single 48" tube fixture, I need to go to a corded mouse.
Gerry :-)}
London, Canada
CaveLamb
> Want to make a very simple hot wire cutter to cut polyurethane foam.
> Probably not more than 12-16" span, max. Found a simple setup on
> Instructables using a 24v 2a transformer and a wall light dimmer.
> Well, I've got a 12v 4a DC power supply, seems to me that this ought to
> be enough, but thought I'd ask.
>
> Thanks,
>
> Jon
Jon,
I'm real curious (after all this discussion) about what you decided
to do?
--
Richard Lamb
email me: cave...@earthlink.net
web site: www.home.earthlink.net/~cavelamb
Jon Anderson
> I'm real curious (after all this discussion) about what you decided
> to do?
It was in one of my replies. After reading about the toxic fumes I'd get
from polyurethane, I suddenly remembered a hacksaw blade I have that
looks like it came from a bread slicing machine, and maybe it did.
Going to make a simple fixture to hold the blade under tension at the
required height, and oscillate the foam past the blade.
I'll post a few pictures to the Drop Box when I get back to fitting the
drawer for my micrometers.
I may yet make a hot wire cutter for some other project, but I don't
have many cuts to make, so the blade should do fine.
Jon
CaveLamb
You WILL get a smoother cut with a hot wire.
A NIOSH mask solves the fumes problem nicely.
Winston
The 1/2" - thick closed - cell foam cuts easily, quickly and
cleanly with a hobby (X-Acto) knife. Tight radii are easy to make.
AMHIKT (Go ahead!)
--Winston
Joseph Gwinn
Doug White <gwh...@alum.mit.edu> wrote:
Same here, although I have young engineers not necessarily fresh minted.
Some people are born with common sense, some learn it (prompted by the
pain of accumulating scars), and some never acquire it.
The most effective approach I have found is to ask people to do simple
crosscheck analyses based largely on first principles. Like
conservation of energy.
Which elicits yet another war story: A now retired optical engineering
colleague of mine would evaluate requests for proposals by applying
conservation of (optical) energy to the requested optical system.
Typically, the minimum scene brightness, maximum allowed entrance
aperture size, and minimum image brightness would be specified, often
indirectly. Optical systems are passive (unless there is an image
intensifier), so the total energy in the image cannot exceed that
falling on the aperture. In many cases, the requested system was
physically impossible. The art is writing the proposal to explain and
solve this without tipping the competition off or embarrassing the
customer.
Joe Gwinn
Joseph Gwinn
cl...@snyder.on.ca wrote:
It was a commercial faraday cage, and I'm pretty sure it was grounded.
If closing the cage door solved the problem, my friend would not have
been given the assignment.
My reason to think that conducted EMI was the issue is that those lamps
are far too small physically to be very good at emitting noise at 1 MHz
and below. Nor would 1 MHz waves fit through the door, so leaving it
open would have little effect, as observed. At 1 MHz, the wavelength is
300 meters.
Joe Gwinn
Jon Anderson
> You WILL get a smoother cut with a hot wire.
For the application, not an issue. The cut surface will be facing down
and not visible.
> A NIOSH mask solves the fumes problem nicely.
Sure, but an expense I don't really wish to incur. I'm only lining 3-4
drawers with foam, it's not a huge project. If I'm getting into
something new and going to do a lot of it, I'll do it right. When it's
just a small project, I'll look to cut corners where I can get away with
it. Frankly if I'd remembered the blade in the first place, I wouldn't
even have looked up hot wire cutters.
Funny now that I think about it, I don't even remember where I got this
blade. Best I can recall, I've had it for 15+ years and never used it... <G>
Jon
Jon Anderson
> The 1/2" - thick closed - cell foam cuts easily, quickly and
> cleanly with a hobby (X-Acto) knife. Tight radii are easy to make.
That's how I'm cutting the pockets for the tools. What I want to do with
the piece I've cut out is slice 3/16 off and stuff that back into the
pocket to cushion the mics. That will also help as a 1/2" deep pocket is
a bit much for most of my mics.
Jon
Winston
> On 11/9/2010 6:18 AM, Winston wrote:
>
>> The 1/2" - thick closed - cell foam cuts easily, quickly and
>> cleanly with a hobby (X-Acto) knife. Tight radii are easy to make.
>
> That's how I'm cutting the pockets for the tools. What I want to do with
> the piece I've cut out is slice 3/16 off and stuff that back into the
> pocket to cushion the mics.
Straightedge slice. Very pretty!
> That will also help as a 1/2" deep pocket is
> a bit much for most of my mics.
That's why I cut additional recesses on
both sides of the tools for my fingers.
It works great!
--Winston
Rich Grise
Bread knife. :-)
Cheers!
Rich
Martin Eastburn
then the variac can be from 0v to say 24v depending on how
it is set up.
Martin
On 11/6/2010 10:48 PM, cl...@snyder.on.ca wrote:
> On Fri, 05 Nov 2010 22:22:15 -0700, Bill Noble
> <Nob...@nowhere.invalid> wrote:
>
>> On 11/5/2010 10:04 PM, Jon Anderson wrote:
>>> Want to make a very simple hot wire cutter to cut polyurethane foam.
>>> Probably not more than 12-16" span, max. Found a simple setup on
>>> Instructables using a 24v 2a transformer and a wall light dimmer.
>>> Well, I've got a 12v 4a DC power supply, seems to me that this ought to
>>> be enough, but thought I'd ask.
>>>
>>> Thanks,
>>>
>>> Jon
>>
>> how are you going to control the voltage on the power supply - you don't
>> want the wire glowing white hot, you want it just warm enough to do the
>> job. Many 12V power supplies are regulated, so it would take a bit of
>> futzing around to make the supply adjustable
> GENERALLY an AC transformer is a better choice - you know there is
> no fancy regulation, and no rectifiers to pop.
> A lamp dimmer can work for controll, but some do not like inductive
> loads like transformers. A simple powerstat / variac controlling the
> transformer works great - and you can use any transformer with
> adequate current output because you have full control of the voltage.
>
> A variac alone will do the job, but DO NOT be tempted to go that route
> as it has no isolation - the cutter will be "live" in relation to
> ground. You NEED an isolation transformer of some type in the circuit.
Wild_Bill
machines.
An old sewing macine could possibly be adapted/modified to cut thin, soft
materials.
When I was making straps and various other accessories for boats that I had
years ago, I stopped in a local furniture upholstery shop to find out what
types of needles they used for thick, multiple layers of fabric, and the
friendly owner was happy to inform me of all sorts of interesting info
regarding thick materials (straps, parachute harnesses etc).
They use needles that cut as they pass thru thick materials, where common
single-pointed needles will get jammed in dense, thick materials.
I was able to gently grind the shanks of the commercial machine cutting
needles so they would fit an old Necchi home-type sewing machine, and the
cutting points made multiple layers of strap material and various fabrics a
breeze for the tiny Necchi motor.
I believe the cutting points were ground similar to an endmill, with the tip
ground as two bevels in opposing directions and with a greater relief angle,
which would likely require a good magnifier and possibly a mini abrasive
disk or stone in a rotary tool.
A cutting point that reciprocates rapidly could be very useful for cutting
fine lines in synthetic materials such as foam. While feeding the material
by hand (without using the sewing machine feed pawls, or thread), closely
spaced perforations/holes will essentially become a cut line.
The freehand feed mode is used for embroidery-type work with thread, such as
creating fabric patches with graphics on them (I watched this being done
years ago).
--
WB
.........
"Jon Anderson" <jande...@comcast.net> wrote in message
news:Us4Bo.364382$Yn5.1...@en-nntp-14.dc1.easynews.com...
Martin Eastburn
Martin