MRC Ampack upgrade: Attn electronics froods
pawlo...@gannon.edu
Actually, this is a rather basic question, and a
scarcely hoopy frood could probably answer it.
Unfortunately, I burned a hole in my towel with
a soldering iron and had to throw it out.
I have an old MRC Ampack that works. It has
a decent power output, which is nice, and a
good-quality rheostat, but poor slow-speed
control on many of my locomotives. It was a
great $2 deal, but I want to make it better,
and here's what I want to do:
http://www.geocities.com/kezelak/ampack_mods.jpg
F1 is some sort of circuit breaker in a glass
envelope.
Basically, I want to make it into a very simple
voltage-follower transistor throttle, and add a
pulse-power switch to cut out one diode. If my
calculations are correct, the circuit should work.
The TIP-120 is supposed to have a gain of at
least 1000, and R3 is there to limit the base
current to no greater than 1.2 mA, which seems
adequate for controlling 1 A with that gain.
The circuit is pretty similar to what my MRC
Railpower 1300 contains, as far as I can tell,
and that gives good slow-speed control, even
without the option of pulse DC for low speeds.
I'm not really worried about heating effects.
I don't go in for coreless motors.
I am going to mount Q1 on an aluminum heat
sink. I could calculate the size needed, but
I'm not going to; I'll just go with "as big as fits"
and replace the $1.59 component if the smoke
comes out.
Any comments on this circuit before I
breadboard it up and try to fry things? It
seems simple, but I'm new at this.
Cordially yours:
Gerard P.
President, a box of track and a gappy table.
jJim McLaughlin
pawlo...@gannon.edu
Good idea.
Crossposted upon suggestion...note that this device
is a model railroad controller.
Transformer T1 delivers 1A.
David Nebenzahl
> I am going to mount Q1 on an aluminum heat
> sink. I could calculate the size needed, but
> I'm not going to; I'll just go with "as big as fits"
> and replace the $1.59 component if the smoke
> comes out.
Plenty good enough. Suggestion: use a simple touch test. Operate the
pack with the cover open and feel the heat sink after some decent
interval of use. If it isn't too hot to touch, you're OK.
By the bye, the TIP120 is rated at 5A continuous, 8A peak collector
current (see http://www.nteinc.com/specs/200to299/pdf/nte261.pdf for
NTE's equivalent part).
Greg Procter
I'd put a ceramic capacitor across the voltage supply as close to the
transistor as possible and a reverse oriented diode across the output
(before the reversing switch. Also, if your output voltage doesn't go
fully to zero then a diode (or two) in series with the output.
Add a few more components and you can have a momentum controller.
Greg.P.
NZ
pawlo...@gannon.edu
> I'd put a ceramic capacitor across the voltage supply as close to the
> transistor as possible and a reverse oriented diode across the output
> (before the reversing switch. Also, if your output voltage doesn't go
> fully to zero then a diode (or two) in series with the output.
>
> Add a few more components and you can have a momentum controller.
GP1:
I assume the capacitor would be a filter, or am I wrong about that?
I'm a little uncertain about filtering, because I don't want to smooth
out my output current too much. I know this can cause more heating,
but that's not something my motors and I have much trouble with.
Our problems are more along the lines of "Oh shite, the Tyco
Plymouth at the end of that smoke trail wasn't powered down, it
was stalled."
Or is the cap for RFI purposes?
What would the diode be for? I'm trying to use this as a learning
experience, as well as a practical controller.
The diode drop is a good plan. I may have to use that with my
KF pack, whose selenium cells are getting replaced with silicon
diodes, because I don't intend to run BLI K4s </snide>. It depends
what voltage drop was allowed for; I'll have to stick a meter on it.
The momentum idea is interesting. I have some old MR articles
which cover simple transistor throttle circuits (good old LHW
and his ability to put basic info, four practical circuits, AND
a plug for his True Action Throttle in three pages of dense
text and small drawings) and I may use it eventually once I
understand the basic principles involved better. I find that a
little experimentation helps me do that. Study a little, think
a little, try a little, repeat as necessary.
Greg Procter
>
> Greg Procter wrote:
> > I'd put a ceramic capacitor across the voltage supply as close to the
> > transistor as possible and a reverse oriented diode across the output
> > (before the reversing switch. Also, if your output voltage doesn't go
> > fully to zero then a diode (or two) in series with the output.
> >
> > Add a few more components and you can have a momentum controller.
>
> GP1:
>
> I assume the capacitor would be a filter, or am I wrong about that?
It's there to cut out high/radio frequency signals, transistors can take
exception to those.
>
> I'm a little uncertain about filtering, because I don't want to smooth
> out my output current too much. I know this can cause more heating,
> but that's not something my motors and I have much trouble with.
> Our problems are more along the lines of "Oh shite, the Tyco
> Plymouth at the end of that smoke trail wasn't powered down, it
> was stalled."
>
> Or is the cap for RFI purposes?
That's it.
>
> What would the diode be for? I'm trying to use this as a learning
> experience, as well as a practical controller.
The diode is there to short out impulses returning from the layout.
Transistors can go pop in response to such impulses and a model railway
can be an electrically noisy place.
>
> The diode drop is a good plan. I may have to use that with my
> KF pack, whose selenium cells are getting replaced with silicon
> diodes, because I don't intend to run BLI K4s </snide>. It depends
> what voltage drop was allowed for; I'll have to stick a meter on it.
A meter! Good idea!
My first transistor controllers were built when I only had old coffee
grinder motors that laughed at fractional voltages. My first decently
motored loco just ran on and on and on ...
>
> The momentum idea is interesting. I have some old MR articles
> which cover simple transistor throttle circuits (good old LHW
> and his ability to put basic info, four practical circuits, AND
> a plug for his True Action Throttle in three pages of dense
> text and small drawings) and I may use it eventually once I
> understand the basic principles involved better. I find that a
> little experimentation helps me do that. Study a little, think
> a little, try a little, repeat as necessary.
>
When you're ready :-)
After the potentiometer output, add a second resistor and put a (say
100uf) cap from between the resistors to the ground.
The loco will now gain and lose speed at a slower than instantanious
rate.
Add a diode between pot and resistor and the loco will accelerate as
before but take forever to brake.
Add another pot (with added resistor) between cap and ground and you
have a brake lever.
Put a switch between the pot and the last resistor and you can switch
the momentum on/off.
Swap the first resistor for another pot and you have variable momentum
rate.
Separate the traction current to the transistor from the control to the
transistor base and you're back to full wave/half wave.
Buy another darlington transistor and parallel it with the existing one,
each fed through one diode and you have variable pulse from half to full
wave at any given setting.
Add momentum to the variable pulse :-)
Hey, you've got a whole new hobby in a plastic box and soldering iron!
Greg.P.
(tongue in cheek)
David Starr
> Folks:
>
That circuit will work, and the emitter follower TIP120 will maintain a
constant voltage at the emitter, no matter how much juice the motor
draws. The theory of emitter followers is simple. Given a constant
base voltage, the emitter voltage will be 0.7 volts (one diode drop)
less than that base. The transistor will flow whatever current is
necessary down from the collector to keep the emitter voltage one diode
drop below the base voltage. Given a constant voltage out of the
emitter, the motor will maintain a constant speed even if the mechanical
load varies from sticky side rods, wheels out of round, grades,
whatever. It's a good improvement and will make loco's run smoother
than a plain rheostat will.
If you have 1 amp going thru the transistor, base current might be as
bad as 2% of that, I.e. a gain of 50. Large signal gain of power
transistors is lower than the small signal gain. To stabilize the
circuit you want current going thru the pot to be large compared to the
base current, so that the base current draw doesn't alter the voltage at
the pot wiper. I'd make the pot more like 500 ohms than 5K, that's only
a 1/4 watt of power in the pot, a 1/2 watt pot will run cool enough and
the standing current in the pot will be 24 mA, large compared to the
estimated 2 mA base current. Then I'd make the base resistor about 100
ohms. The only purpose of the base resistor is to surpress oscillation
in the emitter follower, and 100 ohms is enough to do that job. With
only 100 ohms, the 2 mA base current will only drop 0.2 volts across the
base resistor. With a 5K base resistor, 2 mA base current drops 10
volts, nearly your entire supply voltage, across the base resistor.
Build this circuit and get it running before getting into momentum
throttles. A momentum throttle needs a lot more gain then you get with
a single transistor. The fundamental theory of a momentum throttle is a
large capacitor which changes of throttle setting charge or discharge
slowly. For this to work, the power amplifier cannot draw much current
from the capacitor, which means a high gain amplifier, either several
transistor stages or an opamp. It's a more complex circuit which has a
lot more ways of going wrong. I'd recommend the fairly straight forward
circuit you have for a first try. Do the fancier stuff after you get
something simple working.
David Starr
pawlo...@gannon.edu
pawlo...@gannon.edu
GP1:
The diode idea seems good. I presume it can be fairly small, since it
doesn't handle full output current? I will also consider the RFI
filter.
I suppose the disc cap should be small, so as to shunt high
frequencies
without smoothing off my pulses too much...this is going to take some
figuring out.
DS:
I may have drawn my sketch in a somewhat misleading way, because
Q1 isn't a single transistor, but a Darlingon arrangement, so I
probably
should have used some sort of IC symbol. I looked up the specs to
get the gain of 1000, which is at 3v collector-emitter and 3A current,
but I'm going to test the circuit first. I think I'll hook up some
wirewound
resistors across the output and check voltage under load, and then
perhaps try it with a motor, and if the voltage seems to drop too much
I'll reduce the resistor values and try again.
One thing I'm confused about - isn't the base resistor also there to
limit base current? 100 ohms would do that, of course. I used the
large resistor because
The circuit worked on the breadboard, with no load. This was a
very pleasant surprise.
All the advice is very much welcome. At this point I know enough
to get into trouble. I'm trying to learn enough to get out before
proceeding farther in -- already, I'm doing better than last time I
tried something like this, which was an attempt to build Peter
Thorne's similar circuit in an old MR. It didn't work, but I couldn't
figure out why, so I'm trying to avoid blind copying this time.
David Nebenzahl
> The diode idea seems good. I presume it can be fairly small, since it
> doesn't handle full output current?
Yep; consider the possible sources of reverse-polarity voltage coming
*from* the layout (noise, reverse EMF from loco motors, etc.); there's
not going to be much current coming from them.
> I will also consider the RFI filter. I suppose the disc cap should be
> small, so as to shunt high frequencies without smoothing off my
> pulses too much...this is going to take some figuring out.
0.1 uf. Don't overthink this; it will have *zero* effect on performance
from your point of view.
Greg Procter
A signal diode will do, or an IN400x which is the cheapest I know of.
> I will also consider the RFI
> filter.
> I suppose the disc cap should be small, so as to shunt high
> frequencies
> without smoothing off my pulses too much...this is going to take some
> figuring out.
Just solder a little one in from the junk box.
:-)
>
> DS:
>
> I may have drawn my sketch in a somewhat misleading way, because
> Q1 isn't a single transistor, but a Darlingon arrangement, so I
> probably
> should have used some sort of IC symbol.
I knew what you meant. the normal Darlington represertation has two
interconnected internal transistor bits inside the circle.
> I looked up the specs to
> get the gain of 1000, which is at 3v collector-emitter and 3A current,
> but I'm going to test the circuit first. I think I'll hook up some
> wirewound
> resistors across the output and check voltage under load, and then
> perhaps try it with a motor, and if the voltage seems to drop too much
> I'll reduce the resistor values and try again.
>
> One thing I'm confused about - isn't the base resistor also there to
> limit base current? 100 ohms would do that, of course. I used the
> large resistor because
The base resistor is there to limit base current.
Advance the pot to full voltage without the base resistor in circuit and
a short circuit load and there's full voltage (= infinite amps) between
base and emitter, not a good look!
Ditto if you put the momentum capacitor in there without a base
resistor.
>
> The circuit worked on the breadboard, with no load. This was a
> very pleasant surprise.
Life's like that, it sets you up to expect success and then it runs you
over with a steam roller!
;-)
>
> All the advice is very much welcome. At this point I know enough
> to get into trouble. I'm trying to learn enough to get out before
> proceeding farther in
The components aren't expensive, so as long as you don't set the house
on fire or rub your nose with the soldering iron it's all gain.
> -- already, I'm doing better than last time I
> tried something like this, which was an attempt to build Peter
> Thorne's similar circuit in an old MR. It didn't work, but I couldn't
> figure out why, so I'm trying to avoid blind copying this time.
Which one was that? I built most of them way back and learned enough to
figure out when it was my fault.
;-)
Greg.P.
David Starr
>
> I may have drawn my sketch in a somewhat misleading way, because
> Q1 isn't a single transistor, but a Darlingon arrangement, so I
> probably
> should have used some sort of IC symbol. I looked up the specs to
> get the gain of 1000, which is at 3v collector-emitter and 3A current,
> but I'm going to test the circuit first. I think I'll hook up some
> wirewound
> resistors across the output and check voltage under load, and then
> perhaps try it with a motor, and if the voltage seems to drop too much
> I'll reduce the resistor values and try again.
Testing with a resistor load is a good thing to do. A 12 ohm load
resistor will draw one amp at 12 volts. It will also get hot,
dissapating 12 watts at 12 volts. Motors, so long as they are turning,
don't draw much current unless they are driving a real mechanical load.
For the home workshop it's hard to arrange a suitable mechanical load,
but easy to find a resistor that will take enough juice to stress the
throttle-under-test.
>
> One thing I'm confused about - isn't the base resistor also there to
> limit base current? 100 ohms would do that, of course. I used the
> large resistor because
Not really. The pot should be holding the base at a constant voltage.
The base emitter junction will be forward biased, which causes
conduction from collector to emitter. Enough current will flow thru the
emitter to bring the emitter up to a diode drop or two from the base.
That's why it's called an emitter follower, the emitter follows the base
voltage.
If you have significant resistance in the base circuit, then the
voltage drop across the base resistor lowers the base voltage, which
lowers the emitter voltage. This isn't the end of the world, but the
purpose of the circuit is to keep the emitter voltage steady against
fluxuations in emitter (load) current, so the locomotive motor can draw
all the current it wants to keep turning at a steady speed. The motor
sees variations in mechanical load from bumps in the track, trains
wheels with sticky spots, or stickiness from the siderods, worm gear,
valve gear etc. Nothing is perfect, and the load the motor sees changes
as the shaft goes round. If the motor can draw more current when the
extra load tries to slow the shaft, the motor will run more smoothly.
If the extra current draw lowers the voltage going to the motor it will
slow down. In short, you don't want to limit base current in an emitter
follower.
A darlington pair acts just like a single transistor with the benefit
of much increased gain. The gain of a darlington pair is egual to the
product of the two individual transistor gains. If the bigger power
device has a gain of 10 and the smaller piggy backed darlington
transistor has a gain of 100, the combined darlington pair has a gain of
1000. The downside of the Darlington is doubled base emitter voltage,
but that doesn't matter much in this circuit.
David Starr
>
dgw
<pawlo...@gannon.edu> wrote in message
news:1191617827.5...@22g2000hsm.googlegroups.com...
> Greg Procter wrote:
>> I'd put a ceramic capacitor across the voltage supply as close to the
>> transistor as possible and a reverse oriented diode across the output
>> (before the reversing switch. Also, if your output voltage doesn't go
>> fully to zero then a diode (or two) in series with the output.
>>
>> Add a few more components and you can have a momentum controller.
>
>
> GP1:
>
> I assume the capacitor would be a filter, or am I wrong about that?
Ceramic capacitors have a low enough capaitance that it would have no
noticeable affect on the sinusoidal pulses, but it would greatly reduce
(filter) any high frequency components. I assume something in the 0.01 uf
to 0.1 uf range, voltage rating greater than the output measured in normal
mode X 1.414. Actually, I can't imaging any high frequency stuff coming
back from the layout, but it couldn't hurt, possibly to surpress any
oscillations from the high gain darlington. On the other hand, I wouldn't
make a special trip to buy one. It probably isn't going to be needed.
You could have used just a regular power transistor, with lower value R2 and
R3. It would have reduced the heat in the transistor from about 1 watt per
amp to about 0.3 watts per amp.
I have a power pack with pulse mode but I never looked at the output on a
scope. I thought they used some kind of pulse width modulation, where the
pulse amplitude was always at max voltage, and they went from sliver thin at
the almost zero setting and got wider and wider as the knob was turned up,
until they disappeared at the 100% full voltage point.
With your proposed circuit arrangement, you can run it in the normal full
wave rectifier mode with 120 sinusoidal pulses per second, or the half wave
rectifier 60 sinusoidal pulses per second mode, which I expect will result
in a slower top end loco velocity, but if the loco is running and you switch
from 60 PPS mode to 120 PPS normal mode, I predict there will be a
noticeable sudden increase in speed, if that's objectionable to you.
Also, since the transistor is electrically "hot", you'll need to use an
insulator under it, or somehow insulate the heat sink from ground. And
don't forget the silicone thermal grease under the darlington.
Let us know how it works!
dgw
>> I'd put a ceramic capacitor across the voltage supply as close to the
>> transistor as possible and a reverse oriented diode across the output
>> (before the reversing switch. Also, if your output voltage doesn't go
>> fully to zero then a diode (or two) in series with the output.
>>
>> Add a few more components and you can have a momentum controller.
>
>
> GP1:
>
> I assume the capacitor would be a filter, or am I wrong about that?
>
> I'm a little uncertain about filtering, because I don't want to smooth
> out my output current too much. I know this can cause more heating,
> but that's not something my motors and I have much trouble with.
> Our problems are more along the lines of "Oh shite, the Tyco
> Plymouth at the end of that smoke trail wasn't powered down, it
> was stalled."
>
> Or is the cap for RFI purposes?
>
> What would the diode be for? I'm trying to use this as a learning
> experience, as well as a practical controller.
Gerald, with the proposed circuit, the output current will start to flow
when the poteniometer setting is about 1.4 volt about ground (zero volts),
because the two emitter junctions in series in the darlington each have a
0.7 volt drop to be overcome before they conduct any current. You can
overcome this "dead" range near zero on the pot by adding two silicon diodes
(any small kind) in series, pointing down, between the bottom of the pot and
ground.
dgw
"David Starr" <dstarr...@roadrunner.com> wrote in message
news:4706f4b4$0$7431$4c36...@roadrunner.com...
David: while you have some good points in your analysis, I disagree on a
couple of things.
The output voltage will still drop with increased load due to a dropping
collector voltage, due to voltage sag caused by the transformer impedance.
If a fairly rock solid stable output voltage is desired then the voltage at
the top of the pot has to be regulated. The simplest way is with a zener
diode and resistor. The drawback it that the top end output voltage is then
a lot lower. Plus, even if the voltage were rock steady, the loco that is
binding or going up hill is still going to slow down because of increased
load on the motor.
My other comment is that using a 100 ohm base resistor will blow out the
darlington when the pot was cranked all the way up. I don't know the specs
on the transformer, but a 5 K base resistor may save the darlington in the
event of a short out on the tracks.
Anyway, the purpose of the mod is to make the engine crawl better as a
result of the quasi pulse mode.
>
> David Starr
>
>
dgw
"Greg Procter" <pro...@ihug.co.nz> wrote in message
news:4707D173...@ihug.co.nz...
Gerald and Greg - Greg, you talked about adding a diode in series with the
output IF the voltage doesn't go all the way to zero when the pot is turned
all the way down. I agree with you.....the diode would fix or reduce a
problem like that.
However, the voltage will go all the way down when the pot is turned all the
way down. In fact, the voltage will go to zero when the wiper of the pot
gets down to about 1.4 volts, so, there is no need for a diode in series
with the output. If you use one to stop some phantom reverse voltage coming
from the track, the diode must be speced to handle the full current plus
some. a 1N400X is not beefy enough. Also, if you add a diode on the
output, the output voltage will now drop to zero when the pot wiper gets
down to 2.1 volts. So if you want the output voltage to get to zero when
the wiper voltage gets to the end of its travel, you will need to add three
diodes in series between the bottom of the pot and ground, instead of the
two I suggested in a precious post.
Greg Procter
It's a good theory, and is almost certainly true with straight
emitter/follower circuits but can go wrong once you start adding
momentum etc.
> If you use one to stop some phantom reverse voltage coming
> from the track, the diode must be speced to handle the full current plus
> some. a 1N400X is not beefy enough.
Sure, but I've been doing it for ... umm ... forty years and never had
that diode fail.
> Also, if you add a diode on the
> output, the output voltage will now drop to zero when the pot wiper gets
> down to 2.1 volts. So if you want the output voltage to get to zero when
> the wiper voltage gets to the end of its travel, you will need to add three
> diodes in series between the bottom of the pot and ground, instead of the
> two I suggested in a precious post.
Fair enough.
I've often played with series and parallel resistors and diodes to give
the pot non-lineal characteristics.
Giving the 0-6 volt range to about 60-65% of the pot arc and 6-12 volts
40-35% arc works for me.
David Starr
>
> David: while you have some good points in your analysis, I disagree on a
> couple of things.
>
> The output voltage will still drop with increased load due to a dropping
> collector voltage, due to voltage sag caused by the transformer impedance.
Quite true. No real world power supply can furnish infinite amounts of
current. In fact the original rheostat only power pack relied upon
transformer winding resistance or a circuit breaker to limit current
into a short on the layout.
>
> My other comment is that using a 100 ohm base resistor will blow out the
> darlington when the pot was cranked all the way up. I don't know the specs
> on the transformer, but a 5 K base resistor may save the darlington in the
> event of a short out on the tracks.
Protection is a problem, every railroad has derailments which can short
the rails together. Plus a world of other faults. However, I would not
rely upon a base resistor to limit transistor current. Gain of
transistors is not well controlled. Typically it varies over a range of
3:1 for any single type. The Fairchild data sheet for the TIP120 gives
a gain of 1000, that means all devices will have a gain of at least
1000, and plenty of them will have a gain of 3000. This means a
resistor that protects a low gain transistor won't protect a high gain
device. The resistor that protects a high gain device will lower the
performance of a low gain device. One of the rules of transistor design
is don't design circuits that depend upon the magnitude of transistor
gain. If you do, you find that some of them work and some of them
don't, depending upon the luck of the draw in transistor gain.
The first line of defense is the biggest possible heatsink. Let's
guess a good heatsink could hold the transistor case temp down to 75 C,
allowing the device to survive 36 watts. Assume part throttle where we
drop 6 volts across the TIP120 and supply 6 volts to the track. At this
setting, the transistor can handle 6 amps before going poof. I doubt
the transformer/rectifier can furnish that much current. In short, we
protect against shorts by relying upon a stout well cooled transistor
being tough enough to withstand them.
The best heatsink is the case of the power pack, assuming the case is
metal. For a plastic case, the best heat sink might be a piece of
aluminum the size of the power pack muonted underneath it. Probably the
case has four rubber feet secured by screws to the bottom of the case.
Pull those screws, drill the heat sink to match the case screw holes,
get some 1 inch screws, and some 1/2" standoffs. Mount the heatsink on
the standoffs, the feet on the bottom of the heat sink, and the
transisor on the top of the heatsink. This gives air circulation on
both sides of the heatsink and protects the transistor from mechanical
damage.
Incidently, your idea of putting two diodes in series at the bottom
of the pot is a good one. Without them, track power goes off when the
pot wiper is at 1.4 volts, about 10% of pot rotation. The effect is a
dead spot at the slow end of pot setting, nothing happens for rotating
the pot the first 10%. The two diodes are an elegant solution yielding
nicer control action.
David Starr
pawlo...@gannon.edu
> I have a power pack with pulse mode but I never looked at the output on a
> scope. I thought they used some kind of pulse width modulation, where the
> pulse amplitude was always at max voltage, and they went from sliver thin at
> the almost zero setting and got wider and wider as the knob was turned up,
> until they disappeared at the 100% full voltage point.
>
> With your proposed circuit arrangement, you can run it in the normal full
> wave rectifier mode with 120 sinusoidal pulses per second, or the half wave
> rectifier 60 sinusoidal pulses per second mode, which I expect will result
> in a slower top end loco velocity, but if the loco is running and you switch
> from 60 PPS mode to 120 PPS normal mode, I predict there will be a
> noticeable sudden increase in speed, if that's objectionable to you.
dgw:
My sources say that Fyffe packs put out these kind of variable-width
pulses. I have no idea what happened to Fyffe, but I suspect it had
something to do with suspected heat damage to motors. Whether such
damage commonly occurred is a good question, but the model RR
control universe has always revolved around a massive black hole of
hype -- it's easy to dig up rumors, but real information is harder to
come by, and that probably goes equally for DC and DCC.
I know what you mean about the half-wave pulses - the RMS current
would indeed be halved if I cut out one diode. I thought about wiring
a DPDT switch to use the whole 24V transformer with a half-wave
rectifier, but usually when I've seen pulse-power switches like this
they
are wired the way I have it. It would only be for starting and very
low speeds. I'll see what happens.
If your pack is an MRC Throttlepack or Tech II 1400 or 2400, it's
probably equipped with the same kind of half-wave pulse power
as shown here, though I can't verify that since I've never had one
apart. I actually have some reason to suspect that the Tech IIs
mentioned above used something much like my circuit, or
Peter Thorne's which mine is probably ripped off from -- but then,
how many ways are there to make a voltage follower?
This was Thorne's simple 6-component transistor throttle, Greg P.,
and the one I tried to build before but couldn't. In retrospect, I
probably mixed up the transistor leads. Oh, and of /course/ I
own a meter, which I use for electrical troubleshooting on appliances
and suchlike - I'm an electronics newbie, but I'm not totally
brain dead. :)
Anyway, this thing is going together, bit by bit, in odd moments,
and I should be able to report on its operation in a few days. I'm
still using the 5k pot and 10k resistor, because as it turned out
the Rat Shack didn't have a pot that was any smaller value. Yes,
I do have a Jameco catalog, and my brother has one from Digi-Key,
but let's go with what we can buy in town for now.