J McL:
Good idea.

Crossposted upon suggestion...note that this device
is a model railroad controller.

Transformer T1 delivers 1A.

wrote:
Folks:

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.