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:
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.