Folks:
I have an MRC Ampack that works, but its rheostat provides poor control to some of my locos. Therefore, I am rebuilding it into a simple transistor pack, and this is the circuit I am using, modified after some discussion on rec.models.railroad:
http://www.geocities.com/kezelak/ampack_mark2.jpg
Basically it is a simple voltage follower using a Darlington twin. I am also adding a pulse-power switch to cut out one rectifier diode. The voltage follower seems to work with no load on the breadboard.
Any suggestions, thoughts, etc.?
Edit: The purpose of the protector diode & filter cap were explained here:
http://groups.google.com/group/rec.models.railroad/browse_thread/thread/4c65ec5f744919d4/
If you are going to the effort of building your own transistor throttle why not add a low end speed control adjustment, momentum & brakes, and a real electronic pulse injector. The 1/2 wave “pulse-power” created by using a single rectifier (S2 in the diagram) is murder on both the locomotives motors and the transistor.
Also if one is thinking about the 1/2 wave “pulse-power” why would the capacitor (C1) be included as they are working against each other. For RFI purposes it should be closer to the transistor not AFTER the direction switch. If you are going to do that you shoul put a capacitor on each of the locomotives. That would really cut the RFI to TVs & AM radios more than one where shown.
GD:
Capacitor - okay, I see. You put it close to the transistor to keep the length of wire that acts as an “antenna” to a minimum. That makes sense.
That kind of thing is exactly why I am starting with this simple circuit. I am an electronics newbie who has enough basic understanding to get started. If I begin with this simple circuit, and experiment for a while, I’ll learn stuff, like I did just now, and then I can move on to bigger things. If I try to start with all the bells and whistles, I’d just be following a recipe, and I want to avoid that.
I’ve learned it many times over, and I’ve seen it in others: start small, and grow from there, and you will never lose interest. Start big and you risk becoming overwhelmed and burning out.
Besides, my MRC Railpower gives decent control, and has a very similar circuit, but without the pulse power. I know I won’t have a True Action Throttle, but with any luck I’ll have something better than my Ampack’s rheostat.
I’m not too worried about my motors or transistor. People have used half-wave pulses in this hobby for years without causing all kinds of disasters. I don’t have any coreless motors, just open-frames and cheap cans; the TIP120 Darlington is a $1.50 component, cheap to replace, and will be mounted on a substantial aluminum heat sink, bolted to the metal case, so I should be able to dissipate lots of heat. I will also be doing plenty of experimentation, making sure that any motor heating isn’t excessive.
Edit: As for the capacitor, it is a very small disk, which won’t affect much at the 60Hz frequency
of my pulses.