Re: MRC Ampack upgrade: Attn electronics froods

pawlowsk002@xxxxxxxxxx wrote:


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



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