I purchased a used older MRC Twin Power DC transformer. I can regulate speed only by moving the throttles slightly above the zero mark. After that the voltage immediately jumps to 17+ volts throughout the rest of the throttle range. It is the same on both throttles. Are both rheostats bad and is there anyway to correct this situation? Isn't 17 volts way to high for HO scale locomotives?
Does the power pack use rheostats or transistor voltage control?
Assuming rheostats: Are you testing with or without a load? If you get instant full voltage with a load, the rheostat is probably shorted. Without a load, instant full voltage is normal with rheostats. Remove power and measure the resistance across the output of the rheostat, not the voltage as you move the knob. You should get a reasonably smooth change in resistance if the rheostats are in good shape.
With a rheostat, the voltage dropped in the rheostat is directly proportional to the current being drawn by the circuit. Voltage dropped at the rheostat = resistance tapped times current. If the current in the circuit is zero, there is no voltage dropped in the rheostat. Thus, the output voltage will go from zero to full as soon as the rheostat is moved off the zero post (open circuit).
If you have an engine drawing 0.5 amps and a 50 ohm rheostat - not unusual in the old days - you have to turn the rheostat to about the 18 ohm point to get 3 volts at the rails (9 volt drop in the rheostat), and the engine moving. If your engine draws only 0.25 amps like many modern motors do, rheostat control has a much different start point - you need to be at the 36 ohm point to get the same 3 volts on the rails.
Note that a motor drawing 0.2 amps will have 2 volts (and may start moving) just as soon as the rheostat moves off the "off" post - you don't have enough resistance in the rheostat to control the engine. The advent of efficent can motors that draw less than 0.25 amps made the change to transistor voltage control necessary. MRC power packs from the Tech 2 era onward (all Tech series do) generally used transistors rather than direct rheostats.
The better rheostats used in train control were decidedly non-linear resistance (often touted as wire wound with 3 different wire sizes) to reduce this difference in start points.
The small, cheap transformers used in power packs had/have poor regulation. They were set up to deliver the full 12 volts under load, which means voltage was often 16-17 volts under no load (zero current). The maximum voltage from a power pack will typically be around 14 volts at average loads. Are you actually running your trains at full voltage? FWIW, most 12 volt DC motors will easily survive 17 volts. It is current in excess of their rating that cooks them.
my thoughts and experiences, your choices
Fred W
Fred,
Wow. What a great technical answer. Much more than I expected. Yes, it is an older, gold metal box transformer which I assume would be a rheostat. I was testing at no load which according to your explaination is why I got the 17 volt reading. I will try the rest of your tests just to make sure.
Fair warning - those are probably 40 ohm rheostats, and they will NOT run a modern can motored locomotive at much less than warp speed.
If you're feeling adventurous (and are VERY comfortable working with electrical equipment) you can open the box and check to see if you can connect the 'loose' end of the rheostat(s) to the opposite polarity DC output wire. That will change them to potentiometer mode, and give you a full range of voltage control. It will also cause them to run warm - each will be dissipating about 4 watts of power as heat.
If, in exploring under the hood, you discover selenium stack rectifiers, this would be a good time to substitute either silicon bridge rectifiers or 'home brews' of four 50v3a diodes rigged bridge fashion.
I made those mods to a couple of 1950s power supplies to bring them into line with the 21st century. Now they control both can motors and old power-hog Pittmans with equal ease.
Chuck (Modeling Central Japan in September, 1964 - with power packs that are even older)