Q: How do older DC controllers control the engine speed

DC engines are controlled by the amount and polarity of the power. A controller like the MRC uses a potentiometer to regulate the amount of power sent, and a switch simply changes polarity, causing the engine to change directions.
Reply to
Frank Eva
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Can anyone tell me how older DC Controllers control engine
speed. I know that you supply DC to both rails of the track.
How is speed controlled. I have an old MRC controller and
it seems to only work at ( or near ) full speed. Does it
send out 5 volts at 1/4 speed, 10 volts at half speed and
20 volts at full speed or does it send the speed to the
train in some other method?
Thanks - jack
Reply to
Jack Snodgrass
That is the basic idea. The speed knob is basically a variable resistor in series with the track outputs. On slow speeds the controller has a high resistance. Turning the knob lowers the internal resistance of the pack and lets the locomotive have more power. In essence giving it more voltage.
MRCs normally have a taper wound speed control which means the resistance changes slower at low speeds than high. This is for better locomotive control at slow speeds.
It is possible that a wire in the variable resistor is broken thus cutting off the lower speeds. Does the locomotive do nothing and then jerk forward at a certain point on the throttle setting?
Another possibility is that one of the rectifiers has failed, or that the controller is in "pulse" mode. This would allow only 7 volts or so maximum to be reaching the engine. Check to see if there is a switch marked "pulse or half wave" ,and if so, be certain it is turned off. The only way to tell for certain would be to put a voltage meter on it.
Does the locomotive run ok with other controllers?
Reply to
SleuthRaptorman
The reason that the old MRC controller Jack has only runs modern engines at or near full speed is because, instead of a potentiometer and transistor circuit, it uses a simple rheostat and a rectifier. A rheostat (which is simply a variable resistor) requires a significant current draw to operate. Modern engines often will move on as little as (or less than) 100ma of current. This is too low for a rheostat to be effective, and thus the track gets nearly the full voltage and the locomotive tears off down the track like a drag racer. Awkward... :-)
Reply to
Gary M. Collins
=>Can anyone tell me how older DC Controllers control engine =>speed. I know that you supply DC to both rails of the track. =>How is speed controlled. I have an old MRC controller and =>it seems to only work at ( or near ) full speed. Does it =>send out 5 volts at 1/4 speed, 10 volts at half speed and =>20 volts at full speed or does it send the speed to the =>train in some other method? => =>Thanks - jack =>
Several methods:
a) use a variable resistance to control the power output to the track - since power at any setting is a constant, increased amperage draw by the locomotive means lower voltage --> lower speed. The cheapest train-set powerpacks do this. b) use a variable resistance in a voltage control circuit to maintain voltage output, and vary amperage as needed - best if done via solid state. The cheaper MRC powerpacks use this method. c) generate DC pulses of variable width (duration) and/or voltage - effect is to supply power as needed to loco. The morer expensive MRC power packs do this. d) use a variable voltage input to the rectifier --> constant voltage, variable amperage. Some people have built pwoerpacks based on this principle, using surplus variable transformers.
The fact that your older MRC power pack works only at or near the full speed setting could mean a variety of things, the most likely of which is that your loco needs cleaning and lubricating, or that it simply draws too much power (older locos aren't as efficient as newer ones.) It's less probable that there is a serious fault in the power pack - they are pretty tough, and when they fail the usually fail completely.
Wolf Kirchmeir ................................. If you didn't want to go to Chicago, why did you get on this train? (Garrison Keillor)
Reply to
Wolf Kirchmeir
Direction is controlled by 'polarity', and set using a "reversing switch" (usually DPDT type).
Speed is set by track voltage (12 VDC max, normally), and controlled (in the older power packs) with a variable resistor. Unfortunately, track voltage is a function of BOTH the power pack resistance AND the loco's motor resistance (which varies with load). Hence, speed control was inexact at best. The harder the loco worked (going upgrade) the less power (voltage) the power pack would provide. NOT good!
Newer (non DCC) packs now use solid state electronic components (transistors and/or SCR's) to control track voltage. These set track voltage independently of the loco's motor resistance, and provide far better speed control.
If it's an older resistance (potentiometer or rheostat) type pack, then it may NOT have enough resistance to control the newer more efficient motors. This may explain the speed control problems you describe.
Dan Mitchell ==========
Jack Snodgrass wrote:
Reply to
Daniel A. Mitchell

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