back EMF question

Discussion in 'DCC & Electronics' started by nachoman, Apr 4, 2008.

  1. nachoman

    nachoman Guest

    I have an LED headlight wired on a conventional DC loco operating with a conventional DC powerpack. There is a resistor in series with the LED, and the light illumiinates perfectly in foward. But in reverse, the LED is mostly dark, but it occasionally flashes. I am told this is because of back EMF from the motor.

    I am eventually going to convert to DCC, once I get my trackwork done and decide what system I want. Will this problem still happen with DCC? My guess is no, because the headlight will be wired through the decoder, and the decoder should filter any back EMF from the motor. Am I right on this?

    In the meantime, is there a way to stop the headlight from flashing in reverse under stadard DC?

  2. steamhead

    steamhead Active Member

    Hi...I'm no expert, but.....A diode is polarity-sensitive. When you reverse the track polarity the diode should not light. I doubt that back EMF would be at fault here. Unless you're "coasting" for a considerable time at a considerable speed, the EMF generated is nearly non-existent.
    DCC takes advantage of back EMF to help maintain a constant speed, particularly when heading down a grade. It will not affect the performance of the headlight.
  3. bclemens

    bclemens Member

    Hi, Kevin,
    Anything made out of semiconductor material (transistors, chips, diodes) can be susceptible to CEMF. CEMF or "Back" EMF can happen in circuits that involve inductive loads such as motors. While I don't know how your circuit is wired so I am just speculating, it is possible that voltage spikes are being picked up in your LED circuit. If you are as old as I am, you remember the radio interference that the old point-coil ignition in cars could cause in the speakers. That's the same principle.

    If that is what is happening, here's how to solve the problem:
    Get a small value ceramic disk capacitor. The value is not critical, it could be .01 to .001 microfarad. Break open an old transistor radio or any consumer type audio device and you'll find some in there.

    Simply place it from the anode of the LED (the high side) to ground. You can try just placing the capacitor from one side of the LED to the other.

    The capacitor will look like an open circuit to the normal DC, so the LED won't care if it's there and will function normally. But to any high frequency / short duration voltage spikes caused by motor "noise", it'll look like a short and bypass them around the LED. There is no polarity to that kind of capacitor so it doesn't matter which direction you connect it.

    Good luck!

    (Currently Division Chair of Electronics, Ozarks Technical Community College)
  4. nachoman

    nachoman Guest

    Aha. That's what I was thinking. I'll try the ceramic capacitor, cause I have gobs of them around!

  5. nachoman

    nachoman Guest

    well, I tried a small ceramic capacitor... I think it is like 100 pico farads. That's about the largest one I think I could fit in this locomotive. No dice. I could try to make room for a larger capacitor, but as long as I am not ruining the LED the way that it is now, i think I will leave it be until I convert to DCC.

  6. bclemens

    bclemens Member

    If the capacitor didn't fix it, then the problem must be something different that what I was theorizing...sorry!
    Best wishes,

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