I have a bit of a hybrid project, including my first attempt at adding LED headlights to a dummy diesel.  It's an MTH / Railking SD-9, but what I've added to it are most of the parts from a Lionel Deluxe Upgrade kit for non-powered GP-9 B diesels.  The first part involved taking the electrocouplers from the supplied trucks and transplanting them on the MTH SD-9 trucks - which by the way were a perfect fit.  Then I got to mounting the Lionel LCRX0906 board and getting everything connected.  The electrocouplers work perfectly - the LED headlights, not quite...

For each of the headlights I have two 3mm, 3V warm white LED's wired in series with a 300 ohm resistor also in series - and that part seems to be just fine.  Reading previously about having problems getting the light outputs on some of these boards to work correctly, I also added a 470 ohm resistor directly across each light output to the common ground.

Setting the engine on the track and testing it out, I can only get either the front headlight or rear headlight to work at a given time.  This is where it gets odd...  One or the other of the headlights will work, being able to be switched on and off, and going on and off when changing the locomotive direction.  The other will always remain dark.  However - if I rattle the engine around a bit, the light that previously didn't work will start functioning correctly.  And of course, the one that worked previously will now stay dark continuously.

I initially wired up a test with the components listed using test leads for one headlight, and had no problems.  Is this odd behavior a possible indication that the 470 ohm resistor across the light output to ground isn't sized correctly?  I've checked the wiring and don't see anything loose and all connections seem to be in order.  If anyone has seen this kind of behavior before I'd appreciate the insight!

Original Post

I think you're getting bit by the LCRU oddity with the light outputs.  If my memory serves, that's one that actually doesn't necessarily maintain the same polarity of output for the lights.  The only fix I figured out was a bridge rectifier to make sure the polarity was correct all the time for the lights.  Also, you can use a .01uf 50v capacitor across the light output instead of the resistor, it doesn't dissipate any power and does the same job.

Definitely appreciate the reply...  Polarity is something interesting that I noted early on as I began to tinker with this.  Prior to the odd behavior I described today, the lights worked with their positive connected to ground, and the negative lead(s) connected to the light outputs on the board - the opposite of what I initially tried.  That the polarity isn't necessarily always the same coming off of these boards is definitely something I missed in my previous searches.  I realize I also forgot to mention in my original post that I am operating in a command control environment, not conventional in case that makes any difference.

I'm pretty much a noob at attempting to create electronics such as this - so forgive me...  Does it make a difference where the bridge rectified would be placed, as in between the board and the LED's, or between the LED's and the common ground?  Is there a particular bridge rectifier part number I should seek out for this application?  



It does make a difference, the control of the triacs in the older board design was such that they triggered on any quadrant, hence the polarity in command mode would change at random.  They should actually work in conventional mode as they pass full-wave power.

The bridge would be after the .01uf cap (or your resistor) and before the LED and dropping resistor.

John - just thought I'd let you know that your advice did indeed do the trick!  I had to order the bridge rectifiers and .01uf caps, but after getting them installed and soldered in place my headlights work just like I wanted and no more odd behavior.  


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