# Digitrax DCC Lamp Board question



## JoeG (Feb 3, 2013)

I bought the Athearn Genesis SD70MAC Alaska Engine. It came with a DC quick connect DCC lamp board by Digitrax. I placed a Soundtraxx decoder into the quick connect plug. The engine runs great and all functions work, however, this engine has halogen lights and I wanted to switch them over to LED lights. When I connect the LED's into the FOR and FOF plugs they don't seem to work, but then the halogen lights are in they work fine. Does anyone know why this may be happening.


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## Cycleops (Dec 6, 2014)

What do you mean by the FOR and FOF plugs? You should have yellow white and blue wires coming from the Soundtraxx decoder I think, these are for the lights. If you are connecting the LEDs without a resistor they will blow immediately, maybe that's why you think they aren't working.


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## JoeG (Feb 3, 2013)

Cycleops said:


> What do you mean by the FOR and FOF plugs? You should have yellow white and blue wires coming from the Soundtraxx decoder I think, these are for the lights. If you are connecting the LEDs without a resistor they will blow immediately, maybe that's why you think they aren't working.


FOF and FOR mean (F0) function F means front and (F0) function R means rear. And what I meant by plugs were the four connections at each end of the board, the two connections that are in the middle are the connections for the lights. I have a picture inserted of what the board looks like. As for the resistor, I have tried connecting the LED's with a resistor and without and I get the same results (no lights). I thought I might have blown the LED's but I tested them and they are still working.


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## JerryH (Nov 18, 2012)

Your LEDs are polarity specific while the bulbs aren't. Make sure the you have the LEDs connected in the correct polarity.


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## mopac (Feb 24, 2011)

I have heard the genesis headlights are 1 1/2 volts. Not enough to turn on the LEDs.
Use a volt meter and see what volts are coming off the light leads.


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## JoeG (Feb 3, 2013)

JerryH said:


> Your LEDs are polarity specific while the bulbs aren't. Make sure the you have the LEDs connected in the correct polarity.


I will check the polarity in the LED's. So there is no + or - just polarity?


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## JoeG (Feb 3, 2013)

mopac said:


> I have heard the genesis headlights are 1 1/2 volts. Not enough to turn on the LEDs.
> Use a volt meter and see what volts are coming off the light leads.


Thats one thing I was thinking of too. I think my meter broke so I will have to buy a new one to figure it out.


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## JerryH (Nov 18, 2012)

Yes the polarity is the + and - leads on the LED.


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## JoeG (Feb 3, 2013)

JerryH said:


> Yes the polarity is the + and - leads on the LED.
> 
> Ok thank you for the help!!


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## Cycleops (Dec 6, 2014)

The resistor can go on either, plus or minus.


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## JoeG (Feb 3, 2013)

Cycleops said:


> The resistor can go on either, plus or minus.


I put my resistors on the lights and hooked them up to my decoder and now they work great. However, the resistor gets hot to the touch. I have a warm white LED with a 470 ohm resistor. Im not sure why this is, I haven't run into this before.


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## JerryH (Nov 18, 2012)

I would try a larger resistor say a 680 to 1000 particularly if the LED is on the bright side.


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## JoeG (Feb 3, 2013)

JerryH said:


> I would try a larger resistor say a 680 to 1000 particularly if the LED is on the bright side.


Would that cause the resistor to heat up? I would think you can place any resistor on an LED and it would work, however, I don't know anything about electricity. So I may be very far off.


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## JerryH (Nov 18, 2012)

The resistor limits the amount of current flowing through the LED. When too much current flows it will generate more heat and shorten the life of the LED.


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## JoeG (Feb 3, 2013)

JerryH said:


> The resistor limits the amount of current flowing through the LED. When too much current flows it will generate more heat and shorten the life of the LED.


ahhhh, i figured there was something i didn't understand, thanks for your help. Right after I read your post I ordered 680ohm and 1Kohm resistors.


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## JerryH (Nov 18, 2012)

You may have too small wattage rating on your resistor as well. Here is a calculator. Your 470 should work though.

http://led.linear1.org/1led.wiz


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## JoeG (Feb 3, 2013)

JerryH said:


> You may have too small wattage rating on your resistor as well. Here is a calculator. Your 470 should work though.
> 
> http://led.linear1.org/1led.wiz


Yes, its saying I need 1/2w and I have 1/4w


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## Lemonhawk (Sep 24, 2013)

It's more about brightness. If you need full bright then you probably should change to 1/2 watt, but with a 12 supply a 1k Ohm 1/4 watt resistor is probably a more pleasing brightness. This is a case where you need to please yourself without letting any smoke out!


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## JoeG (Feb 3, 2013)

Lemonhawk said:


> It's more about brightness. If you need full bright then you probably should change to 1/2 watt, but with a 12 supply a 1k Ohm 1/4 watt resistor is probably a more pleasing brightness. This is a case where you need to please yourself without letting any smoke out!


Right now I have a 470 1/4w resistor on a white 3mm led and the resistor is getting hot to the touch.


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## DonR (Oct 18, 2012)

Whoa, Lemonhawk. The wattage of a resistor
is determined by the amperage it must pass.
If a resistor is heating it probably needs to be
a higher wattage.

The resistance (ohms) of a resistor would
control the voltage, thus the brightness of bulbs.
The higher the resistance, the lower the
voltage to the bulb, thus dimming it.

Don


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## Lemonhawk (Sep 24, 2013)

Actually in an LED its current that controls the LED.the resistor is in series with the LED and increasing the Ohms causes less current thru the LED. That's why a n LED driver such as the CL2 outputs 20 ma.


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## JoeG (Feb 3, 2013)

DonR said:


> Whoa, Lemonhawk. The wattage of a resistor
> is determined by the amperage it must pass.
> If a resistor is heating it probably needs to be
> a higher wattage.
> ...


So I should try a 1/2w resistor and see if it heats up?
Well, after using one of the LED ohm/watt calculator it says I need a 1/2w resistor.


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## Lemonhawk (Sep 24, 2013)

I'm a little surprised that the resistor gets hot. Is it so hot you can't touch it or is it just a little warm to the touch? If warm I would leave it alone. Hot and I wonder if something else is going on. Any chance of attaching a voltmeter to the decoder output (voltage across the resistor LED combination, or just across the resistor if that's easier. I don't think I have anything less than about 600 Ohms used with a 12 volt supply and they are all 1/4 watt and don't seem to get warm.


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## JoeG (Feb 3, 2013)

Lemonhawk said:


> I'm a little surprised that the resistor gets hot. Is it so hot you can't touch it or is it just a little warm to the touch? If warm I would leave it alone. Hot and I wonder if something else is going on. Any chance of attaching a voltmeter to the decoder output (voltage across the resistor LED combination, or just across the resistor if that's easier. I don't think I have anything less than about 600 Ohms used with a 12 volt supply and they are all 1/4 watt and don't seem to get warm.


It gets a little warmer then warm but not hot. If I leave my finger on it for about three seconds I have to take it off because its to warm. I wouldn't put it in an engine and leave the headlight on four hours like I do with my other engines. I'll have to check to see what the voltage it. My meter broke so I need to get anther one. I just bought 560ohm 1/2w resistors so I will see how they do, which is what a resistor calculator called for.


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## DonR (Oct 18, 2012)

I agree with Lemonhawk. A single LED of the
size one would install in an HO loco should not
be drawing a current that would cause a 1/4 watt
resistor to get hot to the touch. All of the resistors
used in my LED circuits are 1/4 watt, some are
lighting 4 or 5 LEDs with no problem.

You could have a defective resistor or there
could be some sort of 'contact' that is unintentionally
drawing current thru the resistor.

Don


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## JoeG (Feb 3, 2013)

DonR said:


> I agree with Lemonhawk. A single LED of the
> size one would install in an HO loco should not
> be drawing a current that would cause a 1/4 watt
> resistor to get hot to the touch. All of the resistors
> ...


The wire is magnet wire, would that have anything to do with it?


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## Lemonhawk (Sep 24, 2013)

If the LED is drawing 20 ma then a 470 Ohm resistor would be dissipating under 0.2 watts so a 1/4 watt resistor should work. Something else is wrong. It must be drawing much more than 20 ma. Seems like if the resistor is that hot, the LED would be blown. Possibly a low resistance short some where else?


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## DonR (Oct 18, 2012)

Doubt the type of wire has anything to do with your
situation.

Wish you had a multimeter so we can do some
tests. (you can get one from Harbor freight for 4.95)

If you do, set it to read amps (or ma) and wire
in it series with your resistor, LED and power
source. That will
tell us what current you are drawing then we can
do some other tests to find the 'culprit'.

Don


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## JoeG (Feb 3, 2013)

DonR said:


> Doubt the type of wire has anything to do with your
> situation.
> 
> Wish you had a multimeter so we can do some
> ...


This is the multimeter I bought, I tested the volts and they come out of the decoder at 5.49v. when it was on Dim, and 3v on bright. When I hooked up the LED, with the resistor connected to the negative side it stopped responding (meaning when I put the engine in forward or reverse the light stayed on dim as if it was on rule 17 mode. When I went pressed the button to make the light brighter it did not change intensity.) However, the voltage was around 2v when it was dim. Also, I am not sure how to check for ma with this meter.


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