# Decoder issue fixed, LED resistor recommendation



## Bman (Aug 24, 2010)

I'm not sure what the durability of most mobile decoders are but I have ten (mostly Digitrax) of them that work flawlessly. I was in the process of putting together a GTW GP38-2 (Bicentennial loco). I had installed the new engine, and wired in the new Digitrax DH123D decoder. Well the directional lighting did not work, the front light was always on. Then I pulled another DH123D from a working loco plugged it in to the GP38-2 and had the same problem  After emailing/talking to Digitrax they told me the decoders were broke. I took them back to my awesome LHS and traded them in for new ones. New ones work great. 

I just can't believe that I had two bad decoders, I'm sure it was something that I must have done. 

By the way for LED's Digitrax recommended 1K Ohm 1/4 watt resistors one in the yellow wire and one in the white wire.


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## T-Man (May 16, 2008)

Remember the IC"S are prone to destruction by static charge. Synthetic fabrics without a softner, through a dryer, is enough.

The Resistor makes sense. 18 volts - 3v for an LED is 15 volts / .02 amps = 750 ohms. 1000 ohms is well in the safety margin. That's without the drop in voltage from the track.


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## Bman (Aug 24, 2010)

Interesting, great example of Ohm's law in the model railroad world. I'm a chemistry man so I don't know a whole lot about physics/electricity. In this case what do you run the risk of doing with no resistor or in the other case with to much resistance. I have my ideas but I'm not totally sure.


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## T-Man (May 16, 2008)

More resistance will dim. Less will be a puff of smoke,odor, and no light. Ever again.


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## Bman (Aug 24, 2010)

Thanks for confirming my suspicions


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## T-Man (May 16, 2008)

You said 1/4 watt, well E*E/R= watts. 15*15/1000=.225
My 750 ohms requires a 1/2 watt, 15*15/750=.3, >.25 and < .5
Recently, I have been playing with the formulas.


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