# adding resistors for LED's



## graywolf06 (Aug 14, 2019)

Tried to find an answer in search but could not find one that fit my needs. Question--- is it necessary to put the resistor on the positive wire? Second ?? --- can you just put one resistor on the positive wire since the headlight & the backup light are on independantly and thus could share the same resistor. The reason I am asking is that room in this engine is at a premium ---- GE 44 ton dual motor.


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## T-Man (May 16, 2008)

First question is no. Second question is yes that will work. You should show the circuit to make sure. In order to go back to forward, the light polarity changes. So how do you Id positive? The LEDS could be wired together backwards with the resistor on one wire.

LED Refresher jan 2010

These are soldered to opposite connections but both will light with AC current. This is how your back up light will work.


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## Sevenhills1952 (Feb 2, 2021)

T-man is correct. An LED IS a diode, so on DC the polarity must be correct for LED to light.
Since AC goes positive and negative 60hz, polarity doesn't matter, LED turns on and off 60X/second which is too fast to see it flash.
Current is critical so if LED rated 0.02A (20mA), resistor value is chosen for that.

Sent from my SM-S205DL using Tapatalk


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## graywolf06 (Aug 14, 2019)

T-man, I notice you are showing O Gauge in your photos --I am on HO DCC so therefore not AC voltage. I understand from what you say that I can put the resistor on either leg. You lost me with the second part of the answer about sharing the resistor. The refresher, though thoughtful of you, didn't help me. The engine is being run by a Digitrax SDN136PS decoder.


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## Dennis461 (Jan 5, 2018)

put resistor on blue wire, then split it to go to LEDs.
Blue is positive, white and yellow are the DCC controlled grounds(return).


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## gunrunnerjohn (Nov 10, 2010)

Put a bridge rectifier in front of the dropping resistor for the LED, it'll maintain the same polarity out of the bridge all the time.


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## graywolf06 (Aug 14, 2019)

Thanks, Dennis, this I understand. Thanks for the reply, Gunrunner, Not sure how that is done, but things in this engine are tight and if I was to do that I might as well just put in 2 tiny resistors.


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## T-Man (May 16, 2008)

Glad it's over. Enjoy.


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## graywolf06 (Aug 14, 2019)

I'm glad too ---only looking for an answer that I could understand. Thanks to all.


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## Lemonhawk (Sep 24, 2013)

Since your DCC HO graywolf06, as long as you can assure yourself that only one LED will be on at a time you can use one resistor (I would suggest a 1K resistor) on the common lead to the LED's which I think is + on the decoder with the negative lines going to each LED for control. The resistor calculators attempt to drive the LED a max of around 20 ma which I think is too bright and choosing a larger resistor also extends the lifetime of the LED.


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## graywolf06 (Aug 14, 2019)

Thanks for the clear info, Lemonhawk. I suspect that the only way I will find out if the lights are truly indepedent is to setup an experiment with some common LED's that I have laying around and not sacrifice the tiny ones I have mounted in the engine. I will temporially install those LED's and see what happens.


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