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Resistor calculations for low voltage bulbs on DCC

Posted: Mon Apr 13, 2015 9:57 pm
by Tony_Harris
I am fairly new to DCC, and am installing a Digitrax DH165AO into a Genesis SD70 (pre-DCC socket) loco. The replacement board clips in place and the loco moves. However I have successfully blown the headlamp bulbs by connecting the lamp wires to the same connector tabs on the ‘replacement’ DCC board. Digitrax tell me that I should have connected them to FOF and blue (+ common) using a resistor (obviously?).
Looking at various web sites and using ohms law the resistor value required appears to be:
13.7v – 1.5v = 12.2v @ 15mA = 813 ohms 0.25 watt resistor required in series
Track voltage = 13.7v (from the display on the DT402 controller)
Replacement headlamp bulbs: Miniatronics 1.5v 15mA incandescent bulbs

Maplins have 820 ohm resistors available to fit in series with each bulb. Could somebody please just confirm that I am heading in the right direction?

In case anybody is wondering, I am using bulbs and not LEDs because a) they fit in the headlamps hole b) I have lots of bulbs.

Tony Harris

Re: Resistor calculations for low voltage bulbs on DCC

Posted: Tue Apr 14, 2015 10:18 am
by brianc
There was a good article in Model Railroader October 2014 about choosing headlight resistors.

I am not expert in these matters, but using the example quoted in the article you seem to be on the right lines.

Hope this helps.

Brian Clark

Re: Resistor calculations for low voltage bulbs on DCC

Posted: Tue Apr 14, 2015 1:44 pm
by Mike_R
Your calculations are correct except that the decoder drops around 2v. On the other hand it is better not to run the lamps at full current to give a longer life and allow some leeway if the loco runs on a layout at a higher voltage.

On using LEDs I use these ... cd-55-1654 from Rapid Electronics. Mounted side by side they match the spacing of HO headlights, with the domed heads fitting into the holes. I connect each pair in series, so only one resistor is needed for the pair. I find them a lot brighter than lamps, should last a lifetime and produce almost no heat. I run them well under the max 30mA, usually around 10mA.