NMRA British Region Forum

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
where
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.

thanks,
Tony Harris

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

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 http://www.rapidonline.com/electronic-c ... 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.