Genesis SD75 lights

I was told I need to add resistors to the lights in my new Genesis SD75M's when I install the decoder. Is this true, and if so, what size resistors should I use? Thanks

 

I remember reading an article about superdetailing a UP SD70M, and they used a DCC decoder, giving resistor values. Since the SD70M and SD75M/I are pretty much the same internally, the values should be the same. I believe it was a 620 Ohm (1/2 Watt) resistor for the headlights and rear lights, and a 680 Ohm (1/2 Watt) resistor for the Dtichlights.

I don't remember if they were wired in series or parallel though.

 

Why would you need to do this? What purpose does it serve?

 

There was a thread here recently that explained the formula for determining resistor values. I'm not sure by whom, but he said he could put up a forum that would have a resistor calculator. I don't think he got enough responses to warrant the trouble, though.

 

The reason for using a resistor is that DCC is different than regular DC track. DCC maintains a constant 12 volts in the rails, while you vary the voltage with DC. Most locos come with either 3 volt or 1.5 volt bulbs, so you must use resistors to drop the voltage or else burn out the bulbs instantly. In the case of the Athearn SD70 series, they are 1.5 volt bulbs.

And it was me who posted the formulas for calculating resistor values. Problem is, I'm not sure what current rating the Athearn bulbs have.

 

Thanks for the explanation Harron! I'm not to DCC - yet - but I will be this summer. I'm sure I'll have more questions like this...

 

The Athearn Genesis SD70/75s use 1.5v 12-15mA bulbs

 

So 12V - 1.5 V = 10.5V

10.5V / 10mA = 1050Ohm
or
10.5V / 15mA = 700Ohm

about a 1K resistor should do it to be on the safe side. or down to 680Ohm if you want to drive it harder.

For the power rating of the resistor

10mA x 10.5V = .105W

A 1/8W resistor would work I'd use a 1/4W to be on the safe side still though.

Also you want to wire the resistors in series.

With multiple bulbs controled by the same decoder function wire them in series and subtract the total voltage from 12V and do the math instead of just a single 1.5V

[ 28 May 2002, 07:36: Message edited by: Paul Davis ]