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[jjb62556]

I have a Athearn Genesis SD70M from a friend that want me to put Tsunami 2 sound decoder in it. That is no problem, but it has light bulbs for headlights and ditch lights. My problem is what size resistors do I need in order not to blow the lightbulbs?
Rely don't want to replace the bulbs with LEDS either. Jim

[learmoia]

Why did this get a -1 comment?  Seems like a legit question...

I assume you doing a hardwire instillation to have individual headlight and ditch lights.

Loy's Toys used to make a resistor selector  (Rotary dial with various resistor sizes that you put in line with your circut, and you dial it down to the brightness you want and that is the resistor size you use..)

Without that.. we could try Ohm's law.. Voltage drop across resistor: 12V - Bulb Voltage = Voltage Drop
Voltage Drop / Amp Draw of the bulb (probably .01 or .02/bulb) = Resistor size in Ohms

If memory serves.. LEDs I know it was 1000 (1k)... for grain of rice bulbs used in older Athearns it was in the 550-650 range..

[jjb62556]

Thanks, Jim

[bigdawgks]

Why did this get a -1 comment?  Seems like a legit question...

Just a guess... But it may be because the thread title is just a single character... 

[jjb62556]

That's on me. Push the button before I was done.

[Mark5]

as orginal poster - you can fix it. 

[learmoia]

as orginal poster - you can fix it. 

You Have the POWER!!

[Jim Starbuck]

https://www.ngineering.com/led_calculators.htm

Ngineering has these handy calculators.
Ballpark guess of 12v supply voltage, 1.5v lamp voltage at 20 milliamps brightness according to the calculator would use a 525 ohm 1/4 watt resistor.

[peteski]

@jjb62556 , you can edit your initial post and fix that not very informative subject line.  There is a "Modify Post" link on the top right of your own posts.

I prefer replacing bulbs with LEDs as those are simpler to deal with, and have much longer lifespan. Any chance of doing that?

You didn't mention what voltage those bulbs operate on (if you even know) or whether there is some DCC decoder already installed.  Those would be important clues for assisting you.

Assuming that the bulbs are 1.5V they still could have various current ratings.  I have seen tiny 1.5V bulbs rated at 10, 15, 20 mA.  If you have a 10mA bulb and calculate a resistor for a 20mA bulb your bulb will glow too brightly and will have short life.

As I said, to get things right you need to know the specs of the bulbs, or replace them with LEDs (with typical 1000 ohm resistors in series).