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Lightbulb wiring for ditch light and others Track and electrical/DCC
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Regular poster 212 posts johnybgood18 Regular poster 212 posts
johnybgood18

Hi everyone,

I am really new to all this but I'd like to modify the installed bulblights (1.5V) in an Athearn RTR F59PHI.  The ditchlight are not flashing and I want to make them flash.  I have installed a Digitrax SDH164D in it.  I understand that I will have to reconnect one of each light wire to a separate function and connect the other to the blue common wire.  I have kept the installed board in the loco and used the 9-pin socket.  I also understand that I will need resistors between a decoder (12V) function output.  My question is: if I use the common wire from the loco board but need to use a function from the decoder, would I need to put a resistor or is the board taking charge of dropping the voltage?

Hopefully that makes sense...

Thanks

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Prof_Klyzlr Moderator 7,950 posts
Prof_Klyzlr

Dear Johnny, 1 - Yes,

Dear Johnny,

1 - Yes, _each_ bulb will require a seperate resistor

2 - No, the Decoder will not "lower the voltage" for you,
that's (effectively) what the each-resistor-per-bulb is for
(A quick glance at the SDH164-series manuals suggests that the function outputs are configured for "permanent 12V" operation, they don't have a "LED" or "Low Voltage" mode...)

3 - Function wire --> resistor --> bulb --> common blue wire

Follow this "signal flow" for each bulb, and it'll work a treat

4 - Suggest erring on the side of caution, start with 1K Ohm  (1000 Ohm) resistors.
These may represent "excessive volt drop", but will ensure nothing goes "pop" during your initial testing.

5 - If you're thinking of adding 1.5V bulbs, have you considered LEDs?
LEDs are more robust if used within their spec limits, have a longer "lifespan",
and are available in incredibly tiny sizes...

Hope this Helps...

Happy Modelling,
Aim to Improve,
Prof Klyzlr
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johnybgood18 Regular poster 212 posts
johnybgood18

Thanks

Thanks for the info, it's very concise and easy to understand.

Yes, I did consider using LEDs, but I have about 50 1.5v and 50 3v light bulbs lying around, so I'd like to use them and when they'll burn out, I'll probably replace them with LEDs at that time.  Should I need to install ditch light, I will use LEDs though, as the bulbs I have, although very small, are a bit big for DLs.

I have been able to calculate the ohm rating of the resistor I need ((12v - 1.5v = 10.5)/.035A = 300 ohm) but I'm still not sure about the wattage I would need or how to calculate it.  If you could help me out a bit here, it would be greatly appreciated.

Thanks again,

Christian

You can visit my layout Facebook page: Freelanced Perkins subdivision
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LKandO Prolific poster 3,839 posts
LKandO

Volts x Amps = Watts

12 x .035 = 0.42 watts using your numbers

Alan

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johnybgood18 Regular poster 212 posts
johnybgood18

Pretty easy!

Thanks Alan, so it's (Volt x Amp rating)?  Cool!  In this case, I should be fine with 1/2 watt resistors?  Just double checking if I got this right!

Chris

You can visit my layout Facebook page: Freelanced Perkins subdivision
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