Question for Electronics Experts

[knewsom]

Is there a rule for choosing a resistor when lighting a lamp using half wave lighting on DCC?  I have a Precision Scale N&W 4-8-0 M-2C locomotive that I have converted to DCC.  I used a Digitrax DH163 decoder in the tender, and I wired the front headlight and class lights to work without connecting to the common blue wire so that they would operate on half-wave.  The class lights had to be set up this way as they are grounded to the frame.  The resistor values that I chose (680 Ohm for Class Lights and 820 Ohm for Headlight) work great on DC, but on DCC the headlight is barely visible and the class lights can not be seen.  Do I just need to divide the resistor values in half, or is there some other rule for this?

 

[UpNorth]

Could your decoder be set for LED instead of incandescent.  Soundtraxx I know have this option and the incandescent is very faint when set wrong. Dim is even worse.

[jim22]

That's certainly an unconventional way to power the lights.  You are essentially creating a path from the decoder's power supply back to the frame, which must be connected to one rail.  How do you feel about simply powering them from the rails only?  That would make them constantly on.  Are these 1.5v lamps or LEDs?  Probably the resistors you have would work.

Jim 

[knewsom]

The way that the class lights were set up on the model had one wire going to a 0.75 mm diameter incandesant bulb rated at 1.5 volts and 20 MilliAmps.  They were originally wired in series with the motor and four diodes to give a constant 1.4 volts.  I have rewired them with the dropping resistor and they light fine on conventional DC, but when I try them with DCC there are barely visible.  So it appears that the resistor value needs to be less with Half-Wave lighting, but I am not sure how much less.

[Vail and Southwestern RR]

That's certainly an unconventional way to power the lights.  You are essentially creating a path from the decoder's power supply back to the frame, which must be connected to one rail.  How do you feel about simply powering them from the rails only?  That would make them constantly on.  Are these 1.5v lamps or LEDs?  Probably the resistors you have would work.

Jim 

Not really unconventional.  It is done in other cases.  I think an LED would work better than a bulb, since only being on half the time with an LED looks bright enough.

[jim22]

That one wire that goes to the bulbs, where did you connect it?  To a decoder function output?  I would try connecting it through the resistor to the isolated rail (the one not connected to the frame).  Your DCC system would power the lamp with it's AC signal, and they will be on all the time.  By using a decoder output, you have a screwy return path for the lamp current.  If you want to run the lamp with the decoder, I think you need to isolate the lamp from the frame and connect the common lead for the return path.

Jim 

[UpNorth]

My 2-6-0 are wired in this manner because I did not want to run another wried from bulb to decoder. Digitrax supports this as per their documentation.  They want the blue common present mostly when transponding is involved. 

I found this on the Digitrax site :

" The DH163L0 uses a current limiter set to control one incandescent bulb on the front (FOF) and one on the rear (FOR) and putting the removed number board bulb on either FOF or FOR will result in two very dim bulbs if they light at all, and putting the number board lights on one of the functions will require a resistor.. "

and this:

" Lamps that draw more than 80 mA when running require a 22 ohm 1/4 watt resistor in series with the directional light function lead to protect the decoder. "

Hope it helps.