As of September 2006 this may be old news to some of you but the more recent LLP2K diesels and early Intermountain F-Units have their Headlight bulbs and Number Board bights wired-up together.
Three, and in some cases, four bulbs on these locos are wired together, hot to the track.
This is fine for DC, but in DCC, if connected to FO-F on any DCC Decoder, the combined milliamp rating of the 3 or 4 bulbs overload the Headlight Function and will burn it out.
NCE & Digitrax and Tony’s Train Exchange make decoders specifically formatted for the LLP2K diesels. However, they were designed for locos equipped with one bulb for the headlight and number boards, the
one bulb illuminating everything through a light bar. Now, the later diesels come through with separate bulbs for these purposes. As discussed this can damage the decoder but it also allows you to illuminate the Number Boards and Headlight independently while eliminating the hazard.
For the locos we are discussing, the first thing to do is to separate the headlight bulb from the number board bulbs. The number board bulbs can be left together.
Since 12 volts come off each function output, each Life-Like bulb must be protected. For the NCE P2KSR and Tony’s Trains GP7 decoders EACH bulb must be protected by a 240 Ohm 1/4 Watt Resistor to reduce the output voltage. OR you can change the bulbs to 12 to 16 volt bulbs. Caution! For certain models that have the bulbs close to the plastic shell 12-16 V bulbs may not be substituted because the bulb heat will melt the plastic. Wire the Number Board bulbs to Function One (see decoder directions) and they can be operated independent of the headlight.
The Digitrax DH163L0 decoder has the added convenience of a voltage regulator on F0-F and F0-R. The Headlight and Rear Light are protected as the decoder comes with both F0s pre-set for the LLP2K bulb. HOWEVER, there is no such protection for any of the other functions. The Number Board bulbs need to be treated the same as with the NCE decoder.
InterMountain F Units should be treated in similar manner for amperage rating concerns, however the stock bulbs are 12 volts and DO NOT need resistors. A good fit for these locos is the Tony’s Trains RS2 decoder or the NCE KRS. The dual headlight versions can be left as such or cut the light board and make use of the decoder’s two extra functions, using one for independent control of the Number Board Lights and the second additional function to have a Mars light effect where it’s appropriate for the prototype.