1.5 volt bulbs, how to tell

[karle]

For locomotives like Bachmann steam engines, how can you tell if the factory bulbs are 1.5 V or 16 V.  I want to install a Soundtrax decoder(DSD101LC) which requires a dropping resistor if the bulbs are 1.5V.  I don't want to plug the new decoder into the printed circuit board that came in the loco's tender and fry the decoder.  Also, I assume I have to cut the capacitor on the Bachmann PCB?  Note this loco is about 5-6 years old and has an 8 pin NMRA plug built into the PCB.  Any assistance would be greatly appreciated! 

[Gandy Dancer]

For locomotives like Bachmann steam engines, how can you tell if the factory bulbs are 1.5 V or 16 V.

It should say in the owners manual.   If you still don't have it check the Bachmann web site.

Another way to tell would be to get a 1.5V battery and connect it to the lamp leads.  If it lights you know.

I don't want to plug the new decoder into the printed circuit board that came in the loco's tender and fry the decoder.

The various voltage requirements of the lamp aren't going to fry a decoder.  If the lamp is 1.5V it would burn out the light bulb if installed without the resistor.  It would burn it out so quickly the decoder might not even recoginize the "almost short circuit" was there.  By design one can't even fry a decoder if it is plugged it into the NMRA 8-pin plug backwards.  The loco would just run backwards.

Also, I assume I have to cut the capacitor on the Bachmann PCB?

That would depend on why the capacitor is there, and which circuit it is in.

Personally I often just totally cut out a factory circuit board.  This is especially true if it has a lot of circuitry on it.  This is just because one doesn't know what it really does.  Doing that leaves no question where the wires go and also leaves more room for the decoder & speakers. 

[dstarr]

Light bulbs, how to tell what they are.  Methods

1.  Look for a marking on the bulb itself.  Something that reads "1.5V" or "12v" is good.  A part number can be put into Google and will often yield a manufacturers data sheet.  This Google trick works with a lot of things, IC's, motherboards, transistors, whatever.   Down side, you have to get the bulb out of the loco which can be non trivial.

2.  Literature search.  There ought to be a parts list (sometimes called a bill of materials) which calls out every nut, bolt, washer, boiler, wheel and light bulb needed to build the model in the first place.  Parts lists are very often packed with the model.  If you don't have it or can't find it, try the maker's website, or Google on the "end item"  (locomotive) model number and maker.  

3. Experiment.  Connect a flashlight battery (the old fashioned carbon zinc and the alkaline cells are 1.5 volts, avoid the new fangled lithium batteries which are 3 volts).  If the lamp glows properly on one flashlight battery it's a 1.5 volt bulb.  Or connect the variable DC output of your train power pack to the bulb.  Start off set all the way to zero and inch it up slowly and carefully until the bulb is glowing at regular brightness.  If the bulb glows at a VERY small throttle setting, it's 1.5V.  If you have the throttle all the way up, its 12V.  Be careful, the powerpak will reach 1.5 volts VERY quickly,  if you overshoot you can pop the bulb and then it won't light no matter what voltage is applied.

3.  Expert knowledge.  If the locomotive has a constant brightness headlight the bulb is just about got to be 1.5V.  

Good luck

 

[karle]

Thanks very much for the helpful advice!  Now that I think about it (because of what you have said), it would seem to me if the bulbs were 1.5V Bachmann would have built resistors into the tender PCB or wired them in series in the locomotive body (which I have never disassembled).  So if I plug into the 8 pin plug, the decoder would be outputting power through those resistors.  Thanks again! 

[reklein]

I like Gandydancers tactic of taking out existing circuit boards. That way you know what your working with.

[dstarr]

   Built in resistors are unlikely.  The usually circuit has silicon diodes in series with the motor.  Two diodes in series have a constant voltage drop of 1.4 volts  just right for a 1.5 volt bulb.  The diodes maintain the 1.4 volts no matter what the track voltage may be, giving a constant brightness headlight.  Sometimes a small integrated circuit is used in place of descrete diodes. If the locomotive does not have constant brightness headlamps, the bulbs are most likely 12 volt 'cause it's cheaper, just a bulb, no diodes, resistors, circuit boards etc.  The 12 volt bulbs are as cheap (or perhaps cheaper) than the 1.5 volt bulbs.

  

[modelmaker51]

All of the earlier Bachman Spectrums had 12v bulbs in series with a diode.