I have changing some decoders for a friend at our club and he has an older Ath, Gen. SD-70 which has a the following set up:- The double headlights each have one wire going to the white wire from the decoder which is also connected to the left leg of a T20 transistor. The other wires from the headlights are connected to the centre leg of the T20 and the third leg of the T20 is connected to the blue wire from the decoder.
The rear light is set up the same way using the yellow wire and another T20.
and the ditch lights the same using the green wire and another T20.
( The T20 transistors are oriented with the flat side facing you , I dont know if they are PNP or NPN there are no markings on them )
Given that Ath. uses 1.5 V. bulbs could this circuit be using the transistors in stead of resistors.
I think the "transistors" are actually LM317LZ voltage regulators. Tony's Trains has a diagram for using these to drop the 12+ volts from the decoder down to 1.25v:
http://tonystrains.com/technews/install-lamps-decoders.htm
I've used this technique on locomotive headlights successfully before. The headlights will be slightly dimmer than usual (1.25v vs. 1.5v), but it works and keeps the voltage constant regardless of the track voltage. The trouble with resistors is that they drop the voltage proportionally to the track voltage, so if you normally run on a system that puts 12v to the track, then take it to a layout running a system putting 16v to the track, you run the risk of burning out the bulbs. With the regulator, the headlight output voltage will always be 1.25v regardless of the input voltage.
Hope that helps! :)
Robert Beaty
The Laughing Hippie
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The CF-7...a waste of a perfectly good F-unit!
Then it comes to be that the soothing light at the
end of your tunnel, Was just a freight train coming
your way. -Metallica, No Leaf Clover
Arjay1969http://tonystrains.com/technews/install-lamps-decoders.htm
that's an interesting circuit. the lm317 can also be used as a current regulator to drive one or more LEDs from a varying supply.
but considering the concerns regarding current, why not put the lamps in series and add a pair of resistors to supply the exact voltage required by the lamps/LEDs. This way the current is the same whether there is one or more lamps.
greg - Philadelphia & Reading / Reading
Putting bulbs in series vs. parallel is a whole other discussion, really. If you put the bulbs in series and one of them burns out, how can you tell which one? When one blows, the whole series stops working. If you put the bulbs in parallel, if one burns out you can see exactly which one to replace.
The reason Tony's suggests not using the resistors to set the voltage is because as-is, the regulator will supply 1.25 volts. This is sufficiently low enough to keep the bulbs from burning out for a LONG time.
Now as for using the LM317 to set current for LEDs, I have no experience along those lines...the only LEDs I have installed so far have been in a pair of Athearn F-units for a friend, and I used a dropping resistor.