3 volt regulator for powering dual 1.5volt headlights on DCC locomotives
                 
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This circuit is a way of powering 2, 1.5 volt micro mini 1.2mm diameter bulbs, wired in series and used in the HO diesel's dual sealed beam headlights or tail lights.  It reduces the total current drain (and thus the heat dissipation) compared with using a 1.5 volt regulator, which doubles the current since the bulbs are then wired in parallel.

It uses the popular LM317T variable voltage regulator IC.

Schematic:
RED, BLACK and BLUE refer to decoder wiring standard lead designations.
3 volt Regulator Schematic

The following pictures show the circuit being installed in an Atlas Classic GP38:
(Front is to the right)

Regulator circuit without decoder wires:
Regulator without decoder wires

Decoder installed, white wire goes to the front light bulbs, yellow to the rear light bulbs:
Decoder wires attached
The lamp string common power leads will go to the center terminal of the LM317T instead of the BLUE wire.

With 20 ma bulbs, and the regulator circuit, the lamps and regulator will draw a total of 50 ma if both lights are on.  In contrast, a 1.5 volt regulator powered system will draw about 90 ma. underf the same conditions.

This circuit will also provide more predictable illuminatioon with the bulbs than using resistors in series with the bulbs, since it is controlling the voltage presented to the bulbs, rather than the current.

Another advantage of this circuit is that wiring the bulbs in series allows the wires that join the two bulbs together to be quite short, reducing the number of wires that can get tangled inside the locomotive when the shell is mounted on the frame.  I usually tape those wires inside the roof of the shell to keep the bulbs loosely in place in the headlight housings.
-Jim Fuhrman

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