Making a Constant lighting circuits for DC loco headlights.
Main North HOME.
Most manufacturers of loco these days add some form of constant brilliance lighting and are similar to the “diode circuit” below but with incandescent 1.5 volt lamps. Low voltage 1.5 incandescent lamps have a large variation in brilliance and also in these circuits still have a variation in brilliance from low speed to high speed.
This variation in brilliance and the heat that is generated, I prefer to use LEDs for all my headlights. LEDs are longer lasting, no heat and you can adjust the brilliance with the series voltage dropping resistor. The Prototype White LEDs from DCC Concepts come the correct colour for incandescent lights of the real prototype locos, not with the blue tinge of earlier white LEDs.
Chris Corayer has kindly supplied the details of a better IC than the LM334, the CL2N3 from Supertex available from Mouser Electronics for less than 50 cents, that does not require any external resistor and provides a fixed “constant” current of 20 mAs. The CL2N3 is a two lead IC that will be easier to install than a LM334. The data sheet for the CL2 is at:
http://www.supertex.com/pdf/datasheets/CL2.pdf
- Using a LM334Z Adjustable Current source:
Thanks to Terry Cunningham.
- Shown above is one of the LM334Z/4.7 ohm resistor combinations soldered to one of the motor connections, on a Main West 42 mechanism. These can be mounted anywhere in the loco.
- Connecting the LM334/resistor as shown in the schematic, only one LED will illuminate at a time due to the “diode action” of the LED. The alternate illumination of the LEDs is automatic, when the polarity changes to the track.
- One LM334/4.7 ohm ¼ watt resistor combination per LED headlight.
- If using LEDs you will have to fit a voltage dropping resistor, so the LM334 is the only extra component.
- Using a 4.7 ohm resistor will “regulate” a current of about 12 – 15 mAs of current through the LED irrespective of the voltage applied to the motor.
- I found the LED illuminated long before the loco stated moving.
- This little circuit is a true constant brilliance application more than the diode circuit above and does not “rob” the loco of any speed and hence all modellers that like high speed operation will be happy.
- If the LEDs illuminate in reverse, swap the LM334/resistors around.
- This circuit is so simple, I would replace the incandescent 1.5 volt lamps and the diode board that a lot of later model locos like the NR etc come with.
- Incandescent “constant brilliance” diode board arrangements like in the NR are very poor. You will agree when you add this small circuit.
- Part numbers fro Jaycar are LM334 – ZL-3334 $2.40
- See Jonathan Hilliard ’s article at: http://users.speedlink.com.au/~imra/seminar2.html
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- Using diodes for LEDs & incandescent 1.5 lamps:
These can be 
made
by joining 5 diodes as per the diagram on the left below. For a shorter diode
assembly, in a tighter installation with less room, make 4 legs instead of 2.
Bring LED connections out and insert diode arrangement in heat shrink.
Insert the arrangement in one power pick up lead to the motor. Use 1A IN4004
diodes. May have to experiment with LEDs and motor connections to get correct
motor direction and LED illumination. The idea here is that each diode creates
a voltage drop of about .5 - .8 volt depending on the current drawn by the
motor, the higher the current the higher the voltage drop. The “extra diode in
each “leg” is to isolate the headlights. Note this above arrangement will
reduce the available voltage to the motor by about 3 volts. This is one reason
why loos with constant lighting like the NR, run slower. The above
circuit was tested on a NR drawing 150 - 180 mAs. Five diodes per leg are
required using the 3.5 volt Prototype White LEDs. Hardly any variation in
brilliance between low and high speed. Experiment with the number of diodes and
type of LEDs and 1.5 incandescents.
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