Brightness Lighting
for Model Railroad

As promised, here is my page detailing my use of the LM334Z
as an LED driver in Model Locomotives!

What Continuous Brightness Lighting Means

Continuous Brightness Lighting tries to overcome the prevalent problem of lighting in model locomotives. The problem is that when the train starts to move the locomotive (headlight) is completely unlit, and as the train speeds up (i.e. higher voltages applied) the light gets brighter. The worst part is, even at top speed the light is usually nothing more than a dull orange glow from the outside.

Some Previous Solutions

There are various solutions to the problem, but so far none of them have been satisfactory from the modellers perspective. Previous solutions are almost always unsuitable for smaller scale modellers as well, simply because of the size of the solution.

I have always felt the use of LEDs has been a great option. Not only do they run cool, but they never need replacing. Being plastic, they can also be highly shaped to fit the particular application. Many have felt the same but the problem is how to drive them? LEDs require a certain voltage to make them start working, and past this will consume all available current. Typically a resistor would be used to resist the current flowing through them. This stops the LED from burning out, but leads to the same problem we had from the lamps. The LED only lights dimly at low voltages, and gets progressively brighter as the train is sped up. We should also put a diode inline with the LED, so that reversing the locomotive, and the back EMF generated by the motor, does not damage the LED. Now our LED will not even start to light until we are moving along at some speed.

Voltage Regulators

While voltage regulators have some acceptance in the railroad modelling community, they are a very poor solution to the problem. Most have dismal performance, and are difficult for modellers to employ. I have posted some information here, if you are interested in reading the drawbacks of this approach.

My Solution, the LM334Z

The solution I have found is a small chip from (nominally) National Semiconductors, the LM334Z. This comes in a very small package (TO-92), smaller than the LED. The LM334Z is a "programmable current source", what this means is that the chip will allow only a certain current through it, irrespective of the voltage. "Programmable" only means that you can set the amount of current yourself. This is done with a single resistor. The unit can take forward voltages up to 40V, and needs no protection against reverse voltages (i.e. the locomotive is running in "reverse") for voltages up to 20V. Both of these exceed the nominal 12V of model railroads by a very comfortable margin.

The unit will only light in "forward", giving directional lighting without any additional diodes (which cause voltage loss).

end transmission

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