Regulating/capping voltage for a lamp

[TIDexter]

I'm working on a project for my model railroad and I would like some help.

I have a locomotive with a DC motor that runs it. This motor is powered by a power supply that ranges from 0-12VDC to control the speed of the train. The DC motor can pull as much as 1.5A.

Also in the locomotive is a lamp that is connected in parallel with the motor to simulated headlights. Currently the lamp starts off dim with low voltage and obviously gets brighter with higher voltages as the train moves faster.

What I would like for the lamp to do is reach its full brightness as soon as possible, say at 2V from the power supply, and remain that brightness for higher voltages. That way the lamp pretty much stays the same brightness no matter what. What happens below 2V I'm not picky about, whether the lamp is off or it dims in brightness, doesn't really matter. Basically, as long as the lamp is full bright at any speed of the train.

From what I can tell the best way would be to get a 2V lamp and limit/cap/regulate the voltage to the lamp so it stays at 2V. So how would I go about doing this? I think a zener diode might do what I want to achieve but I'm not sure of it handles variable voltages or the somewhat high current. I've also at the 78** series of voltage regulators but they all need a higher input voltage to work, which is not ideal.

Recommendations? Am I even going about this the right way?

 

[jstamour802]

I think a zener diode would work out just about right. You'll want to calculate the power so you size the zener properly, but I doubt the lamp draws that high of a current.

There is probably some resistance limiting current to the lamp, so a smaller zener would probably work ok. You can also add a resistor to further limit the current, although it may adversely affect the turn-on voltage for the lamp.

Just make sure you put the zener in a part of the circuit that only affects the lamp, otherwise you may shunt voltage for the moving parts and cause the train to move at permanent low speeds!