Continuous
Brightness Lighting
for Model Railroad
Locomotives...



Circuit Details...
Details about the technical aspect of the circuit,
and schematic diagram.




Circuit Details

The LM334Z is a constant current source which requires only a single resistor to set the current. While the specifications for the chip state that it is only programmable to a maximum of 10mA, we will be running it at 15mA. This is quite acceptable for our use, and we will not be exceeding the units rated Power Dissipation of 400mW. The chip is wired completely inline with the LED. One of the advantages to this design is that you do not need different components to drive different LEDs. The nature of the circuit means that it will equally well drive a 1.7V Red LED or a 3.6V White LED without any modification! As the chip is completely protected by design from reverse voltages up to 20V, we do not need to add any reverse protection diodes (which drop the voltage).

At the level we will be using the chip it will not even get warm. One thing to note is that the low value of the resistor means that it is difficult (if not impossible) to get in very accurate tolerances. The resistor type is of the 5% tolerance type, which means the value may actually be 5% off from the specified value. This would never cause concern, but it may be beneficial to measure the resistor's resistance with a multimeter and use the lowest value to gain up to a couple milliamps in current if you are trying to get the brightest light.

Circuit Schematic






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