RECOMMENDATIONS FOR USING LEDs

CURRENT LIMITING (Resistors)

For all LED’s some form of current limiting should always be provided, typically a Resistor.

To determine the ideal resistor value, use OHM’s LAW: V = I x R or R = V / I

V = the Resistor Voltage Rating in Volts, I = the current in Amps, R = the resistance in Ohms

The LED will use part of the applied voltage, and the resistor must use the rest when they are connected in series.

A typical white LED uses up 3.5 volts, while a typical red LED only consumes 1.8 volts. If excessive voltage is forced on any LED, it will generally draw excessive current and burn out. The series resistor prevents this.

Example: The voltage across a white LED and a resistor in series is 14 volts, and you want a current of 3 milliamps (mA) through the LED. Being connected in series the resistor and LED will have the same current but divide the voltage. The LED uses 3.5 volts and the resistor gets the remaining 10.5 volts.

The appropriate ideal resistor for 3 mA in this example is R = V / I = 10.5 / .003 = 3500 Ohms (3.5 KOhm/14V).

LED’s have a limited ability to withstand reverse voltage polarity.

White LED’s are generally easily destroyed by static electricity.

REVERSE VOLTAGE PROTECTION FOR LED's INSTALLED IN LOCOMOTIVES HEADLIGHT

To prevent inductive voltage spikes from the motor from reaching and destroying the LED.

RECOMMENDATIONS:

For an LED in a DCC decoder circuit, the decoder circuit will serve to isolate the LED from the motor = no further precautions are needed.

Normal DC motor circuit = Protection should be provided in the form of a diode reverse connected across the LED (a reverse voltage will be shorted out by the reverse diode, preventing it from reaching the LED).

The Locomotive lighting circuit has two LED’s, forward and reverse headlights = One LED will serve as the reverse protection for the other if the LED’s are directly connected together and share a common resistor. If they don’t share a single resistor, each needs a protective diode.

The Locomotive has two separate lighting circuits and/or if one LED is removed, the other LED is unprotected = Protection should be added in the form of a rectifier diode.