Diode on a relay?

Do I need a diode on a relay that's only switching the dome light? (Ford Ranger...one bulb)

Thanks,

Ken

You should always add a diode across the coil of any relay to supress spikes when it de-energizes.  These spikes can go back and possibly wipe out whatever is driving it.  The coil stores a voltage, just like a capacitor.  When it is de-energized, the coil discharges, creating a reverse polarity spike, induced back into the line. During normal operation, the diode is reversed biased and is effectively out of the circuit.  When the relay is de-energized, the diode is forward biased to the spike and the spike is shorted out.

Cathode (band)  to coil +12VDC

Anode to coil GND/negative lead.

Use a 1N4000 series diode. I like to use IN4007.

- Jim

Thanks Jim,

I'll head to Radio Shack.

Ken

I wanted to correct myself on my explanation....when the coil is energized, a magnetic field is created which pulls in the relay.  Its when this field collapses (relay denergized) is what generates the pulse/spike.  Similar principle like when you apply voltage to a motor to make it spin.  You can remove the applied voltage and spin the motor shaft and generate a voltage out.

Good luck with the project!

- Jim