Diode on a relay?

Printed From: the12volt.com
Forum Name: Car Security and Convenience
Forum Discription: Car Alarms, Keyless Entries, Remote Starters, Immobilizer Bypasses, Sensors, Door Locks, Window Modules, Heated Mirrors, Heated Seats, etc.
URL: https://www.the12volt.com/installbay/forum_posts.asp?tid=62564
Printed Date: July 18, 2025 at 4:25 AM

Topic: Diode on a relay?

Posted By: jumargibbs
Subject: Diode on a relay?
Date Posted: September 12, 2005 at 5:07 AM

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

Thanks,

Ken

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Ken

Replies:

Posted By: sharc
Date Posted: September 12, 2005 at 7:53 AM

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

Posted By: jumargibbs
Date Posted: September 12, 2005 at 8:47 AM

Thanks Jim,

I'll head to Radio Shack.

Ken

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Ken

Posted By: sharc
Date Posted: September 12, 2005 at 9:21 AM

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