Diodes on relays?

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=17533
Printed Date: May 24, 2024 at 8:40 PM

Topic: Diodes on relays?

Posted By: Yoshi_D_Dino
Subject: Diodes on relays?
Date Posted: August 13, 2003 at 8:53 AM

Quick question, what is the general rule on when it is ‘required’ to use diodes on the coils of a relay?

Generally I've only seen them on starter and ignition interrupts of alarm system relays.

Or when the alarm system does not have a dedicated ground output for an armed status and needs to share the ground output from either interrupts.

Other applications would be to just isolate multiple sensors on shared trigger wires.

Cheers!

Replies:

Posted By: muldrocks
Date Posted: August 13, 2003 at 12:44 PM

There are 2 ways you use diodes in conjuntion with relays - in series and in parallel. You have hinted at one - the series one, when you want multiple sources to operate one relay, or one source to operate multiple devices.

The other, and most important is the parallel one. Here the diode is connected the opposite way to "normal", and its role is to clamp the back EMF from the relay as it is de-engergised. Without the diode to clamp the EMF, which can climb to hundred's of volts, any semiconductor in the circuit can get damaged by the excessive voltage which it can't withstand.

Hope this makes sense

-------------
John Muldrock
Hawera
New Zealand

Posted By: mobiletoys2002
Date Posted: August 13, 2003 at 1:16 PM

also most newer relays already have a diode within the relay to supress the back emf,but it is always a good idea to make sure the relay has one already or if not add your own diode with any relay application.