Diodes on relays?

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Yoshi_D_Dino

Member - Posts: 22

Joined: July 29, 2003

Posted: August 13, 2003 at 8:53 AM / IP Logged

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!

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muldrocks

Member - Posts: 3

Joined: August 09, 2003

Location: New Zealand

Posted: August 13, 2003 at 12:44 PM / IP Logged

Hi

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

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mobiletoys2002

Gold - Posts: 1,050

Joined: April 12, 2002

Location: United States

Posted: August 13, 2003 at 1:16 PM / IP Logged

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.

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Diodes on relays?

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