relay/diode problem

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steelroe

Member - Posts: 17

Joined: April 03, 2008

Location: Ireland

Posted: April 03, 2008 at 12:36 PM / IP Logged

I have the following problem when supplying power to a 12v relay with a diode fitted across the coil, it keeps blowing the fuse of the circuit from which I am taking the coil supply. No other connection have been made on the relay. Would an incorrect diode cause this problem.

Help Appreciated

KPierson

Platinum - Posts: 3,527

This member consistently provides reliable information

Joined: April 14, 2005

Location: Ohio, United States

Posted: April 03, 2008 at 1:01 PM / IP Logged

Is the diode facing the correct way? If the diode is backwards it will short out the supply. The "stripe" should be on the 12vdc side. The non-striped end should be on the ground side.

Kevin Pierson

steelroe

Member - Posts: 17

Joined: April 03, 2008

Location: Ireland

Posted: April 03, 2008 at 1:15 PM / IP Logged

Thanks for the reply

Yes it is the right way round. I am not sure of the rating of the diode though.

howie ll

Pot Metal - Posts: 16,466

Joined: January 09, 2007

Location: United Kingdom

Posted: April 03, 2008 at 1:34 PM / IP Logged

Diode rating shouldn't matter its only stopping back EMF accross the relay's coil, check your relay wiring.

steelroe

Member - Posts: 17

Joined: April 03, 2008

Location: Ireland

Posted: April 03, 2008 at 1:39 PM / IP Logged

All I have connected is the coil + and -, 12V to the stripe end of the diode and grounding the other end. Could the diode be faulty.

KPierson

Platinum - Posts: 3,527

Joined: April 14, 2005

Location: Ohio, United States

Posted: April 03, 2008 at 2:06 PM / IP Logged

Does it work with the diode removed? It could either be a bad relay or a bad diode. If the relay energizes with the diode removed I would replace the diode. If the relay blows the fuse with the diode removed I would replace the relay.

Kevin Pierson

steelroe

Member - Posts: 17

Joined: April 03, 2008

Location: Ireland

Posted: April 03, 2008 at 2:11 PM / IP Logged

I tihnk I found the problem. Tried it without the diode and worked fine as before. Then iInoticed the relay is fitted with a LED (light emitting diode) and resistor. So is this causing the problem in that when connected the second diode would be acting against the LED. Can anyone explain what is happening here.

Would the LED be connected in the opposite way to the diode I am fitting?

i am an idiot

Platinum - Posts: 13,672

Joined: September 21, 2006

Location: Louisiana, United States

Posted: April 03, 2008 at 2:30 PM / IP Logged

The resistor and the LED will not cause it to blow a fuse. Either the diode you are using is shorted or it was in backwards. You stated that it was installed properly, so it has to be shorted.

KPierson

Platinum - Posts: 3,527

Joined: April 14, 2005

Location: Ohio, United States

Posted: April 03, 2008 at 2:43 PM / IP Logged

If the LED is wired backwards from the diode then that can be your problem. Try connecting the diode the other way around, and switching your + and - to match the new diode configuration.

Kevin Pierson

howie ll

Pot Metal - Posts: 16,466

Joined: January 09, 2007

Location: United Kingdom

Posted: April 03, 2008 at 5:09 PM / IP Logged

For what its worth, European relays or so I was taught back in the day always used 86 as pos for coil and 85 as neg. Bet the existing LED and diode are that way, use a meter acros the coil to find out. A standard automotive relay should be under 100ohms across the coil. Going back to my first sentence, if you follow that convention you should be OK, secondly if it already has a diode you shuldn't need another!

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