relay/diode problem
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=103675
Printed Date: July 19, 2025 at 8:52 AM
Topic: relay/diode problem
Posted By: steelroe
Subject: relay/diode problem
Date Posted: April 03, 2008 at 12:36 PM
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
Replies:
Posted By: KPierson
Date Posted: April 03, 2008 at 1:01 PM
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
Posted By: steelroe
Date Posted: April 03, 2008 at 1:15 PM
Thanks for the reply Yes it is the right way round. I am not sure of the rating of the diode though.
Posted By: howie ll
Date Posted: April 03, 2008 at 1:34 PM
Diode rating shouldn't matter its only stopping back EMF accross the relay's coil, check your relay wiring.
Posted By: steelroe
Date Posted: April 03, 2008 at 1:39 PM
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.
Posted By: KPierson
Date Posted: April 03, 2008 at 2:06 PM
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
Posted By: steelroe
Date Posted: April 03, 2008 at 2:11 PM
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?
Posted By: i am an idiot
Date Posted: April 03, 2008 at 2:30 PM
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.
Posted By: KPierson
Date Posted: April 03, 2008 at 2:43 PM
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
Posted By: howie ll
Date Posted: April 03, 2008 at 5:09 PM
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!
Posted By: steelroe
Date Posted: April 03, 2008 at 5:19 PM
Will check all this tomorrow. In the mean time would you guys have any idea on how to solve this one https://www.the12volt.com/installbay/forum_posts.asp?tid=103673&KW=steelroe related to the same install.
|