Do I Need Blocking Diodes?

Printed From: the12volt.com
Forum Name: Relays
Forum Discription: Relay Diagrams, SPDT Relays, SPST Relays, DPDT Relays, Latching Relays, etc.
URL: https://www.the12volt.com/installbay/forum_posts.asp?tid=142411
Printed Date: April 28, 2024 at 11:01 AM

Topic: Do I Need Blocking Diodes?

Posted By: xcalibr1
Subject: Do I Need Blocking Diodes?
Date Posted: January 11, 2017 at 9:46 PM

I want to tap into the ground inputs on a body control module on a 2002 Jeep Grand Cherokee for the turn signals and hazard lights. I want those grounds to ground the coils of relays to cut power to DRLs anytime the turn signals or flashers are on. My question is do I need to use blocking diodes to protect from any surges going into the BCM when the coils are de-energized even though I am on the ground side of a circuit? Im using 4 relays with the maximum of 2 relays being used at any given time on a ground signal. Either side turn signal will engage one relay per side and the flashers will engage 2 relays at the same time. If I need diodes then what exactly would I need and which direction do they get installed? I have installed many remote starts in my vehicles but never did anything that used diodes so I am clueless as to their use and sizing.

Replies:

Posted By: the12volt
Date Posted: January 11, 2017 at 10:02 PM

Use diodes across the coils of the relays with the cathode side of the diode (side with stripe) on the relay coil terminal you supply 12V + and the anode side of the diode to the other coil terminal of the relay where you supply ground from the BCM.

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Posted By: xcalibr1
Date Posted: January 12, 2017 at 6:34 AM

Im sorry for being stupid here. As I said I have never wired anything quite like this. But the ground is not coming from the BCM. When you engage the turn signal lever or flasher switch you are completing a circuit to ground for the BCM on any of the 3 wires. The BCM is not supplying the ground. Does that change anything? I also need to know what diodes to use. I can post a pic of wiring schematic if that helps. Thanks.

Posted By: the12volt
Date Posted: January 12, 2017 at 6:44 AM

Whether the ground signals you are using to trigger the relays are coming from the BCM or going to the BCM, the diode connections will be the same.

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Posted By: xcalibr1
Date Posted: January 12, 2017 at 7:24 PM

I found these relays that already have diodes in. Makes it easier for me. Should work right? Would it be overkill to still put diodes directly into the 3 wires between the point where I tap in and the BCM itself?

Posted By: the12volt
Date Posted: January 12, 2017 at 7:42 PM

Putting the diodes inline would be of no benefit at all. They will only suppress a spike when placed across the coil.

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Posted By: xcalibr1
Date Posted: January 12, 2017 at 7:51 PM

Okay. That shows what I know. I was only thinking of it as a way of making sure positive voltage could in no way ever reach the BCM.