diode across relay

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=54298
Printed Date: May 13, 2024 at 6:35 PM

Topic: diode across relay

Posted By: Reeceddogg
Subject: diode across relay
Date Posted: April 20, 2005 at 4:35 PM

i recently rewired my headreast lcd to include a relay so they shut off automaticlly with the car but now when i turn the car on the get a voltage spike and i know this will eventually damage the screen. how would i use a diode on the relay to stop this
Relay wired:
85- ground
86- remote turn on
87- 12 volt constant
30- to screens

Replies:

Posted By: Montrealler
Date Posted: April 20, 2005 at 7:57 PM

Install the diode to catch the back emf across the coil (pin 85 and 86 )

IT WILL SEEM STRANGE BUT DO THE FOLOWING

Put the diode Kathode(white stripe) at pin 86
Put the diode Anode (black side) at pin 85

When the coil stop to be energized sudenly the electromagnetic field wich is still present will go back in the coil an re-transformed into power

the diode will short the feedback pulse wich is always generated in the opposite polarity of the actual trigger of the relay.

This is why the diode seem to be install in reverse.

Posted By: joebiv318
Date Posted: May 03, 2005 at 9:53 PM

He said when he turns the car on he gets the spike. Not when he turns it off, does this matter?

Posted By: ff-mike
Date Posted: June 13, 2005 at 4:29 PM

Good catch there joe

I would start with a noise filter on the output and see if that reduces the noise