send 12V+ when continuity to ground is broken

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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=30099
Printed Date: July 18, 2025 at 4:15 PM

Topic: send 12V+ when continuity to ground is broken

Posted By: midaryl
Subject: send 12V+ when continuity to ground is broken
Date Posted: April 10, 2004 at 9:29 AM

I went temporarily brain dead. What I'm trying to do is send 12 volts thru a wire when continuity to a ground is broken, Could someone point me to a diagram or help me pin this out?

Replies:

Posted By: mladams
Date Posted: April 12, 2004 at 11:59 AM

what you want to do is wire the 12 volt constant to pin number 30

next you wire the device your looking to power to pin 87A

bridge power from pin 30 to pin 86

next run the ground that will be broken from pin 85

that should do what you're looking for

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Matt Adams
Electrical Engineer Major
York College of PA
Chevy Cavy Rally Sport

Posted By: robbie2883
Date Posted: April 12, 2004 at 1:24 PM

It's more complex that that. If you do it that way, the coil will put a constant draw on the battery and drain it.

What exactly are you trying to do? is this a door trigger or something?

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if its jammed force it - if it breaks in the process it needed to be fixed anyway
posted_image

Posted By: scorp133
Date Posted: May 06, 2004 at 3:56 PM

I am looking to do something similar. I have a trunk release button that I want to be disabled when a (-) lead from my alarm grounds. I had 86 and 30 on constant 12v+, 85 to my alarm ground when armed wire, and 87a to the trunk switch. I thought it worked, I could pop my trunk and when I armed the alarm it broke the circuit, but when I disarm my alarm and 85 is ungrounded for whatever reason the coil is not interrupted and continues to keep 30 connected to 87 (which is nothing). If I pull the relay it will reset and I can use the switch again, how do you make the coil at rest without a switched +12v?

Posted By: chriswallace187
Date Posted: May 07, 2004 at 12:16 AM

Here's what you can do - assuming it's a positive switched trunk release. 85 to the ground-when-armed, 86 and 87a to the switch side of the trunk relase solenoid wire, and 30 to the solenoid side of that wire.

What happens then is whenever the trunk release button is pushed with the alarm disarmed, it works as normal.

With the alarm armed, the shortest path to ground is across the coil of the relay, therefore when you press the button it activates the relay and cuts off power from the solenoid.