relay on for 1 sec after power loss

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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=125872
Printed Date: May 13, 2025 at 6:25 PM

Topic: relay on for 1 sec after power loss

Posted By: sk8rjess
Subject: relay on for 1 sec after power loss
Date Posted: January 26, 2011 at 10:43 AM

i'm running a carpc and my startup/shutdown controller just stopped working.
i can use a relay to turn on the pc no problem, but turning it off is my issue. when i turn my ignition off, i only need the relay to initiate for about 1 second, then stop sending power. i figured i'd need a capacitor but what size? and someone correct me if i'm wrong please :)

Replies:

Posted By: i am an idiot
Date Posted: January 26, 2011 at 11:00 AM

a 10,000 mic cap paralleled across the coil of a relay should give you about a second. You will also need to put a diode in series with the power wire, before the cap and relays coil. The diode will prevent the cap from trying to power whatever is up line from the relay.

Posted By: hotwaterwizard
Date Posted: February 16, 2011 at 12:43 AM

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John DeRosa (Hotwaterwizard)
Stockton California
When in doubt, try it out !

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