Constant to momentary

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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=73790
Printed Date: May 17, 2025 at 9:29 PM

Topic: Constant to momentary

Posted By: powersnet
Subject: Constant to momentary
Date Posted: March 02, 2006 at 3:44 PM

I need to build an automotive relay that will hold the coil for three seconds. I have seen diagrams on this but don’t know how to choose the right components, capacitor and resistor. I would also like to know how it works

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Replies:

Posted By: Hornshockey
Date Posted: March 02, 2006 at 6:40 PM

There is a setup on the relays page to the left. Basically consists of a relay, a capacitor and a resistor. If I'm not mistaken it explains on that page exactly how it works. If you need a 3 second pulse, a 7500 microfarad cap will give you about 3 seconds delay. Just wire the cap and resistor in parallel on the ground side of the coil.

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Posted By: powersnet
Date Posted: March 06, 2006 at 12:17 PM

I have used the diagram on constant to momentary output and can't get it to work. I am using a light to simulate the device 12V+ on 87 and positve light lead on 30 and negative light to ground. the coil is diode protected. I am using 1k microfarad cap and a 330 ohm resistor which only gives me a short pluse. By all calculations I should get 3 seconds. I have tried everything I can think of, but no positive outcome!

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