Capacitor to hold relay energized?

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Forum Name: Car Security and Convenience
Forum Discription: Car Alarms, Keyless Entries, Remote Starters, Immobilizer Bypasses, Sensors, Door Locks, Window Modules, Heated Mirrors, Heated Seats, etc.
URL: https://www.the12volt.com/installbay/forum_posts.asp?tid=10287
Printed Date: May 07, 2025 at 2:00 AM

Topic: Capacitor to hold relay energized?

Posted By: Toolrocks
Subject: Capacitor to hold relay energized?
Date Posted: March 02, 2003 at 1:12 AM

Hey I'm new here so please be gentle. I tried a search but didn't find anything relevant to my question. I'm hoping to keep a relay energized at least 5 seconds after it loses its original "turn on" signal. I'm pretty certain I can wire a capacitor across the coil but I don't know how to calculate the C's value. Can you please tell me the formula or another/better way? The formula would be most appreciated so I can vary the time. Thanks in advance and this is a great site.

Replies:

Posted By: Velocity Motors
Date Posted: March 02, 2003 at 6:52 AM

Capacitor & Coil Calculator

Here you go, it wasn't in the forum but in the site menu on the left under Calculators.

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Jeff
Velocity Custom Home Theater
Mobile Audio/Video Specialist
Morden, Manitoba CANADA

Posted By: the12volt
Date Posted: March 02, 2003 at 8:19 AM

That calculator will not work for your application and there is not yet one available on this site. However, you can find the values of the capacitor and/or resistor you need fairly quickly by experimenting. Start with a 5K µfd capacitor without the resistor as shown below. The resistor is only needed to bleed off the charge of the capacitor and will shorten the length of time the coil of the relay will stay energized. If the delay time is shorter than you want with a 5K µfd capacitor, try a 10K µfd capacitor until the duration meets or exceeds the desired duration. If the duration is exceeded, begin adding different value resistors starting with higher values at first to shorten the duration until you reach the desired duration. You could also use a variable resistor as an attenuator to dial in the exact desired duration once you've exceeded it with a large enough capacitor.

Substitute your pulsed output where the diagram shows 12V+ in from a turn signal. Terminal 30 is your output.

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I'll work on a finding a formula so I can write a calculator for this purpose and post it here.

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Posted By: Toolrocks
Date Posted: March 02, 2003 at 10:44 AM

Thank you both very much. I will try this as soon as I can get some components. I know that in the past I've seen a graph showing the discharge rate of a capacitor, I just can't find it now. Thanks again.