constant to momentary relay issue

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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=135176
Printed Date: May 14, 2025 at 3:01 PM

Topic: constant to momentary relay issue

Posted By: lexelite
Subject: constant to momentary relay issue
Date Posted: October 30, 2013 at 1:06 PM

Its good to be back on this forum - it has been a while. Recently, I used the relay diagram here to build a constant to momentary relay setup:

posted_image

The diagram is fairly simple. I used these components for the 10K resistor and 1K uF capacitor:

[URL]https://www.radioshack.com/product/index.jsp?productid=2102507[/URL]

[URL]https://www.radioshack.com/product/index.jsp?productid=2062309[/URL]

The issue I am having is that the relay coil is energizing when I place power on terminal 86/87 and providing 12V on the common 30 terminal. However, the relay is not deenergizing and releasing the arm back onto 87A as designed. The circuit stays on as long as power is applied to 86/87.

I troubleshot it for about 30 minutes and did not come up with any clear reasons. I did confirm that the correct cap lead (rubber) was positive, soldering was solid, among other things. Can anyone please point me in the right direction?

Replies:

Posted By: lexelite
Date Posted: October 30, 2013 at 4:13 PM

I think I may have figured it out. According to my research, I believe I may have erroneously used a 10 ohm resistor, instead of a 10K ohm resistor. Which would prevent the capacitor from properly charging and de-energizing the relay coil.

Posted By: oldspark
Date Posted: October 30, 2013 at 4:40 PM

Try it without the re4sistor. Usually the cap will discharge thru the relay coil (which is only about 50 Ohms upwards).

Posted By: davep.
Date Posted: October 31, 2013 at 11:08 AM

lexelite wrote:

However, the relay is not deenergizing and releasing the arm back onto 87A as designed. The circuit stays on as long as power is applied to 86/87.

It IS working the way it's designed to. The capacitor doesn't begin to discharge until power is removed from terminal 86.

Posted By: davep.
Date Posted: October 31, 2013 at 11:15 AM

Never mind. My mistake. I NOW see this is supposed to be a constant to momentary pulse, not a momentary to constant.

The capacitor isn't charging enough to deenergize the coil. A 10 ohm rather than a 10K Ohm resistor would cause this.

Sorry for the confusion.

Posted By: lexelite
Date Posted: October 31, 2013 at 12:24 PM

davep. wrote:

Never mind. My mistake. I NOW see this is supposed to be a constant to momentary pulse, not a momentary to constant.

The capacitor isn't charging enough to deenergize the coil. A 10 ohm rather than a 10K Ohm resistor would cause this.

Sorry for the confusion.

No worries - no harm, no foul. Thank you for trying to help. I am fairly confident in my understanding and skill with relay. Simple error using a 10 ohm resistor in place of 10K ohm resistor. That is how you know you are tired :)