momentary relay

Printed From: the12volt.com
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=99833
Printed Date: June 23, 2025 at 6:46 AM

Topic: momentary relay

Posted By: firetrader
Subject: momentary relay
Date Posted: December 09, 2007 at 10:49 PM

Hi All, 1st post,

I am restoring an old 1964 fire truck that has a brass bell with a 12 volt Heavy duty acutator,
Looking at the relay pict's for momentary switching the capacitor size is not listed.

I will be running a 60Amp relay, the orignal capacitor from the truck was a 4uF 400VDC 640W paper capacitor, what should I use as these are no longer available.

Thanks

Andy

-------------
Andy
Fire Trader Australia

Replies:

Posted By: i am an idiot
Date Posted: December 10, 2007 at 7:33 AM

The capacitor allows the coil of the relay to be energized until the capacitor stores a charge, thus de-energizing the coil. The resistor bleeds off the charge of the capacitor when positive voltage is removed from the other side of the coil. You can increase the output time by simply changing the value of the capacitor. This one will give you about a 1/2 second output.
posted_image

There is a 1000 MicroFarad capacitor in this diagram. As stated above this one will give you about a half of a second. You can change the time by changing the value of the Capacitor. 470 Mic cap should give you 1/4 second. A 2200 Mic cap should give a little more than a second.

Posted By: dualsport
Date Posted: December 11, 2007 at 8:27 PM

How does that 60A relay in your application operate? Is it supposed to be a momentary output controlled with a switch?

The 4uF, 400V (not sure what 640W means in relation to a cap though), may be connected across the contacts to prevent arcing and burning of the relay contacts from the big bell, but that doesn't do anything for timing. Are you sure you want a momentary relay?