proper type of relay for this

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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=112591
Printed Date: July 22, 2025 at 8:15 AM

Topic: proper type of relay for this

Posted By: rfhelp
Subject: proper type of relay for this
Date Posted: March 23, 2009 at 12:52 PM

I am using an rfid module to trip a standard automotive relay. I have been told that this can damage the rfid board over time so I need to use a low power relay. I was given a radio shack relay number 275-249 which is a 5a mini pcb relay dpdt. I am told I need this to trip the larger automotive relay.

DPDT 5A at 240VAC/24VDC

Nominal coil rating of 12VDC, maximum 15.6VDC
Pickup/dropout voltage 9.6/0.6VDC

Nominal coil current 60mA

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Being in Canada I cannot find this relay locally and was wondering if there is "NEWARK" relay that is the same. I have called three diff electronics shops locally and they have been no help.

Replies:

Posted By: rfhelp
Date Posted: March 23, 2009 at 12:56 PM

sorry

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Posted By: dualsport
Date Posted: March 24, 2009 at 1:00 AM

You'd be much better off using a transistor driver instead of trying to drive the relay directly from your circuit. It'll draw much less current from your rfid, and protect it from the voltage spikes that the relay will put out when it turns off.
You'll be able to use any standard relay instead of needing to find a specialty low power relay.

Posted By: i am an idiot
Date Posted: March 24, 2009 at 2:29 AM

Does the RFID module supply a switched ground or positive output? If ground out, you can use the following diagram. The transistor is available from RadioShack for $1.59

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If the module provides a positive output, purchase One of these instead. and let me know. I will have to get you a new picture.

Posted By: KPierson
Date Posted: March 24, 2009 at 5:38 AM

The size of the relay coil won't dramatically effect the destructive coil breakdown voltage. Instead of trying to find a "different" relay simply put a diode across the coil to supress the spike.

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Kevin Pierson