Relay Education

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=146269
Printed Date: May 03, 2025 at 12:06 AM

Topic: Relay Education

Posted By: rcuestas28
Subject: Relay Education
Date Posted: June 11, 2020 at 3:53 PM

What is the purpose of pin 87A on a relay and when would you use it?

Replies:

Posted By: the12volt
Date Posted: June 11, 2020 at 3:59 PM

There are many applications that utilize terminal 87a of an SPDT relay. Start here: https://www.the12volt.com/relays/spdt-and-spst-automotive-relays.asp. Then follow the links under "Relays and Relay Wiring Diagrams" near the bottom.

-------------
posted_image

the12volt • Support the12volt.com

Posted By: rcuestas28
Date Posted: June 16, 2020 at 3:23 AM

Okay. I'm understanding that a 5 pin relay will always have either 87a or 87 energized depending on if the switch is on or off. Whicu makes sense for high/low beams.

In the scenario of connecting a 5 pin like a 4 pin: (A relay with pre-installed wires in it)

30 - car battery with in-line fuse
86 - ground
85 - 12v switch box
87 - accessory light
87a - (not connected and not exposed)

Does that mean that the relay will still draw current when the vehicle and the switch is off?
Or will this function just like a 4 pin since 87A is not connected to anything?

Posted By: i am an idiot
Date Posted: June 16, 2020 at 4:03 AM

Zero current draw in this application.