Clarification on 87a Terminal of 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=131333
Printed Date: May 02, 2024 at 4:12 PM

Topic: Clarification on 87a Terminal of Relay

Posted By: gordyg
Subject: Clarification on 87a Terminal of Relay
Date Posted: May 02, 2012 at 11:44 AM

I am installing a set of air horns on a truck and I would like to confirm a connection on the relay please. This air horn set comes with two compressors driving one trumpet each.

The instructions are not very detailed, but the box does show a diagram which instructs to connect the relay as I have drawn in the image below.
My concern is: will both compressors work simultaneously with one compressor connected to 87 and the other connected to 87A?
I have never used the 87a connection before and my understanding is that 87 or 87a alternate (I could be wrong). If the way they have instructed correct, I would like to know for my understanding what the advantage is of using 87a connection opposed to connecting both compressors to 87 terminal.

Could someone please clarify this for me?

posted_image

Replies:

Posted By: awdeclipse
Date Posted: May 02, 2012 at 12:13 PM

That diagram would have compressor on 87a running with the relay open (no closing of the switch on terminal 86) and the compressor on 87 running when closing the switch (turning off 87a compressor) Like you suggest, both compressors should be connected to terminal 87.

87a is the NC or normally closed terminal. It is pretty well defined in the relay section of the site.

Relay Section

Also, be sure you fuse according to wire size used.

Posted By: gordyg
Date Posted: May 02, 2012 at 12:31 PM

Thanks for your clarification.

Posted By: i am an idiot
Date Posted: May 02, 2012 at 7:13 PM

https://www.bcae1.com/relays.htm

Posted By: oldspark
Date Posted: May 02, 2012 at 11:20 PM

Some "5-pin" relays are dual-output types with two 87 terminals.
(Warning - some relays like Hansa have them incorrectly labelled as 87 & 87a.)

Posted By: howie ll
Date Posted: May 03, 2012 at 5:46 AM

With two connected terminals they'd both be 87, with isolated, 87b.

-------------
Amateurs assume, don't test and have problems; pros test first. I am not a free install service.
Read the installation manual, do a search here or online for your vehicle wiring before posting.

Posted By: oldspark
Date Posted: May 03, 2012 at 6:09 AM

If that's in response to my comment, I agree, but I'm just warning...

Not that it bothers me - I never paid attention to the numbering - just their housing diagram.

Posted By: howie ll
Date Posted: May 03, 2012 at 6:10 AM

I test them naah.

-------------
Amateurs assume, don't test and have problems; pros test first. I am not a free install service.
Read the installation manual, do a search here or online for your vehicle wiring before posting.

Posted By: gordyg
Date Posted: May 03, 2012 at 11:34 AM

Just to add, I tested terminals 30 and 87a with an ohmeter and it is a closed connection, 87 and 30 is open. Below is an image of the drawing on the relay itself.
So if I were to connect both compressors to terminal 87 I have to make sure I am not overloading the relay.
I dont know the current draw of these compressors, trying to find this info from the ebay seller.
posted_image

Posted By: howie ll
Date Posted: May 03, 2012 at 12:07 PM

About 15 amps each your near the limit for a single relay.
A single 4 terminal relay rated at 40amps would be a better bet.
Unless of course this is the relay they gave you.

-------------
Amateurs assume, don't test and have problems; pros test first. I am not a free install service.
Read the installation manual, do a search here or online for your vehicle wiring before posting.

Posted By: gordyg
Date Posted: May 03, 2012 at 12:41 PM

Agreed, thanks for your help.