spdt substitute for spst?
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=124310
Printed Date: May 09, 2025 at 11:09 PM
Topic: spdt substitute for spst?
Posted By: relaytrouble
Subject: spdt substitute for spst?
Date Posted: November 06, 2010 at 4:33 PM
Hi, I'm not sure I've got this straight. The app is aftermarket motorcycle horns, wired with an SPST relay that seemed to go bad. So I accidentally ordered an SPDT relay. If I understand properly, I can use the SPDT and just have nothing attached to 87a and it will work like the SPST relay?
Please advise. Thank you.
Replies:
Posted By: Ween
Date Posted: November 06, 2010 at 6:03 PM
you are correct
Posted By: relaytrouble
Date Posted: November 06, 2010 at 7:43 PM
Thank you for the reply. Now I feel okay using the SPDT relay.
Posted By: oldspark
Date Posted: November 07, 2010 at 1:27 AM
Since SPST & SPDT relays are often the same price, I tend to get SPDT and not use 87a.
Hence if a circuit with SPDT fails due to 87a failure, I can just swap relays. (Not that I recall that happening for eons.... if ever?)
That does assume the same footprint however - thee are SPSTs with TWO 87 outputs - the 2nd being the 87a on the SPDT. That can be a bit sparky if 87 is +12V & 87a is ground. (As well as improper functioning...)
In summary, what you did by accident is what clever people do. (Unless there is significant price difference that outweighs the "one part for both" advantage.)
Posted By: howie ll
Date Posted: November 07, 2010 at 2:51 AM
Yes just ignore 87a.
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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: howie ll
Date Posted: November 07, 2010 at 2:53 AM
Sorry, I also agree, relays (decent brands) are rated 15 million + cycles, make sure your replacement is mounted vertically, terminals down, away from any dirt/heat/moisture, even in a plastic bag! If not spray over with a water retarder.
-------------
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.
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