30 vs 87
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=130936
Printed Date: May 08, 2025 at 5:24 PM
Topic: 30 vs 87
Posted By: anderson3754
Subject: 30 vs 87
Date Posted: March 17, 2012 at 8:35 PM
Is there a general rule to follow when to supply constant power to #30 to energize #87, versus constant power to #87 to energize #30.
I see diagrams using both diagrams to control headlights, coolant fans, fuel pumps, etc.
Regards
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Playing the ultimate game every day
Replies:
Posted By: howie ll
Date Posted: March 18, 2012 at 2:43 AM
No, technically or by ISO convention, 30 not 87 is power input.
BUT when using a 5 pin as a changeover or off switch e.g. door locks, immobiliser, then because 87a and 30 are connected then 87 becomes power and 30 becomes the output.
By convention, 86 is + coil,
85 is - coil.
This one is more important because some relays have built in diodes to prevent back feed.
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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: KPierson
Date Posted: March 18, 2012 at 8:42 AM
The "problem" with applying power to Pin 30 is that on a standard SPDT relay when you have power on 30 and the relay is not energized you have a hot lead on Pin 87A. Assuming everything is installed properly this isn't a concern as 87A, if not used, should be insulated, but none the less this isn't an issue if you apply power to Pin 87 due to the fact that if the relay is not energized Pin 87 is physically disconnected and therefore you will have no hot leg to worry about.
Either way will work - I have always used Pin 30 as the "input" and the throws (pin 87 and 87A) as outputs but there is no "rule".
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Kevin Pierson
Posted By: anderson3754
Date Posted: March 19, 2012 at 7:54 AM
howie ll wrote:
No, technically or by ISO convention, 30 not 87 is power input.
BUT when using a 5 pin as a changeover or off switch e.g. door locks, immobiliser, then because 87a and 30 are connected then 87 becomes power and 30 becomes the output.
By convention, 86 is + coil,
85 is - coil.
This one is more important because some relays have built in diodes to prevent back feed.
Howie II.. thanks again. I very new at this relay stuff, but I find them very interesting in the various ways to use them. I am aware that some relays have this diode requiring #85 to be the ground - coil, and #86 to be the + coil.
Very nice forum learning a lot here. ------------- Playing the ultimate game every day
Posted By: anderson3754
Date Posted: March 19, 2012 at 8:03 AM
KPierson wrote:
The "problem" with applying power to Pin 30 is that on a standard SPDT relay when you have power on 30 and the relay is not energized you have a hot lead on Pin 87A. Assuming everything is installed properly this isn't a concern as 87A, if not used, should be insulated, but none the less this isn't an issue if you apply power to Pin 87 due to the fact that if the relay is not energized Pin 87 is physically disconnected and therefore you will have no hot leg to worry about.
Either way will work - I have always used Pin 30 as the "input" and the throws (pin 87 and 87A) as outputs but there is no "rule".
Okay great that helps me lot. So depending upon the application 30 and 87 can be used for 12v power in to best serve your need.
In the past i have used #30 for 12v constant power in, fused of course, and always deleted or removed the wire from 87a.
I actually have one more question about 87a but I have try and word it correctly hopefully later this afternoon.
Regards ------------- Playing the ultimate game every day
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