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· Premium Member
1,746 Posts
Discussion Starter · #1 · (Edited)
Automotive Electrical Basics #2 - Relays

A few years (!) ago I posted this:

With some of the current posts, I thought it was about time for another installment.

Basic of Basics – Relays

What is a Relay??
In simple terms a relay is an electromagnetic switch that allows a small capacity circuit to switch a MUCH larger circuit on and off.

Why they are used:

In automotive applications mostly for cosmetics and costs. The designers would much rather have a nice looking, dinky switch on the dash/console than a massive clunky thing on the panel, and not having to use large gage wire for long runs ($$$). The cost of a relay and socket are MUCH less than the cost of large gage wire, and are safer to boot!

The most common relay in use today are the little Bosch “ice-cubes”. Everybody uses them, and for good reason. They have a 30A capacity, are small, and use standard readily available crimp on connectors. Here’s an explanation of what the numbers mean:

#30 = Relay Common ("C" or Com)
#87 = Normally Open (NC)
#85 = Relay Coil
#86 = Relay Coil
#87a = Normally Closed (NO)

(2nd #87 = Normally Open (some relay models have two switched contacts)).

#30 – Common ("C" or Com)
This is the un-switched lug, or “supply”. This is where the fused/breakered heavy gage positive feed wire from the battery would connect to.

#87 – Normally Open (N.O.)
This is the switched contact. This is where your load connects to (if you want to turn it “ON” when the relay trips). This terminal will supply the positive current/voltage to the load.

#87a – Normally Closed (N.C.)
This is the Un-switched contact. This terminal is connected to #30 when the relay is “at rest” or not tripped. This is where your load connects to (if you want to turn it “OFF” when the relay trips). This terminal will break the positive current/voltage to the load.

#85 – Relay Coil
This is ½ of the electromagnet coil that trips the relay. This terminal is connected to ground.

#86 – Relay Coil
This is the other ½ of the electromagnetic coil. This is where your “dinky little toggle switch” sends positive 12 volts to.

Relay Rules:
#1 – Any REPEAT ANY wire supplying terminal #30 MUST be protected by a fuse or fusible link. Why?? Fire prevention and safety. With no fuse the circuit will cook until something melts – usually the wire, but after the rest of the car is on fire…….

#2 – Any wire connected to #30 AND #87 OR #87a must be of equal or larger size than the wires that are on the load. Why?? A smaller gage wire has less current capacity, and will overheat (melt) when the load exceeds its capacity.

As suggested (thanks BTW) heres a picture of a Bosch relay in a generic circuit, courtesy of Painless Products....

· Premium Member
1,218 Posts
Thanks for the sequel John! Your first post was very helpful to me when I read it this summer. It's good to have guys like you on this site who are willing to share their knowledge and expertise to help the rest of us. A little electrical education goes a long way. :thumbsup:


· Registered
3,604 Posts
This is always a fun topic, good job covering the basics.

I would note that relays are often used these days because the switching is controlled by computers in many cases.

I might suggest a second chapter on relays covering power switched versus ground switched circuits. Also a set of references for further study, for example there are a few web sites with example applications for relays.

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