Relay Basics
by Chet Walters

RELAY BASICS - 101
Some may not know what a relay is or how it works. Maybe a little parable. There once was a little man named Jon whose job it was to lift a weight of 50 lbs. He could do this all day long with no trouble. But, one day the boss came up and said that Jon now had to lift 200 lbs. But Jon knew he could not lift 200 lbs without eventually breaking his poor little back. So, he got his friend, Hercules to lift the 200 lbs each time that Jon tapped him on the shoulder. So Jon was able to lift the extra weight through Hercules' strength and Jon did not have to lift any weight at all.

Let's say you want to add some extra horns to your truck. Your horn switch (Jon) was designed little but it only had to supply current (50 lbs) to your dinky little stock horn and can do that easily. However, your new horns are bigger and require more current (200 lbs). If you simply hook up the new horns to your existing wiring and switch, then your switch will burn out rather quickly. How to get around this? Install a relay (Hercules). A relay is a mechanical or solid state SWITCH which is triggered (tapped on the shoulder) by current supplied to its trigger terminal (86). When current hits that terminal it closes the switch inside the relay, be it mechanical (by energizing a coil magnet) or solid state (by tripping a transistor). The other terminals of the relay then feed current from the INPUT terminal (30/51) through the now closed contacts to the OUTPUT terminal (87). Your old horn switch does not feed current to your new horns, it only triggers the switch inside the relay. The switch inside the relay feeds current to your horns from a completely different source (ie battery).  The wires to the relay triggers can be of very slight gauge (22-24) while the wire that actually work the accessory (in this case horns), can and should be of heavier gauge (18-10).

To wire up your new horns so that your little switch and slight gauge wires can work themsafely, wire them as illustrated. Your old horn wire that went "hot" when you pushed the horn button is hooked to the TRIGGER terminal (86). When you hit the horn button, the button only needs to supply a small amount of current to trigger your relay. Hook up heavier wire thru a fuse directly from the battery to the INPUT (IN) terminal (30/51) and then hook your new horns "hot" to the OUTPUT (OUT) terminal (87). Terminal 85 is common ground. Now, instead of asking your weak and EXPENSIVE horn switch to work the increased load, your strong, CHEAP and easily replaceable relay does all the heavy lifting. 

Relays will also fail after a time. Some are even rated by the number of times they will close a circuit before burning out (common relays are usually rated to 100,000 operations). The advantage is that a relay is normally cheaper and easier to replace than a switch. 

A common Bosch style auto relay is shown above along with the functions of the terminals.
One should note that many relay packages are incorrectly labeled. The package that this relay came from labeled terminal 87a as "power to lamps" just like 87 when the 87a terminal is actually "normally on" and goes cold when the relay is triggered. One should test these terminals to determine what each actually does before making a permanent installation. Usually, if the center terminal is labeled ON THE RELAY as "87a" then it is usually a normally on terminal and will go hot (complete the circuit with terminal 30) when the trigger is cold. If it is also ON THE RELAY labeled as terminal "87" then it will usually be a normally off just like the outer 87 terminal. These 87 & 87a terminals can be used creatively as input to feed current from two sources to the 30 used as output.  One should always mount a relay of this type with the terminals down and it helps to seal the seams around the case and terminals with silicone seal. These relays are sensitive to moisture and they will corrode if directly exposed.

 
Another type of auto relay (Conduct-tite #84607) is shown here.
One should test these terminals to determine what each actually does before making a permanent installation. Terminals on this three post relay are labled here with the functions of the terminals matched to the more common 4&5 post relay above. These terminals are not interchageable and the case provides the ground. This relay appears very similar to a typical auto horn relay, but most auto horn relay completes the ground circuit and is unusable in other applications. One should always mount a relay of this type with the terminals down and it helps to seal the seams around the case and terminals with silicone seal. These relays are sensitive to moisture and they will corrode if directly exposed.