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The Bosch Relay Unraveled

Contributed by
Jolly Goodfellow
Craig Ueltzen

A Relay is just a remote switch, or a way of operating a heavy load with a vary small load.
Load is the amount of amps running through the circuit. Here's Some Photos of Bosch Relays.

A Sample of Bosch type Relays, most of these are Bosch Brand.
You will note that some have metal cases, some plastic. Some
have mounting tabs, other don't. There is even 6 volts relays.

They come in many brands & are used in quite a few different vehicles. Ford's version is a gray
relay with no mounting ear & is used in the new under-hood wiring modules. GMC & Chevy uses
them too, but they are usually black. There are cheap look-a-likes too, but the numbers on the
terminals give them away. Most after market relays will have a mounting tab for mounting it with
a screw. Always mount them with the terminals facing down, or they will get water inside & fail.
They make the small ones to handle loads of up to 30 amps, & the big one can handle 75 amps. The
small ones can be normally open, (open contacts) or have an open & closed set of contacts.
The bigger relays only have the one set of normally open terminals.

The innards of the relay. When power is applied across the #85 & #86 terminals, current flows through
a coil of small wire. This wire is about 100 feet long, & is usually 28 gauge wire. This builds up a
magnetic field in the bar it's wrapped around, & the steel plate snaps to it. When the power is off,
the spring pulls the plate back away from the magnet bar. The "click" is the plate slamming into the
magnet as it turns on. It doesn't "click" when turned off, because the plate swings away from the
magnet without hitting anything.

Note: When the power is applied, the coil sets up a magnetic field in it's windings. When the power is
removed, the field collapses, & a reverse current of high voltage will "kick back" (voltage spike).
This is called counter EMF, & is how your ignition coil works. If your fingers are across the coil
terminals when the power is removed, you will get shocked!! This has not be a big problem with most
circuits. You can now get relays with a built in resister to prevent the voltage spike.

Click on Relay for Larger Picture

Below is a couple of circuits to help understand how the relay works in real life. Relays are used to
transfer high current. A Lot of vehicles make use of the "ground-to-turn-on" circuit. If one of the
relay coil terminals have battery power all the time, the ground-on circuit is how it's wired. Most
horn relays are wired in the ground-on method. The steering wheel contact touches ground & turns on
the horn. The horn relay is used because the 15-20 amps from the horn would arc & quickly destroy
the contacts in the steering wheel.

You will note that most of the Bosch relays have a diagram on the side of the case, this will show you
how to wire the relay. There will be four or five post on the relay, #30, #85, #86, #87 & #87a.
The path for the heavy load will be from post #30 to Post #87 or #87a. Post #85 & #86 are the relay
operating circuit. If you have a post #87a this is the "Normally Closed" (NC) post & #87 if the
"Normally Open" (NO) post. #30 is the Common Post (Com). #85 should be power to the coil & #86
the ground for the coil. Some relay will have two #87 post instead of a #87a post.

Click on Relay for Larger Picture

the Bosch Relay can be had in 6, 12 & 24 volt versions. The number on the top of the case will tell you
the voltage, amperage, like 12V 20/30A. Also there may be a part number & brand name. on the Bottom
by the blades you will see the numbers for the terminals, refer to the diagram on your relay for
style of Relay. Relay can be used for many operations, like switching on lights, or redirecting
power flow. You can even built a holding circuit with two relays. Most of the projects below use
one of more of these relays, be sure to use the right type of relay called for.

You can buy these relays from most any well stocked parts store or online here ~~~> Keefe Performance.

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