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Discussion Starter · #1 ·
I was looking for information on 12V relays and found this very informative post on the net, and want to share it with you.

There seem to be many members who don't know what a relay is, or don't know how to wire one in. this guide is here to help solve that issue...

what is a relay?

-a relay is a device that allows you to control a high-current electrical load with a low-current electrical 'signal'. they are usually electro-magnetic, but are also available in solid-state forms. they can be used with a switch (to allow control of a high-current load with a small switch) or they can be hooked up to a switched power source in the car like the ignition or accessory power circuits (to allow power to be switched on/off automatically with the ignition key).

Why do I need a relay?

-when hooking anything up to a car's factory wiring, it's important to remember that factory wires are designed to carry the load of only the factory installed components. they are not 'general use' power circuits like the power outlets in your house. for example, the ignition (IGN) circuit is designed to power the car's ignition system and nothing else. hooking up a high-current device to this circuit can create a fire hazard. by using a relay, you can use the IGN circuit to control a high-current device without directly powering it from the IGN circuit itself.

Is a relay hard to hook up?

-no... most relays require only 4 wires.

Where can I buy a relay, and how much do they cost?

-you can buy standard automotive relays at radio shack, autozone, walmart, or at almost any electronics store or automotive store. a typical SPST relay will cost about $5(US).

I bought a relay, but I don't know how to hook it up. all I see are a few weird numbers and strange symbols, what do they mean?

-a standard bosch-style relay will have 4 or 5 numbered leads (30, 85, 86, 87, and sometimes 87a). why they picked those numbers, I have no clue; but I can tell what they hook up to.

-30 = constant [positive (+)] power (usually wired directly to car battery)

-85 = coil ground (wired to the negative (-) battery terminal or any grounded metal panel in the car)

-86 = coil power (wired to the control source. could be a switch, or it could be the car's IGN or ACC circuit.)

-87 = switched [positive (+)] power output. (when the relay coil is powered, lead/pin 87 is connected to lead/pin 30)

-87a = [on 5 lead/pin relays only] this lead/pin is connected to lead/pin 30 when the coil is NOT powered.

here is the bottom view of a standard 4-lead Bosch-style automotive SPST relay

here is what happens inside the 4-lead/pin SPST relay. on the left, the coil is NOT powered. on the right, the coil IS powered. notice the switch changes positions when the coil is powered. when the coil is powered, pins 30 and 87 are connected. when the coil is NOT powered, then pin 30 is not connected to anything, therefore it is in the 'off' position.

here is the bottom view of a standard 5-lead Bosch-style automotive SPDT relay. you'll notice that the only difference is the addition of the 87a lead/pin.

here is what happens inside the 5-lead/pin SPDT relay. you'll notice that instead of an 'off' position, there is an electrical contact (87a). on the left the coil is NOT powered, and pins 30 and 87a are connected. on the right the coil IS powered, and pins 30 and 87 are connected.

here is a typical setup to control power to a carPC via the car's ACC/IGN circuits. this is generally use for laptop installs or 'no-write' setups (not good for normal carPC because abruptly cutting power without proper shutdown can cause data corruption);

I want to use the relay to turn on/off with the car. how do I hook that up?

-connect lead/pin 86 to the car's IGN, or ACC circuit. these circuits can be tapped into in the wiring harness that goes to your car's key switch. the ACC circuit can also be tapped into at the fuse box or in the stock radio harness.

What is the difference between the ignition (IGN) and the accessory (ACC) circuit?

-they are both powered when they key is in the 'RUN' position, and they are both not-powered when they key is in the 'OFF' position.

-the 'ACC' circuit is powered when the key is in the 'ACC' position, but is not powered when the key is in the 'START' position. when the key is turned, power to the relay will turn on as the key passes the 'ACC' and 'RUN' positions, then turn off in the 'START' position, then turn back on as the key is released (springs back to the 'RUN' position. this is not an issue if you're using a manual on/off switch, or have a delayed on startup controller.

-the IGN circuit is powered when the key is in the 'START' position, but is not powered when the key is in the 'ACC' position. this circuit avoids the issue stated above, but requires that the key is left in the 'RU' position if you want to use the carputer with the engine off. on some cars, this can burn out the ignition coil.

I bought a 5-pin relay, can I still use it as a simple on/off switch instead of a changeover switch?

-yes. simply leave pin 87a disconnected.

My relay has more than 5-leads/pins?

if your relay has more than 5-leads/pins then is is most likely a DPDT, MPDT, or MPMP relay (M=multiple). it will work the same way, it simply has two or more separate switched inputs/outputs inside it. regardless of the number of switched contacts, it will still use a single coil, so it will still be controlled by a single power source.

Are there others types of relays other than the standard 'bosch style automotive relay'?

yes, just like regular switches, there are several different types, sizes, and styles of relays. some have multiple switched contacts, some are circuit-board mountable, some have high-current capacity, some have low-current capacity, some use higher or lower voltage coil power (generally you would only use a 12v coil in a car).

What is a 'valet switch'?

-the 'valet switch' is a standard SPST switch wired between the coil power source and pin 86. it allows you to leave the relay turned off, so nobody (including a 'valet' driver) can turn your carPC on. how effective this will be depends on how well you hide the switch (it should be accessible, but hidden from plain sight).

does a relay take the place of a fuse?

-no! a relay provides no protection from overload or short-circuits.

do I need to use a fuse if I use a relay?

yes! you must still fuse your power wires!


295 Posts
Nice! I always find the symbols rather cryptic.

I installed a fusebox kit from Eastern Beaver, it included a relay and all wiring, which makes it easy. The 'key on' power signal for the relay I got from the rear tailight wire. Relay (small black box) fit nicely under the RH sidepanel.

The fusebox I placed in my round headlight bucket, where it has been very handy for adding electrical stuff. Power outlet for GPS, battery monitor, Vapor speedometer, heated grips, amber running light. Two outlets are power on all the time and six, controlled by the relay, are key on/off.


2,171 Posts
Petrus, GREAT post! This information will help me alot I am sure. Have used relays many times but only when they were provided in kits. Your presentation gave me the knowledge to now experiment on my own. Thanks...

PJungnitsch, nice wiring setup, your post and pictures are much appreciated as well.

Contributions like these take time, but in my opinion, make for a great forum. Gerry
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