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How to wire a relay

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How to wire a relay

Post by ctchme » Tue May 10, 2011 3:53 pm

Hey guys, I thought i might add my 2 cents worth on the site, so here is a brief write up on why you should be using a relay for different accessories, and how to wire them up

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 jaycar, dick smith, supercheap auto, repco, bursons, etc, and usually only cost $5 - $10

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).

-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, or when you want to switch from using one accessory to another.

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.

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.

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 wire (pin 30)

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Post by tassuperkart » Fri Jun 01, 2012 9:49 am

Good artical m8

Forcd4 wrote:Oh fuk no dude it's you a again, the oracle.

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Post by Forcd4 » Fri Jun 01, 2012 7:03 pm

Can I just add its a good idea to steer clear of these integrated fuse/relay modules they selling now, sure they are convenient but fuses are there to protect the cable so the protection must be at the power source..... Don't run 10 feet of cable from your battery to a fuse, that first ten feet shorts out and you have a fire on your hands. Fuse at or as close as possible to the supply?

I love relays, one of the best electrical components out there.... So many applications, it would be god for pics not just to switch a higher current but hold in contacts, lock out contacts latching relays, oh relays do it for me.
" Friends may come and go but 200lbs is always 200lbs " - Henry Rolland -

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Re: How to wire a relay

Post by Kimmo » Tue Jun 26, 2012 12:45 am

ctchme wrote: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.
One kind is called an optocouple, which uses a LED and a photoreceptor to switch a big transistor or something like that; I've seen em switching 240VAC, which is pretty cool.

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