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Yeh, what he said. :D Think of it as a switch that's activated by energy.
 

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A little more info: A relay is usually used to power up devices that require a lot of current. A relay is actually a switch that is closed by an electromagnet. The switch that operates the relay sends current to an electromagnet in the relay and closes the circuit. A relay can be placed closer to the device it is going to energize (ie the engine compartment), making the wiring run from the power source to the device more direct. The switch to operate the relay can be placed farther away, like in the passenger compartment; that way the wire to power up the device doesn't have to run from the power source to the switch and back to the device. Another advantage of using a relay is that the switch that operates the relay will last longer because it is only carrying the small amount of current required to open and close the relay.
 

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A solinoid on a starter works very similar, the principle is roughly the same, however the solinoid (relay) is only energised and allowing the high current through when the key is turned to the start position. Only small gauge wires come to the starter switch in the car, as opposed to the large cable going to your starter solinoid.
 

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Discussion Starter · #6 ·
got it. now how do you know when you should use a relay ie, radiator cooling fans? also how do i know what size? to buy? i know this is really basic stuff for most of you, can you suggest a site or book where i can look up what i need?

thanks john
 

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Any standard auto relay should do,most are 30amp,bosch is a typical relay found in autozone and such places.

If you just buying one or two for a basic upgrade just go to the parts store,but if your going all out and getting 5 or more i'd try either online or an electrical supply company,alot cheaper.
 

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Relays:
A relay is nothing more than a heavy duty “remote control” toggle switch. By activating a relay, you energize an electromagnetic coil that either engages or releases a set of heavy-duty contacts. You use a small device to turn a HUGE device on or off.
Example: The starting circuit. A starter solenoid is just a relay on steroids. The key switch in the dashboard or column couldn’t possibly handle the current needed to crank over your 11.5:1 BB! The switch in the column merely makes/breaks the circuit to a BIGGER heavy-duty switch that energizes the starter motor.

Electrical Basics
 

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Its basically an electrically operated switch.

Chris.
 

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Originally posted by john6066:
thanks guys. a lot to digest. sounds like relays are the way to go. i installed a cooling fan and did not use a relay. opps i'll get on it.

thanks again
When you change your electric fan to run with relays, you should notice an increase in fan speed due to less voltage drop.
Keep the high current wiring as short as possible and use a heavy guage wiring like 8 or 10 gauge, depending on what your fan draws to keep voltage drop to a minimum and fan speed high.
Some of the better fans require more then a 30 amp relay. The high speed of a taurus fan or a Mark VIII fan require a bigger relay then 30 amps.
Also a free wheeling diode wired accross the fan's electrical terminals, but between the fan and relay will help your relays live a longer life.
Some HD relays for motor circuits allready have the free wheeling diodes installed in them at the factory e.g. Bosch 75 amp motor relay.
 
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