how to use relays/fuses

[Travisfab]

Rookie question here:
I am wiring up an electric fan today and had a few questions. What exactly does a relay do? How do I know how large of one I need. Also with fuses, how do you know how big of a fuse you need, say if you were wiring 3-100 watt lights or a race radio. Is there a way to measure the amps they pull to get an idea of the fuse you would need?
Thanks

[MH20]

Watts = Volts*Amps, so Amps = Watts/Volts

So for 3 100W lights it would be [3*(100)]/12 = 25Amps

And a relay is basically just an electrically activated switch. It makes it so you dont have to run high current through your switches, etc.

[KitRacer]

Not to disagree, but your average 12V battery is more than 12 volts. I think its more like 12.5 or more. It doesnt matter as much for this calculation, but can through things off when calculating other ways.

[SpareChangeRacng]

If you put a voltometer on the battery when the car is running you'll be getting about 14.2 or so volts (which I hear is normal) - should this effect the equasion?

[tkr]

You should set up the circuit for the lowest possible voltage supply/highest possible amp draw. Many alternators under load at idle aren't putting out much voltage...if any. On top of that, there is a momentary spike in amp draw when the load is initially powered. In my opinion its safe to use 12v as a guideline and err on the high side of the amp draw.

[drtdevil93]

relays are useful as you can control a high current device without running high current somewhere unfavorable. for example: my removable dash with switches for my hella lights. i dont want to run a high current line inside and in the dash, so i run it directly to the light, and interrupt it with a relay. i then have a small 18 ga. wire from the switch to the relay. also nice that you dont have to run 10 14ga. wires into the cab.

erik

[michael]

This is mainly a car audio site, but it has lots of basic DC info:

Basic DC (http://www.bcae1.com/)