What is a relay ?

A relay is like a switch, with a set of contacts. But instead of you using your finger to change the position of the switch, you apply power to a coil. They can be AC or DC at the switch (coil) and have a single throw, double throw, triple, quad, etc.. Which means a DPDT, Double Pole (meaning two sets of switches, Normally Open, Normally Closed, and a Common) Double throw, meaning both at the same time. They each (each side of the relay) has a Common, Normally open, and a Normally closed set of contacts. The Coil (input voltage) when supplied, or taken away changes the state of the switch. An example would be, say I had a 12vdc relay (coil rating), and the contacts where rated at 10amps (two sets, DP). By supplying the 12vdc to the coil, you could switch 2 lights at the same time, as long as you don't exceed the 10 amps per contact (2). Relays come in all kinds of ratings; from the coil voltage, and type, to the ratings of the contacts, usually by amps, or mille-amps.

Jack

There are two answers: just for your information, a relay is one or more switch contacts that are operated by an electromagnetic coil. There are also solid state equivalents that don't use moving contacts and coils. But in both cases the 'contact' outputs are controlled by an electrical signal on another input. The input and outputs are isolated, allowing them to operate on different voltages and current levels.

The practical answer for your situation is: A relay is that thing that a qualified electrician will be installing per the requirements of the applicable electrical codes. If you don't know what a relay is, you probably shouldn't be installing one in your home wiring. By the way, relays that are capable of controlling significant loads like A/C units are called contactors. They work on the same principle, but are rated for heavy line voltage loads.

On Wed, 19 Nov 2003 6:46:06 -0800, benbadger wrote (in message ):

A relay, in your particular situation, is an automatic switch which is powered by the switch that turns on your AC. The relay would then turn on power to the fan.

In other words, you turn on one switch (the AC). This act turns on both the AC and the relay, which would turn on the fan, as well.

Wiring is pretty straightforward, and if done at the low-voltage level at the thermostat (35 vac, here in the USA, I think), would keep your (inexperienced) hands out of the really dangerous stuff (ie, 120 and 240 vac).

Get a friend who knows about such things, or a professional electrician to help you if you're not qualified.

Good luck,

Many thanks to all. I'll give what you say careful thought.

Thanks again.

Ben