In a nutshell a relay is a electromechanical device which allow you to
switch a much higher amount of current in which the switch cannot.

All mechanical switches have contacts in them. When you thow the switch
the contacts meet together and pass the current. However the size of the
contact (and the type of metal) determines the current carrying
capacity. Exceeding this capacity can cause the contact to heat up and
even melt. In order to be able to use the same switch to activate the
device (say higher wattage head lights) you add a relay.

Well the switch is now use to switch on the small amount of current
needed by the coil in the relay. This is typically a fraction of an amp
in common automotive relays instead of tenths of amps. The current
energize the coil which cause it to behave like a magnet. The
electromagnet closes a set of contacts which can carry significantly
higher current than the switch can.

There are other functions of relays besides this basic function, such as
able to switch multiple tracks of current using one actuating signal.
Some relay can toggle or latch.

- Vince

http://gipsyflies.home.comcast.net
1989 Vanagon GL Camper
1993 Mazda Miata (for sale)
1996 Land Rover Discovery
2005 Mini Cooper S

-----Original Message-----
From: Vanagon Mailing List [mailto:vanagon@gerry.vanagon.com] On Behalf
Of The Shaws
Sent: Wednesday, September 15, 2004 9:47 PM
To: vanagon@GERRY.VANAGON.COM
Subject: What does a relay do?

I hope you experts don't mind me asking this question but I have read
about this several times on the list and would like to understand the
idea.

I have read 2 relay situations:

1. Installing headlights with greater intensity. Use a relay...

2. Install a relay in the fan switch to prevent the switch from burning
out...

So my understanding is that the addition of lights with a greater energy
demand draws more power than the wires can handle... so what does the
relay do?

thanks

peter