When to use a relay??

[bski224]

Doing a whole wiring harness on my 72 Z28 and was wondering if I need to use relays, Not sure what they do?? I do have elec fuel pump, elec fans, elec H20 pump. Whats the main purpose of a relay??

[MADMIKE]

Relays are used to remotely control other powered systems. Lights, fuel pumps, fans, etc. The benefit is the power circuit itself does not have to run to the control unit,(which would also require a higher amperage control unit) and suffer voltage drop from having to go from the power source to the control unit and then out to the device(light, fan, horn).

Example:
Older US cars tend to have the battery up front, lights up front and the light switch near the driver. So for the lights to operate;
The light switch would close the circuit.
This would allow current to flow from the battery, to the light switch and then back out to the lights.

Thats fine and dandy and works. But the battery is usually less than a foot away from the lights, yet the power has to travel anywhere from 10-15 feet just to power a light that is less than a foot away. Wasteful.

D/C voltage is susceptible to voltage drop over short distances, where A/C can go up to 100' before having to de-reate(size up) the wire.

A relay can remove the above mentioned examples excess 10-15 feet of wire. Remove excessive overloading of the light switch itself.(ever had an older car with blinking head lights due to a well worn headlamp switch) Reduce wasted power, and weight from having to run long/heavier power wires.

With a relay control circuit, fractional amperage can be used to control a heavy amp load, safely and efficiently. Thus you can have the power wire going directly to the lights, switched by a relay, which itself uses minimal power. Allowing more power to feed the lights, which in turn ,even in stock form, will be noticeably brighter. This also helps remove excess charging/discharging of the battery which can be wasted HP as the engine has to keep the Generator/Alternator charging the battery.

[marc_b]

you want to use relays for all those items. A relay is an electric opperated switch. It allows a small amount of current to throw the switch (relay) on and allow a large amount of current flow through the switch (relay). You don't want large amounts of current flowing through wires under your dash or switchs to opperate those items. Also make sure to use fuses along with the relay.

[smhigh]

The BEST information and kits for electrical components are at MAD Electrical. I've talked at length with Mark, the owner, and his knowledge is invaluable. Here is a link to his site concerning relays.

http://www.madelectrical.com/catalog/rly-1.shtml

[bigblockcamino]

Go to the yards and get a bunch, they are all over. I snagged like 4 or 5 of them for my fans/fuel pump/etc

[TnBlkC230WZ]

If you have a small switch that needs to turn on a big load or you have multiple sources turning on a load, use a relay.

Also, if you have a ground source that needs to be a positive source, you can use a relay.