Question to all you wiring experts.

[pigeon]

i was wondering, and i know many people here should know this...

what is the purpose of a relay?

why doesent current go straight from the switch, to the load...

an example would be a car horn
you push the horn, the relay clicks, then the horn goes BEEEEEP

what is the downside of making it go straight from the switch to the actual horn?

[websurfer970]

well a horn circuit isnt as simple as that, if there were no relay, you would have to run larger gauge wire up the steering column and back down, through the firewall and to the horn, using a relay eliminates the need for that much wiring.

[blue LEDz]

well a horn circuit isnt as simple as that, if there were no relay, you would have to run larger gauge wire up the steering column and back down, through the firewall and to the horn, using a relay eliminates the need for that much wiring.

yep.

if you didn't use any relays in your car, you'd have like 60 amps worth of juice flowing through your dash to all the switches. that's a fire hazard waiting to happen. the relays under the hood do just what websurfer970 stated. smaller wire means less cost, and MUCH lower current through the interior of the vehicle. it takes such a tiny amount of juice to flip a relay, and that relay can switch on 30 amps for a standard automotive relay.

[goldstar]

A relay is an electro-mechanical switch that is designed to pass heavy currents through contacts that can be energized (made to open or close) by the use of an electromagnetic coil which can be operated by a very small current. Relay contacts can be normally open (NO) or normally closed (NC) and energizing the coil shifts the contacts from one state to the other.

Here's an example of how and why a relay would be used in a headlight circuit. Assume we're powering the high beams of two H4 bulbs. At 12 Volts, the total current draw is 10 Amps. Ideally, to minimize voltage drop, two short, heavy gauge wires should be run from the batt + term direct to the bulbs. In order to control the bulbs, a high current relay is placed somewhere in the line between batt and headlights.

The headlight switch is connected to the coil terminal of the relay. Since the coil draws very little current, the headlight switch can be a low current device and the wire connecting the switch to the relay can be relatively small gauge. The switch itself can be connected to any convenient voltage source behind the dashboard. Turning on the switch sends current to the coil which causes the relay contacts to close and turns on the headlights. The coil circuit and the contact circuit are completely electrically separate. In actual practice, two relays would be used, one for the high beams and one for the low beams. Headlight circuits generally have two switches, an on-off and a hi-lo beam but the principal is the same as far as the relays are concerned.

Without the use of relays, a heavy gauge wire or wires would have to run from the batt back to the instrument panel and be connected to a heavy duty headlight switch capable of carrying very heavy current. From the switch, the heavy gauge wires would then have to run forward to the headlights. This arrangement would require a long wire run leading to considerable voltage drop and a reduction in light output compared with the set-up using relays.

For the same reasons, other heavy current draw devices which have to be operated from the cockpit (such as horns) use relays.

[pigeon]

very very well explained
thank you

now, another electrical question, if you have a moment to answer it...

many people have tried to explain it to me, but i cant seem to 100% grasp the difference between voltage and amperage...

i think i might have it...

is it comparable to psi and cfm
psi being voltage?
cfm being amperage?

[goldstar]

Usually a water analogy is used to explain electricity. Consider water flowing through a pipe. Voltage is analogous to the PRESSURE of the water and current (amperage) is similar to the AMOUNT of water flowing through the pipe in a given unit of time. Resistance would be represented by the diameter of the pipe: for the same pressure, a smaller diameter pipe would limit the flow and a larger diameter pipe would permit a greater flow in the same time frame.

Since 1 ampere is defined as 1 coulomb of charge per second, you are roughly correct using your air analogy. Voltage would be similar to air pressure in PSI and since CFM represents a specific quantity (or weight) of air moving in a specified time period, it is similar to amperage. Again, duct size would be related to resistance.

Interesting idea using an air analogy to explain current flow, I wouldn't have thought of it myself.

02 DX Millenium Red

[websurfer970]

yes yes, very good, text book answer