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HOW TO WIRE A RELAY
I see way to often people adding new high current circuits to there rigs with no relays to operate the device.
This will lead to problems up the road and a unreliable electrical system.
Most aftermarket switches (rocker, toggle micro,etc) can usually only handle a max of a 10 amp load, after
that you are causing damage to the switch. The idea behind having a relay installed is to take the heavy
current away from the switch. This will allow much smaller gauge wiring to be done under the dash and keeps
the heavy gauge high current wires out from under the dash where they do not belong.
An example....
A pair of small fog lights usually draw from 8 to 10 amps. If the lights were only run through a switch,
the switch would burn out in a short period of time. By using a relay, you end up moving the heavy current
through the relay and not through the switch. A normal 5 terminal relay will only draw about 0.2 to 0.3 of
an amp to turn on. so that's only a max of .3 amps that is going through the switch. The switch will last a life time.
You can get relays that can handle a 30 - 80 amp load, very cheaply.
Most general purpose relays have a set of normally open contacts and a set of normally closed contacts
and are wired internally as shown in the picture posted below. The two terminals that turn the relay on
and off are (85 and 86), The terminal for your main power in is (30), and the terminal for main power
out for a normally closed circuit is (87) . If you want a normally open circuit you would use (87A)
instead of 87. Some relays reverse the 87 and 87A markings.
When wiring your electrical system. The correct way to wire things like fuel pumps, cooling fans, fog
lights, nitrous controls, etc. Is to use a relay to control the circuit. This will save you a lot of grief up the road !
Also this is something easy to remember how to determine how much current (amps) there will be in a circuit
Current = Watt’s / Voltage
Watt’s = Voltage x Current
Dome light
5 watts = 12 volts x current
current = 5 watts / 12 volts
= 0.416 amps
Headlights
55 Watts (per light) = 110 Watts
110 Watts = 12 volts x current
Current = 110 Watts / 12 volts
= 9.167 amps
Most vehicles when running are about 13.8 volts so to change it a bit
2 lights 100W so total watts = 200 Watts
Voltage = 13.8 Volts
Current equals = 200 watts / 13.8 Volts
Therefore total current = 14.49 Amps
Now that is not taking into account voltage drop over the distance of the line but it will tell you what size fuse you will need for the circuit and then you can use this chart to find out what size wire to use.
Remember to put the fuse as close to the power source as pratical,, protecting the wire from damage.
Another Version.
This way lights are switched on/off by key
By putting switch in ground wire, you can locate the relay by the 12v source.
keeping the live wire length to as short a length possible.
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