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Automotive Wiring Tips - Using Relays
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 Things To Consider When Wiring Accessories...When wiring anything in your vehicle that draws heavy current such as high powered
offroad lights there are a few things to consider. Number one, make sure you use
wire that is rated for the amperage that the accessories is going to pull.
It is
always better to have wire that is OVER rated rather than wire that is not rated high
enough. If wire is used that is not rated to handle the current that your accessory
will pull, the result could be overheated wires that could melt the insulation, causing a
short or worse yet it could result in a fire. If you know how much current your
accessory will draw you can determine what gauge wire is appropriate for your application.
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Personally I like to use wire that far exceeds the current draw of my accessory.
It's overkill but in a few applications I've used heavy gauge stranded industrial wire
with water and chemical resistant insulation. That way there is no question as to whether
the wire is rated high enough or not. If this approach is taken, it is very wise to
place a fuse at the battery end as close to the battery as possible. Most wire in a
vehicle, if shorted out, will burn up before the battery overheats and possibly
explodes. If wire that is over-rated for vehicle use is used and a short occurs, a
short will most likely result in damage to the vehicle of some sort unless a fuse is put
in line as close to the battery as possible. With the fuse there, in the case of a
dead short, the fuse will burn out first before any damage could occur.
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 With accessories that pull a lot of power it is always better to get your power
directly from the batteries positive terminal rather than tapping into the existing fuse
block or wiring harness. In most cases the vehicles existing fuse block is not rated
to handle the additional load of high powered accessories such as offroad lights.
If
you are the kind of person that likes to add all kind of goodies to your vehicle it might
be worth installing an additional fuse block that handles non-critical items like offroad
lights, CB radios, power inverters, etc. This additional block can then be powered
by a heavy duty wire capable of carrying the current required of all the accessories on the block. Be sure to fuse the block at the battery. |
 In almost every case where high current is
required the switch use to turn on the power should not handle the load.
That is
better left to a relay. What is a relay? A relay is a device that, through a
magnetic induction coil, turns on the power for you. The switch that is installed in
the cab of your 4x4 actually only powers the relay itself which draws very little
current. In most installations a 30 AMP relay from Radio Shack (Auto Relay
Cat. Number 275-226) will do unless your amperage demands exceed 30 amps.
I like to
used a lighted switch in the cab to let me know if driving lights are on. |
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Offroad Lights or Accessory Wiring Diagram using a 4 -
Pole Relay Relay
Click to Enlarge
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 The method I use for wiring the
lights and other external accessories, for the most part, follows the diagram pictured
above. As in the diagram a wire is run from a 12 volt power source to the switch
in the cab and out to the relay placing a fuse at the source of the power.
(Follow
the relay's wiring schematic when connecting the wires to the relay) One of the
relays terminals goes to ground. Then run a heavy gauge wire from the battery to the
relay placing a 30 Amp fuse in line very close to the battery. Do not connect the
power to the battery until all wiring is done. Then I run a single heavy gauge wire
out to the lights or other accessory. If installing lights, split it into two leads at the
lights. If you do this be sure the wire is rated to handle BOTH lights since it will
carry the current of both. The diagram shows two leads coming from the relay.
Then I run the second wire of the lights or other accessory to a good ground on the frame
of the vehicle. If the wires will not be soldered together and crimped connectors
will be used it's a good idea to put a dielectric paste on the connectors where they come
in contact. This will prevent corrosion as time passes ensuring a good connection.
Then double-checked all wiring before plugging in the power.
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Ohms Law
 Ohm's
Law Defined
Ohm's Law defines the relationships between (P) power,
(E) voltage, (I) current, and (R) resistance. One ohm is the resistance
value through which one volt will maintain a current of one ampere.
( I ) Current is what flows on a wire or conductor. Current is measured in
(A) amperes or amps.
( E ) Voltage is the difference in
electrical potential between two points in a circuit. It's the push or
pressure behind current flow through a circuit, and is measured in (V)
volts.
( R ) Resistance determines how much
current will flow through a component. Resistors are used to control voltage
and current levels. The higher the resistance, the smaller the amount of
current is allowed to flow. Resistance is measured in ohms.
( P ) Power is the amount of current times
the voltage level at a given point measured in wattage or watts.
V = I x R
I = V / R
R = V / I
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Alternative Offroad Lights or Accessory Wiring Diagram
using a Relay
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