Old Trucks: Using Relays to Control High Amperage Circuits

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John Niolon



One of the things we have to contend with when
restoring or rodding these old trucks is the wiring
(or lack thereof). How many of us have
looked under the hood or dash board to find a mess that can only be compared to
a backlash on an old casting reel. Bare conductors, frayed and missing sections
of insulation, Wire nut connections and I’ve even seen a patch made with
stranded clothesline wire wrapped with vinyl tape. In addition to the disrepair,
most of the older models were 6 volt systems and even when new were marginal
performers. As the years added their
toll to the wires, insulation, connectors and fuse blocks the performance
degraded exponentially. There are many
aftermarket solutions to the problem that include everything you need to rewire
your trucks except the beer, the colorful language and the fast pain relievers
like Tylenol and Advil. Headaches are
included in the kit.


While some of us find the wiring simple and
straightforward, there are many who regard anything electrical as akin to
alchemy and the black arts performed by wizards and sorcerers.. The new kits come with pre labeled wiring and
cover most every device you can add onto our vehicles but sometimes leave a
little to the imagination when it comes to the actual hookup of devices and
especially to high amperage parts like sound systems, horns and auxiliary


The normal #18-14 gauge wiring in automotive
systems isn’t up to the task of high amperage draws and will usually let all
it’s smoke escape in short order. But,
they are sized properly for operating relays that can handle the switching of
these big boys of the electrical group. 


A relay is nothing but a remotely controlled
switch. The contacts of the relay can
transfer the high current from the source to the device without burning out or
burning up. These contacts are operated
by a magnetic field that is produced by a coil. When the coil is energized the relay contacts move from one point to
another, and either turn on a device or turn it off. There are variations to that theme, but
basically that’s it. In the diagrams
below I’ll show the basic parts of a relay and how they work together to do the


Figure #1&2 shows the basic diagram for a switch. Figure #1 shows a single pole single throw (SPST) switch similar to what
you use every day to turn on a light bulb.




When the switch is “closed” it makes a connection
between the two blue dots (or contacts) and power flows from the power source to
“D” the device (a light, or motor, or radio, or whatever). Normally the switch is activated by flipping
a lever or pressing a button or in some instances, turning a key.


Figure # 2 shows a variation on the basic
switch. It is a single pole double throw
switch (SPDT) and switches the power from the source to one of two contacts”¦ if
the switch is moved to contact “a” then power flows to it’s device and of course if the switch is moved to
contact “b” it’s device is powered.. The
variations on this configuration as well and are nearly endless”¦ more contacts
can be added as well as switch arms but the all basically work this way.. Simple, right ???? The power or current that a switch can handle
is only limited by the wire size and the size of the contact points in the
switch. Of course, the larger the wire
and contacts”¦ the more power or current the switch can handle. There are tables everywhere on the net that
will tell you what size wire you need to handle what amperage (current) based
on the length of the wire. Switches are rated the same way and are labeled with
their maximum use on their cases.


a general guideline you can use for automotive wiring

lengths are generally less than 25′ and don’t offer that much voltage drop at
that length


18 AWG –4
amps or less
16 AWG -10  amps
14 AWG -15 amps
12 AWG ““20 amps
10 AWG ““25 amps
8 AWG  -30 amps



Now, returning to the relay”¦


I know you all remember 3rd grade
science experiment in magnetism. You
took a big nail and wrapped a whole gob of insulated wire around it then
connected the wire to a battery”¦ and walah !!! the nail turned into a magnet. You learned
that the battery voltage created a magnetic field around the nail and coil of
wire”¦ an electromagnet. You spent the
afternoon picking up gem clips and magnetizing your buddy’s watch. Relays use an electromagnet to close and open
the switch or switches they control. Look at figure #3 this is the coil we just talked about. Now look at figure # 4 and see how it is
incorporated with a switch.. when the coil is energized the magnetic arm of the
switch moves between the contacts and
you have a relay.



In most situations the arm is spring loaded and is
not touching a contact (in a SPST relay) or in contact with one of the contacts when the coil is not energized. When the coil is energized it will pull the
arm to the other contact. Again there are variations”¦..there are different
configurations of relays referred to as “˜normally open’ and “˜normally
closed’ which refer to the position of
the arm when the coil is not energized. Normally open the arm will be away from
the contact (in a SPST relay) or not
connected to the device contact in a SPDT relay. Normally
closed relays
 are just the opposite
and actually open the circuit (remove the power) when the relay is
activated. For our purpose here we will
deal with “˜normally open’ relays. Later on I’ll give you some links to more
relay information. There are literally
dozens of configurations and relay circuits that can allow you to do most
anything you can think of”¦ turning on a light, changing direction of d.c. motors, actuating alarms, making windows work up and
down, give your neighbor next door a little jolt when his t.v
is too loud”¦. well most anything, anyway.


Note: for
our discussion we’re assuming that the relays we are using are in automobile
applications with a direct current (battery) power source”¦ coils are dc devices only. But, relays can be used in a.c applications also. The only requirement is that the coil voltage must be converted to
dc. A simple diode added in series with
the coil voltage will convert the a.c. to d.c. and the coil will work properly.


So, in our simplistic example”¦ look at figure #
5. When the coil is not energized the
arm is not touching the contact (red dotted line) when power is applied to the coil the arm is
pulled magnetically to the contact completing the circuit from the power source
to the device “D”. That’s all there is
to it. 


Ok, so now if the coil stays energized the device
will always be on and run till the power source is removed or exhausted in the
case of a battery source. That’s
probably not practical”¦ we need a way to turn the coil on and off. Now start thinking in terms of your truck”¦ The power source (for both the relay and the
device) has to be the battery, right ?? You have positive wiring and negative”¦ but the negative is the ground in
the truck or the chassis and frame “¦ so
half your wiring is already in place isn’t it ???



In our truck situation the circuit would look like
figure #6 below”¦ we’ve added a switch
between the battery and the coil allowing us to activate the relay when we
choose. When the switch is pressed the coil circuit is closed”¦ the arm on the switch
 is closed completing its circuit and the
device is turned on.


Notice again in figure %#6 that I now show the
relay as “˜enclosed’ these high power
devices are usually enclosed in a plastic case about 1″ square with a
convenient mounting tab at one end. Most aftermarket automotive relays are
rated at 20, 30,40 or higher amperages.


The device itself might be a 50 amp device but the
only wiring that must carry that load is from the battery to the relay and from
the relay to the device”¦ the control wiring (coil circuit and switch) can be
small since the coil itself will need less than ½ amp to actuate. And, when the relay is mounted close to the
device you cut down on the amount of heavy wiring that is needed. An example
would be headlights”¦ without a relay a heavy wire would have to be run from the
battery connection point thru the firewall to a switch that could handle the
amperage.. then from the switch back thru the firewall down the fender and to each headlight. But, as stated before, with a relay all the
wire running from the relay to the switch and back can be light guage wire.


In today’s vehicles, lots of circuits are operated
by relays”¦ cooling fans, head lights, heater fans, radios, door locks, wipers,etc. And,
with an endless variety of relay options and configurations you can turn on or
off devices, turn on with timed delays, turn them on(or off) at certain
temperatures warn you when things aren’t right (door ajar). They are very useful tools. Trying to run headlights on # 14 wire will
accomplish two things”¦. Dim headlights and dangerous circuits which can lead to
a cab full of smoke or a fire. Relays in
these high amperage circuits protect your truck and give the circuit the power
it needs to perform properly.


Now”¦ two more things”¦


#1 this
little black box has terminals on the bottom that you attach your wiring to
with quick connect crimp on terminals. And, the terminals are numbered”¦ not logically like you’d think. I think the guy that came up with the
numbering used last months lottery numbers. The standard numbers for most relays  are 30,85,86, 87 (and sometimes) 87A


Here’s the hookup”¦


30 ““ the power lead from the battery goes here (for
the high amp device)

85 – this is
the ground side of the coil”¦ wire this to a chassis ground

86 – this is
the power lead to actuate the relay coil (from the switch)

87 – this is
the hi amp output to the device

87A ““ is for the second contact in a SPDT relay
(when switching between

 Two sets of contacts)





#2 I did not
show any protection for the circuit at all (to keep is simple). Normally you should put a fuse holder and
fuse in the line between terminal 87 and the device. (or between the power
source and terminal 30 if it’s more convenient. This gives you over current protection and prevents the circuit from
releasing all it’s smoke. (not a good thing) Figure 8 below shows the fuse in the device line between the battery and the relay”¦ although as mentioned above”¦it could go between the
relay and the device also. The important
thing is to have it in there in case there is a short circuit in the wiring or
device. The fuse will blow and save the
circuit and you from a embarrassing situation. It wouldn’t be cool to pull up at the drive-in with a cab full of smoke
and you emerging smelling like burnt insulation.




Now this should have de-mystified the whole booger-bear
of relays. They are very simple devices
and are one of the most useful tools in the electrical department of your


There are lots of good resources on the net if you
want to investigate relays further”¦. Two excellent sites are “The Install
Doctor” who offers a pdf
file with literally dozens of relay examples http://www.installdr.com/TechDocs/999404.pdf And The 12 Volt page who not only
gives relay info but alsio basic 12 volt dc
circuits many of them  are vehicle specific”¦ http://www.the12volt.com learn all of this and YOU will be the relay guru
that everyone comes to for help.





All International Rights Reserved. This document may not be copied
or published without prior written consent of the author.

This information was done by me and for sharing amound
FTE users. I only ask that if you reproduce it give me credit for it and if you
make money from it? give me my percentage.

Since I have no way of knowing your level of competence,wiring,
welding or cutting skills or mechanical ability?.there are no guaranties or
warranties either verbal, written or implied with this article. Pictures were
made at different stages of construction and all parts in pictures may not be
complete in each shot, but I’ve tried to make the plans or drawings complete to
the best of my ability. If you have questions or see mistakes or problems, let
me know by e-mail and I’ll make the corrections if possible..

Use these ideas at your own risk. Modify them at your discretion
and to suit your purpose. Your mileage may vary. batteries not included. much
assembly required. Wait one hour after building to enter the water, additional
charges may apply. not all applicants will qualify for advertised A.P.R.. for
ages 10 to adult. Side effects are
comparable to placebos. Do not take drugs when building or operating machinery”¦just
say no !


John Niolon


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