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A 355nation Tutorial presented by
captainrab, brewer15650

Tutorial Name
Relay 101 basic's for Auxiliary loads

Relay 101 basic's for Auxiliary loads


Why do I want to use a relay and do I really need to? Anytime you want to switch a device which draws more current than is provided by an output of a switch or component you'll need to use a relay. The coil of an SPDT or an SPST relay that we most commonly use draws very little current (less than 200 milliamps) and the amount of current that you can pass through a relay's common, normally closed, and normally open contacts will handle up to 30 or 40 amps. This allows you to switch devices such as headlights, parking lights, horns, etc., with low amperage outputs such as those found on keyless entry and alarm systems, and other components. In some cases you may need to switch multiple things at the same time using one output. A single output connected to multiple relays will allow you to open continuity and/or close continuity simultaneously on multiple wires.
Below are a few relay basics along with probably the most common circuit asked about on the forum

HOW DOES A RELAY WORK?​

SPDT Relay: (Single Pole Double Throw Relay) an electromagnetic switch, consist of a coil (terminals 85 & 86), 1 common terminal (30), 1 normally closed terminal (87a), and one normally open terminal (87) (Figure 1).

When the coil of an SPDT relay (Figure 1) is at rest (not energized), the common terminal (30) and the normally closed terminal (87a) have continuity. When the coil is energized, the common terminal (30) and the normally open terminal (87) have continuity.

The diagram below center (Figure 2) shows an SPDT relay at rest, with the coil not energized. The diagram below right (Figure 3) shows the relay with the coil energized. As you can see, the coil is an electromagnet that causes the arm that is always connected to the common (30) to pivot when energized whereby contact is broken from the normally closed terminal (87a) and made with the normally open terminal (87).

When energizing the coil of a relay, polarity of the coil does not matter unless there is a diode across the coil. If a diode is not present, you may attach positive voltage to either terminal of the coil and negative voltage to the other, otherwise you must connect positive to the side of the coil that the cathode side (side with stripe) of the diode is connected and negative to side of the coil that the anode side of the diode is connected.





Real world scenario for adding auxiliary loads.

The remote turn on lead can be fed from an ignition wire power source then add a switch in series with this line so that you can control the load. This is handy for trucks that don't have fog lights but would like them. Take power from an ignition source run that to a switch in the cab then out to pin 86 on the relay, ground pin 85 and the relay will turn on when your switch is on. In addition once the ignition is turned off then then the relay will turn off. Having said that, calculate the current draw of your newly added load and make sure the relay can handle that current. Add another power source from the side of the mega fuse that doesn't connect directly to the fuse box. Fuse it properly and size the wire to handle the expected current. Attach that contact power feed wire to pin 87. Run power from pin 30 to the newly added loads such as fog lights. Make a ground connection to the other wire of the fogs lights and your in business

 

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I have one question. I am a complete idiot when it comes to electronics. I purchased two lights to mount on brush guard along with a wiring kit.

I understand all of the wiring besides one thing. What do I need to tap into for the switch? My switch has a prong for a ground, the power source from the relay, and then a third one. That is where I am slightly confused. I just don't know what exactly to tap into.

I mean the harness seemed pretty idiot proof just I confused myself. Switch has LED light for when on. And in the picture basically just asking what would be the easiest place for the "12 volt ignition source"
 
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