How To Wire a Relay


Relays are often misunderstood but are very simple in design and easy to use. In this video we give you an overview of how a relay works and why you should use them when installing electronic accessories in your car. Especially if you have a car with an older electrical system that wasn't made to take the load of todays accessories.

Let’s wire up a relay. I’d like to demonstrate something for you. I have an electric fan, connected to a power source and a meter that will show how much power is being drawn. I’m going to turn the fan on and see what it draws. Put my clamp on here and see that it’s drawing 3.5 to 4 amps. Try to remember that for a minute. Remember what is inside the relay. We have a switch that goes off and on, we also have an electromagnet. We cover this in the previous video, Relays - How They Work . The industry has decided on 2 different ways to label the little terminals on the relay. They either use numbers, in which case the switch is labeled 87 and 30/51 and the magnet is labeled 85 and 86. Or they use letters, calling the switch C1 and C2 and the magnet W1 and W2.

I’ve made a bigger image of the relay so you can see it a little better. Here’s your power in, 87. And your power out, 30/51. Our magnet is powered here by ground. To show you what happens, I’m going to put a bit of negative, or ground, to one end of the magnet. I’m going to use another wire to bring power to bring power from the switch inside the car to the positive end of the magnet. Now listen carefully, see if you can hear this. The relay is working.

Now all we have to do is connect power to it. I’ll give power right here. If I were to put a fuse in this circuit I’d put it in this line. Now I’ll go over to the other side and I’m going to use alligator clips. You’d use something a bit better in your car when you’re doing it for real. Now I have power in, power out to the fan. I’m set up to the switch inside and I turn the switch on. The relay is closed and it’s working.

I’m going to take a look and see what the lead is to the fan. Coming in between 3.5 and 4. Let’s look and see what’s happening with our relay here. The relay is drawing about 0.2 amps. The actual fan is getting between 3.5 and 4 amps running in and running out, but the load on the switch inside is about two tenths of an amp. That’s the value of a relay. Now if you’re going to be putting a horn, or lights or an electric fan or any other consumer you can use a relay to do it and that way protect the electrical system in your car.

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