Automotive Relays - Garage - Universal Tech

If you work on cars you’ve probably heard of relays; in fact, we often cover using relays in our tech articles. Following detailed instructions to wire one can work, but understanding what makes them tick allows you to take full advantage of these amazing little creatures. While there are many uses for relays, the most common is for switching a device such as a cooling fan, fuel pump, horn, nitrous solenoid… almost anything that draws high amperage. The reason is simple; relays allow us to control an accessory that draws a lot of juice through a light-duty remote switch or switching device. This basic use of relays is what we will focus on throughout this article. Just think of a relay as heavy-duty electromechanical switch, made up of an electromagnet and a set of contact points. Still a little confused? Don’t worry; we’ll break this down further as we dissect one of these little buggers to show you its internal functions.

01 Looking at the base of most relays, you will find between four and five terminals, each with a corresponding number. For a basic switching application, only the outer four terminals need to be used. For this article, just think of the terminal in the center as a middle child and ignore it.

02 On the side or top of the relay there is usually a schematic to help you decipher what each number/terminal represents. Don’t worry if this schematic doesn’t make sense right now. Return to it after our hands-on explanation and it should come together.

02 On the side or top of the relay there is usually a schematic to help you decipher what
03 We brought in our friend Wally to help explain the concept. The simplest way to think of a relay is as a switch controlled by a little helper. In this case, Wally will be our helper.

03 We brought in our friend Wally to help explain the concept. The simplest way to think
03 When we tell him to flip the switch to the on position, he does so.

04 Now, let’s take a look at the inside of a relay. Since the cost of installing a Wally into each relay would be very expensive, manufacturers use an electromagnet instead. Hopefully, most of you have played with an electromagnet as a kid and understand that it’s just a wounded coil around an iron core that creates a magnetic field when fed power and a ground. In the case of our relay, the two electrical terminals to activate the electromagnet can be seen here. Following the leads to the bottom of the relay, one leads to terminal “85” and the other to terminal “86” (take our word for it or look at the schematic). Thus, we can establish that terminals “85” and “86” are used to activate the relay’s electromagnet.

04 Now, let’s take a look at the inside of a relay. Since the cost of installing a Wally
05 Here we’ve taken apart a spare relay to give you a better look at the electromagnet. The plate we bent over to the side is called the armature.

05 Here we’ve taken apart a spare relay to give you a better look at the electromagnet. T
05Think of it as an arm pivoting on its elbow. At the far right of the armature, you can see a round contact point. When the electromagnet is activated, the armature gets attracted to the magnet, swinging on its elbow to switch the contact point. The second image should help you visualize how this works.

05 Think of it as an arm pivoting on its elbow. At the far right of the armature, you can

Automotive Relays Universal Tech Close Up

06 This is a close-up of the contact points. These contact points make up the switch portion of the relay. Think of this as a heavy-duty switch capable of handling 30 to 40 amps (depending on the rating of your relay). To help put this into perspective, a typical automotive fuel pump or radiator cooling fan draws 15 to 20 amps).

07 To put this theory into practice, let’s connect power and ground to the terminals leading to the electromagnet (“85” and “86”).

07 To put this theory into practice, let’s connect power and ground to the terminals lead
08 The first image shows the armature in the “NO” (normally open) position before power to the electromagnet is applied.

08 The first image shows the armature in the “NO” (normally open) position before power t
08 The second image shows the electromagnet activated, pulling the armature and the contact point over to the left to close the circuit.

08 The second image shows the electromagnet activated, pulling the armature and the conta

09 Now let’s look at how the current flows in and out of the switch. Following the contact point on the armature, we can see that it leads around to terminal “30”. Imagine feeding ( ) 12V power from the battery into this terminal.

09 Now let’s look at how the current flows in and out of the switch. Following the contac
09 The current will come around and end at the center contact point. Since this is a normally open switch, current cannot jump across the air gap to the left contact point until the electromagnet is activated.

09 The current will come around and end at the center contact point. Since this is a norm
10 But once power and ground are supplied to the electromagnet, the armature swings over and closes the circuit allowing current to run around and down to terminal “87”. If we had a fuel pump or a fan hooked up to this terminal, it would now be supplied with power from the battery (coming in from terminal “30”).

10 But once power and ground are supplied to the electromagnet, the armature swings over

11 Here you can see the typical electrical connections to a relay. The thin red and black wires supply power and ground to terminals “85 and “86” (to activate the electromagnet). The thicker red wire supplies ( ) 12V power from the battery to terminal “30”. This will feed whatever device is connected to the switched terminal “87” (blue wire).

11 Here you can see the typical electrical connections to a relay. The thin red and black
12 Here’s our final layout. The switch now controls the thin red wire to activate the magnet in the relay.

12 Here’s our final layout. The switch now controls the thin red wire to activate the mag
12 When it’s flipped over, power flows from the battery and out to the light bulb. The advantage of using this setup in a car is that activating the electromagnet in a relay does not require much current at all. This allows a low-current switch or even a computer to remotely activate a device that draws high current (up to 40 amps). Again, please keep in mind that we have only covered the basic use of a relay in this article. Relays are available in various configurations and can be used to do much more. Hopefully, we’ll get a chance to show you some more advanced uses in a future article. Good luck with your next wiring project.

12 When it’s flipped over, power flows from the battery and out to the light bulb. The ad