Help with a relay ? (2 Viewers)

[aktundra]

I am going to install an onboard air system. When I look at various wiring diagrams for installing relays on the internet, I get confused. (Apparently I must have understood them at sometime in the past, because I have some extra relays in my tool box !! ). There is a pressure switch in this circuitry, but for my questions it’s not relevant.

The dash switch to engage the compressor’s clutch will receive “switched 12v”, from a convenient fused circuit on the dash. When the dash switch is closed it will send 12v to pin 86, and 87 will relay to the electrical clutch. But I’m not sure I can visualize what the connections on pins 30 and 85 will look like.

30 IS THIS CONSTANT 12v, DIRECT FROM THE BATTERY, WITH ITS OWN FUSE ?
85 ground feed from clutch compressor. DOES THIS CONNECTOR ALSO GO TO THE BODY GROUND ?
86 12v feed from the switch
87 12v feed to the compressor’s clutch

 

[ppc]

@aktundra Here is a much simpler diagram, note the switch on the ground side of pin 85, light is your compressor:

 

[jonheld]

@aktundra Here is a much simpler diagram, note the switch on the ground side of pin 85, light is your compressor:

View attachment 3409753

Please remember to add a fuse off the positive battery terminal.

 

[CharlieS]

I think you are showing two different diagrams, one 12v + switched and the other ground switched.

Either way, the idea is that there is a electromechanical switch inside (an electromagnet that closes a connection) and allows yopu to switch the power from the battery to your circuit, instead of running the 12v through your switch and then on to your accessory. The switch doens't get fried, and power goes where it needs to.

Your switch just energizes pin 86 with 12v power and the circuit between pin 30 and pin 87 is connected, so you get 12v from the battery. Or your switch grounds pin 85 doing the exact same thing.

The higher amperage your accessory draws the more important that relay becomes.

And if you literally don't know which physical pin (30, 85, 86, 87), it is typically molded into the relay base right beside the pin itself.

Also, you just want a normally open, single pole, single throw relay. They make several other types.

 

[ToyotaDon]

Terminal 85 is not a ground feed from the compressor clutch. All load components in an electrical circuit require a 12V positive (power) and negative (ground) connection to operate. There are two load components in this relay controlled system, the relay coil which is between terminals 86 and 85, and the primary load which is illustrated there with a light bulb but would be the compressor clutch in your situation. The diagram which shows one black wire to the light bulb and terminal 85 simply means both need to be connected to ground. The red wire to terminal 30 and the switch show that both need to be connected to power. The better these connections, the more reliable your setup will be, so a fused wire to terminal 30 and a dedicated ground wire on terminal 85 would be best.

 

[LandLocked93]

Why isn't the pressure switch relevant in this scenario? Just curious.

 

[tmxmotorsports]

Why isn't the pressure switch relevant in this scenario? Just curious.

Good point should be if there is an on board air tank.

 

[ppc]

Why isn't the pressure switch relevant in this scenario? Just curious.

Switch or pressure switch no difference in wiring. The OP has a lack of understanding but wanted to know the basics of wiring in a relay into a circuit. Don't need to overwhelm him as the point with every option possible that makes basic learning confusing.

 

[aktundra]

The pressure switch is essential to the system, but isn’t relevant to how to ground & how to energize the relay.

I am converting an extra rear defroster switch on the dash to energize the system. When depressed, the switch’s light will come on, and send 12v to the pressure switch. If tank pressure is under maximum, the pressure switch will close, and send 12v to the relay

The diagram seemed to imply there is steady (fused) 12v at pin 30, regardless of the switches. And seems to imply that the compressor clutch (light bulb), is grounded at the relay. Both looked odd to me.

Thanks to all that contributed. I think I got it !!!

 

[gummycarbs]

The diagram seems to imply there is steady (fused) 12v at pin 30, regardless of the switches. And seems to imply that the compressor clutch (light bulb), is grounded at the relay. Both look odd to me.

Because that diagram is an abomination and whoever made it should feel ashamed.

It's not really showing that both are grounded at the relay, but rather that EACH GOES TO GROUND. They can share a wire going to ground, each can have its own wire to ground, or whatever.

Here's what matters: a relay allows you to switch a HIGH CURRENT circuit by completing a LOW CURRENT circuit. If 12V passes between pins 85 and 86, this causes pins 30 and 87 to be connected. It doesn't matter HOW you complete the circuit across pins 85 and 86. USUALLY 86 goes to a switched 12V input, and 85 goes to ground, but it doesn't have to be that way[1]. All that matters is that you've got 12V on one side and ground on the other, with whatever switch on whatever side.

Pins 30 and 87 can provide 12V to a device. Or they can provide a ground to a device that already has 12V going to it.

Simplified: when you put +12V on one side of pins 85 and 86, and ground on the other side, it wakes up a little gnome, who proceeds to throw one of those Frankenstein switches, thereby connecting pin 30 to pin 87.

[1] the exception is when the relay has an integrated diode. These are used to prevent voltage spikes when switching, which can cause pitting on the contacts, leading to failure. In this case, pin 85 has to go to ground.

 

[VonArnim]

Remember a relay is just a little switch that is controlled by an electric magnet. It needs POS and NEG to energize the magnet to switch. The beauty is that you can run them via a positive trigger source or a negative trigger source, to energize the magnet and cause it to switch.