How a relay works

[Ricko1966]

I've been on several posts where people have been trouble shooting this or that,but it's clear they have no idea what a relay is,how a relay works,or why they need a relay. I am not teacher or an engineer just a dumb old mechanic trying to give you guys the benefits of what I've learned. There's a pic you can blow up to understand this. A relay is just a switch controlled by an electro magnet when the magnet is powered up,it opens or closes a switch to turn something on or off.Why would you use a relay?,to turn things on and off at a certain time with 1 switch powering multiple relays. Another one would be the electromagnet uses very little current but the switch in the relay can handle lots of current. Your starter solenoid is a good example,your ign switch in the crank posution powers the electromagnet which closes the contacts and the starter cranks. The ignition switch would burn up trying to pass starter current. The relay pictured is a super common relay. 85 and 86 are the electro magnet. 30 is power in 87 and 87a are power out. If you look at that relay you can see power goes from 30 to 87a when the relay is off, it goes from 30 to 87 when the relay is on. Let's say I thought that relay wasn't working and I know it controls the fuel pump when the keys on. I can put a piece of wire in place of that relay, jump 30 to 87 my fuel pump should run. If it doesn't the relay isn't the problem. To test that relay if I want to I can check continuity from 30 to 87a without the electro magnet,it should have continuity, now power and ground 85 and 86 your continuity should change to 30 and 87.
When you jumper a relay, really your jumper should have a fuse on it,in case there's a short on the output wire. I rarely do it that way it's a paper clip or whatever is closest to me. Ymmv

 

[SquareRoot]

And then there's this....where the big boys play. I've been wanting one of these.

 

[SirRobyn0]

@Ricko1966 Thanks thread will be good to reference for folks that need it in the future. One thing that has always blown me away is how some folks don't seem to even have a basic understanding of how a relay works, but it is literally on the side of most relays!

 

[CalSgt]

And then there's this....where the big boys play. I've been wanting one of these.

I've been eyeing those too

 

[Ricko1966]

And then there's this....where the big boys play. I've been wanting one of these.

I just looked it up,Only way I'd have that is if some one gave it to me. All circuits are 20 Amp max a 5.00 dollar relay is rated for 40 amps. My other concern is its not modular. You lose one circuit and you have to replace the whole thing,or ad a 5.00 dollar relay. I bet if you look you can find a nice relay plate that will look good,be cheaper,and function better.

 

[AaronW]

Thanks for this post. some of us are electrically challenged. I had to wire a rotary 3 phase converter into my wood shop, a while back. Luckily any visible smoke was merely what was coming out of my ears, and not from bad wiring.

Aaron

 

[Ricko1966]

@SquareRoot I'd much rather have something like this.Anyway just my 2 cents, more circuits,higher amperage,and more serviceable,just makes more sense too me.

 

[SquareRoot]

They make them in higher amps. That's just the pic I grabbed off the web. One modular unit yes- the circuits are individual. Just a little pricey.

 

[Bextreme04]

@SquareRoot I'd much rather have something like this.Anyway just my 2 cents, more circuits,higher amperage,and more serviceable,just makes more sense too me.

This is exactly what I have in my truck. Added two fan controls, choke power, trailer power, and a switched IGN power all for $40. I can't think of a single reason I would want that MSD one...

Another thing we ran into at my work(Aviation engineering stuff) was that one of our dealers wanted to use a relay to change the power feed to a device between a main ship battery and an emergency back up battery. They failed to realize that if you are trying to use the relay in a state where it is switching power away when the master power was off... it would drain the battery powering the relay switch in a very short amount of time.

Most relays take about 100mA of load to stay energized. That's 0.1 Amps of continuous draw. Which isn't much, unless you are drawing it all of the time. Most 12v automotive batteries are going to be in the 75-80 Ah range. That means they can support an 80 Amp load for ~1hour, or a 1 amp load for ~80 hours. But really you probably wont have enough to start the engine with less than 50% charge, and the battery manufacturers say you shouldn't take them down more than 10% to avoid damaging the battery. So with some math(I know... I'm sorry I brought math into this), you can calculate that keeping a relay energized with the engine off would take about 2 weeks to kill your battery enough to not be able to start, and that's just to hold the relay on, not to actually power whatever it runs too.

So all of that to say, pay attention to how you use relays. Try not to wire it in such a way that the relay needs to be engaged when the engine isn't running or the ignition switch is off.

 

[Blue Ox]

Repeating something that was drummed into me by a GM electrical engineer...

"A relay allows a low current circuit to control a high current circuit."

Just keep that in the back of your brain while you're working through this.



Carry on.

 

[Hunter79764]

If you learn by seeing and touching vs reading, take one of those cheapie relays and pop the cover off. Most can be exposed easily, look at the contacts, then click it on an off by applying 12v between 85 and 85, so you can see the arm click.


Little known fun fact, HVAC relays that run on 24V AC will also run on 12V DC, and some commercial units will have relays with 2 or 3 circuits all running on the same coil. I have one that turns on my high speed fan power from fused battery, breaks power contact to my low speed circuit (I had issues back-feeding power), and triggers the ground to my idiot light for temperature when I get to 210°, all on one relay. Heads up, the amp ratings aren't usually as high.

Something like this, ordering new the 12VDC coil is easy to get, but if you scrounge it from AC units, 24VAC works just as well.

https://www.grainger.com/product/OMRON-Flange-Mount-Relay-11-Pins-1YDA4

 

[Grit dog]

Sidebar relay question since the experts are gathered here…
Bought a Scamazon headlight relay kit and installed it recently.
Aside from get what you pay for like the main trigger plug being pinned 100% incorrectly (easy fix after a half hour of wtf diagnosis), it has a little voltage bleed over from low beam mode into the high beam headlights.

Seems rather unlikely it’s a consistent partial incorrect contact in one of the plugs or splices. So the obvious issue (to me) could be in the relay(s).
I’ve never seen a relay that wasn’t just on/off or fried. Haven’t had time to dig into it but what’s the likelihood that it’s just a bad relay that is as cheap as the rest of the components?
(This seems like something I’d just “throw parts at” and gamble the $10-20 if I can’t find another relay to swap in in my parts bins.

 

[Hunter79764]

I'd pop the cover and inspect it for sure. The other thought coming to mind is maybe a stray wire or two from a bad crimp that is making "some" contact to another pin? Hard to say, voltage bleed isn't a real common thing and could be a pain to track down... FWIW, I've run similar stuff usually without much issue. A headlight relay harness shouldn't have to cost $100, its $10 in components plus a little time to build, $20 isn't totally unreasonable.

 

[oldretiredafguy]

Sidebar relay question since the experts are gathered here…
Bought a Scamazon headlight relay kit and installed it recently.
Aside from get what you pay for like the main trigger plug being pinned 100% incorrectly (easy fix after a half hour of wtf diagnosis), it has a little voltage bleed over from low beam mode into the high beam headlights.

Seems rather unlikely it’s a consistent partial incorrect contact in one of the plugs or splices. So the obvious issue (to me) could be in the relay(s).
I’ve never seen a relay that wasn’t just on/off or fried. Haven’t had time to dig into it but what’s the likelihood that it’s just a bad relay that is as cheap as the rest of the components?
(This seems like something I’d just “throw parts at” and gamble the $10-20 if I can’t find another relay to swap in in my parts bins.

You hit the nail square on the head with the explanation of the source "Scamazon".

 

[Grit dog]

I'd pop the cover and inspect it for sure. The other thought coming to mind is maybe a stray wire or two from a bad crimp that is making "some" contact to another pin? Hard to say, voltage bleed isn't a real common thing and could be a pain to track down... FWIW, I've run similar stuff usually without much issue. A headlight relay harness shouldn't have to cost $100, its $10 in components plus a little time to build, $20 isn't totally unreasonable.

Ha the cheapo relays have a clear cover of all of things. They look fine? Nothing is glowing like the f ed up headlights lol
I ran thru the wiring and there’s nowhere where the wired could touch except in 2 of the headlight plugs and those are wired clean, even if one of them was wired wrong!
I just need to put a different relay in and see.
They’re not brighter that’s for sure. Although there was no issue with the existing headlights I kinda thought they’d be little brighter getting lower straight from the battery so to speak