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How Do These(Automotive) Type of Relays Work?

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It has a coil circuit which operates a magnet which pulls the contacts of a larger capacity circuit together
They are pretty cheap ,separate the base and pins from the top cover and you will see all
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Ian Rogers

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You can get 5v relays, but these aren't used in the automotive industry.. a 12v relay has a coil with an impedance of approx 120, and the 24v has an impedance of approx 240. thus both take about 100mA to energize.. (they will of course activate with a much lower voltage, but the current will increase ).

Also remember the 20-30A rating is for the switched side of the relay, and nothing to do with the activating side.

Standing corrected..... I actually thought I typed decrease.... thanks Colin (Its a good job someone else is watching)
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Why don't you keep all this in the same thread? Soon you will have a half dozen threads all helter skelter with a single objective. This tends to confuse those who are trying to help you with your project. I suggested using an automotive relay for the reasons I mentioned in the last thread. I mentioned why I suggested their use and told you that tomorrow morning I would give you a circuit to use that should do as you wish.

This is the series of relay I had in mind. Note there are flavors that handle 50 (or more) amps for your load. No, they are not like the SSR. They are regular relays. They do however use a 12 volt coil and their merit is they are designed to handle large current, which was also explained. The relay(s) I just linked to use standard 1/4 inch spade lug connectors which can be had at any auto parts supply store just like the relays. Also, per KISS there are sockets available.

Now if there was something myself or another member explained to you in your previous threads, that you don't understand, it would be to your advantage to ask those who explained it rather than start new threads. :)

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Why don't you keep all this in the same thread?


The reason I start new threads is due to the fact that I do not want the thread to get off topic by asking questions that may bring the thread off topic. Here in this thread I had one objective, explain what an automotive relay is, nothing more, nothing less. The other thread was only related to E-Stop switches, again, nothing more nothing less. Same scenario happens with the other threads as well.

By nature I prefer to be an organized person, and I extremely hate for threads to end up 15+ pages long and contain content that doesn't even relate to the original post. This creates two issues:

1. It creates an issue for me as I have to read back through all those pages if I ever need to confirm something that was said, which can take time and energy.

2. It creates an issue for future users who may have similar questions who may just need a simple question answered, and do not care to read a thread that's swaying back and forth between topics.

As long as the thread is going in a linear fashion towards a solution, that's fine, but again, this is just a personal preference as I know I'll have to revisit most of these threads, and it's so much easier and "quicker" to find information when I separate different main components.

So once I'm fully aware of how most automotive relays work from this thread, I can continue on with the E-Stop thread.
Else I would end up with 8 additional posts (as can be seen from this thread currently) and probably more as I still need to ask some questions about this automotive relay, and 3 weeks from now when I revisit this site and I needed to confirm/check something someone else said relating to automotive relays, it would possibly take quite a bit of time.

As long as I'm not double posting and bringing up the over all main objective (my main project) in every single thread, I think it's a great organized way to do things... :)


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(they will of course activate with a much lower voltage, but the current will increase ).
This is incorrect. The current will decrease.

A relay will operate on a lower voltage but as you lower the voltage, there will come a point when the will NOT operate. Once you find this voltage-point, you know how much tolerance you have with UNDER-VOLTAGE.
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I'm with RS14 EXCEPT the opening statement in this thread is probably what caused the irritation:

I see that it can handle 20-30A, however, I do not understand how much voltage is needed to trigger it...12v maybe?

may have been better said as:

I see that it can handle 20-30 ADC, however, I do not understand how much voltage is needed to trigger it...12 VDC maybe?

It was covered implicitly with the (Automotive) hint.

I THINK what Ron's concern is that you may be UNAWARE that switches and relays have AC and DC ratings. They also have HP or Horsepower Ratings.

That's the piece that's MISSING and causing concern in this thread.

Now when you bring it back into the old thread we might still say it's unsuitable and you should consider using an automotive external starter relay such as this one: https://www.autopartswarehouse.com/...093010841W41260599c6350&apwkwd=starter+relays

So, where we are going is that relays/contactors and switches are application specific.

Now, I'll get wierd, because it probably belongs in the other thread and add a Reversing switch as a possibility: https://www.industrial-electronics.com/motor_control/3h_Reversing-Motors-Drum-Switch.html

So, yea hopefully you'll understand how a relay operates, but realize that it won't be appropriate for your motor load. I do need to re-phrase that: The initial relay in question could be used for reversing as long as the motor is stopped. but it should not be used to start the motor.

So, maybe Ron was looking for something like:

"Can you give me some ideas on how to do ......"

And answer our questions. The more we know, the better we can help.

A final word on "standard". it's very dependent on the industry your in. For instance BLACK is the HOT wire in the US 120 V wiring. Heating wiring uses codes like R Y W G for RED, Yellow, White and Green. Now someone decides we need a C or Common wire at the thermostat, so (what color is it?)

A 24 VDC coil is common in process control systems
A 24 VAC coil is common is Home HVAC systems
A 12 VDC coil is common in a car
A 120 VAC is "common" for industrial control circuits..."in the US"
Split phase 240/120 is "common" is residential (yes, there is 3 phase residential)

I'm using "common" in place of "standard".

Where seeming good logic can go wrong. I have a 2 HP 120 VAC motor and there is 746 W/HP so I can use a lighting tohggle switch rated at 15 A right? WRONG!!!
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Seems their site is under maintenance at the moment so will have to check it out tomorrow. But overall, and I may be wrong, it seems some are saying these relays are triggered from 12v , and there are some that are triggered from 24v?

I just thought that meant, those are the types of systems these relays are made for and anything above that would destroy it.



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The data sheet say they are for Resistive loads which a motor isn't. Time to ask: What is the nameplate info for the motor?

Yep, some are triggered from 24 VDC.

I'll also point you here: https://www.golfcarcatalog.com/catalog/index.cfm?fuseaction=product&theParentId=1863&id=5514 Look at the links at the bottom. The pic has a 48 V coil and I would probably assume the one for sale has a 36 v coil.

I'll post this https://www.electro-tech-online.com...N010-Contactor-Fuse-Diode-Lessons-Learned.pdf link directly because it's TOO important.

See, things can get messy for fwd and rev: https://www.golfcarcatalog.com/catalog/index.cfm?fuseaction=product&theParentId=1430&id=5617
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You can buy them by the box at auto junk yards, most 70's-80's european cars had many Bosch relays in them. They are just as common as a Bosch "junior timer" fuel injector socket.
Some (but not very many) relays have non standard pinout because they have an extra coil terminal in lieu of the NC contact.
Some also have built in back emf diodes or resistors across their coils and could be polarity sensitive.
Note that the NC current rating is typically less than the NO rating. The relay shown above is 50/30 amps.
For extreme environmentally challenging applications, you can buy sealed versions of this relay. Good for boats.
Newer cars still have relays but are about 1/2 the size of these.
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