Question about technical limitations of 12v relay switches

[TimGavinq]

Hi, I'm new here, I've just started a project that has me building an electrical circuit for the first time and I don't really know what I'm doing! I'm hoping someone can answer a quick question.

I have a 12v electric motor (has two power levels, at 9A and 20A and it was suggested to go with a 25/30A circuit breaker) and I'm looking to install some kind of wireless switch so I can turn it off and on using a remote. Maybe later down the line, I'd like it to do more than that, but for now, all I need is a single button to push to turn the motor off and back on. My understanding is that I'm looking for something like this. However, all of the affordable relay switches with remotes that I can find are rated at around 10A. I have found some that have a higher amps rating, but they're all significantly more expensive (and this project has already burnt through more cash than I anticipated!).

My question is rather simple and might sound pretty basic to a lot of you, but I am new to this and want to double-check that this is the case. So, am I correct in assuming that a 10A relay switch cannot be used for a motor that produces 20A? Or, does the 10A that the relay switch refers to relate to something else altogether and it's fine to run a 20A load through it? Assuming this is not the case if for the sake of saving money I was to only use the motor on the lower setting (9A), is that cutting it too close to the 10A rating of the relay - as in, should I allow for a safety buffer, or is it safe to go right up to the 10A limit?

Thanks in advance for helping out someone completely new to all this!

 

[rjenkinsgb]

10A is the current limit for the relay, to have a reasonable life.

Also depending on the relay and circuit board design, some chinese modules such as the one you link to may not even stand that for very long, as the PCB connections may not be big enough to take 10A without getting hot.

The simple solution is use a module like that as a slave switch to control a separate high-power 12V relay such as an automotive type.

 

[TimGavinq]

Also depending on the relay and circuit board design, some chinese modules such as the one you link to may not even stand that for very long, as the PCB connections may not be big enough to take 10A without getting hot.

The simple solution is use a module like that as a slave switch to control a separate high-power 12V relay such as an automotive type.

Thanks for this suggestion. Two questions from this.

Firstly, do you know how to tell the quality of the PCB connections from just viewing a product online, or is that a bit difficult? Is it more just a matter of cheap Ebay relays = bad quality and more expensive relays from special electronics stores = more likely to have higher quality PCBs?

Secondly, do you happen to know of any resources you could point me to where I can learn about how to go about making the slave switch you've referred to? I know what you mean by the automotive relay, just not sure I properly understand what you mean by a slave switch and how I would put together something like this.

 

[rjenkinsgb]

If you are only using the remote as a pilot relay, the current ratings are not really important.

This is an example of using a 12V car relay with a switch; just use the wires from the remote operated [slave] relay contacts in place of the switch:

https://mgispeedware.com/wp-content/uploads/2019/05/relay-socket-wiring-diagram-4-pin.jpg

The only thing that is missing is a "flywheel diode" - a generic rectifier diode such as a 1N4000 series (anything from 1N4002 to 1N4007) connected across the relay coil.

That allows the energy stored in the coil to be safely dissipated when the switch opens, and prevents contact arcing or burning.
Connect it with cathode (end marked with a bar) to the positive side of the relay coil.
More info on that here (there are various names for the function).

What is a Flyback Diode?

You see them next to DC relays. How do they work?

www.douglaskrantz.com

A similar thing may be needed across the output from the power relay, to the 12V motor, if that does not have built-in suppression or electronics.
In that case it would need a rather larger diode or another form of snubber circuit.

 

[TimGavinq]

Thanks, I read some info about the flyback diodes - I appreciate that.

 

[Diver300]

The flyback diode slows down the rate at which the current reduces in the relay coil. That can slow down the rate at which the contacts open, which isn't a good idea with large loads, as it can lead to contact burning.

There are a few alternatives to a flyback diode, but all will increase the maximum voltage that the switching device feeding the relay is subjected to. However, with each alternative, the maximum voltage is quite well defined and is far less than you would get without any flyback diode.

The simplest alternative is a resistor in parallel with the relay coil. If you have a resistor whose value is 10 times the relay coil resistance, the maximum voltage that the switching device will be subjected to will be about 10 times the supply voltage, so about 120 V for a 12 V supply. The coil current will reduce about 10 times as fast with the resistor in place as it would with a flyback diode.

A resistor like that will take about 1/10th of the power of the coil, in addition to the coil power. If that matters, you can put the flyback diode in series with it.

The 10 times factor is not fixed. The larger it is, the larger the maximum voltage and the faster the relay turns off.

Another alternative to a flyback diode is a TVS or transient voltage suppressor. Here is an example:- https://uk.farnell.com/taiwan-semiconductor/sa36ca-r0/tvs-diode-500w-36v-bidir-do-204ac/dp/2677629

That will limit the voltage to about 36 V so that the coil current will still fall about 3 times faster than it would with a flyback diode. The voltage suppressor won't take any current during normal running. You need to have a bidirectional TVS.