Question about technical limitations of 12v relay switches

Thread Starter

TimGavin

Joined Aug 1, 2020
6
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 is suggested to go with a 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 max load, 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!
 

MisterBill2

Joined Jan 23, 2018
6,785
Starter Solenoids are not intended for continuous duty and they will burn up after a few minutes. So the solution is to have the remote control device operate a 30 amp 12 volt relay. Those relays are available at many auto parts stores, with the benefit of being a quite common item available all over the USA. They are also relatively compact, being a black cude shape just a bit over one inch on each side. Bosch and Echlin are the two brand names that I can recall, but there are others as well.
 

shortbus

Joined Sep 30, 2009
7,857
Starter Solenoids are not intended for continuous duty and they will burn up after a few minutes.
You're mixing up a starter solenoid with a starter relay. This is a picture of a starter relay -
1596290199040.png They are made for continuous use, like in electric winches, snow plows, etc.
 

MisterBill2

Joined Jan 23, 2018
6,785
There are certainly a number of different versions available, and at least one is for short duty only. I know because a friend installed one to connect a second battery for charging in his motor home a while back. It burned out because the coil overheated.
And that is a much higher current rated device thean the 30 amp cube relay that I mentioned. Those devices, as shown, are good for a lot more than 30 amps.
 

Reloadron

Joined Jan 15, 2015
5,623
I would start with, as suggested, a 12 volt 30 amp automotive relay which can be had at any automotive parts store. You can buy some with the socket included or just buy some 1/4" terminal lugs at the same store. I would go with the Yellow lugs (higher current) and wire things with #10 thhn wire. Eventually you can control the relay On/Off with any number of remote controls like this one on Amazon. Matter of fact Amazon has everything and anything you could need. 12 volt automotive relay. There isn't really much to doing what you want to do.

My bad, I had AWG 12 and changed it to AWG 10 just to allow more overhead.

Ron
 
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MisterBill2

Joined Jan 23, 2018
6,785
The benefit of using #10 wire will be that it tends to conduct heat away from the crimp-on terminals better, and for this application that will be useful. I do suggest soldering the crimped connection to further assist in conducting heat away.
 

bwilliams60

Joined Nov 18, 2012
1,332
With relays, the 12v 30A Bosch style relay will work well for you. It is a 4 or 5 terminal relay and you can buy them cheap or go to a wrecking yard or garage and get them for free. They are in abundance. To learn more about relays, visit www.12voltplanet.co.uk and find the section on relays. It shows the different kinds and is a great tutorial.
As for starter relays or magnetic switches, thet are not continuous duty relays. They are for intermittent use. You can however buy continuous duty relays and they do have very different operating characteristics and specifications. Google continuous duty solenoids and you will see what I mean.
To answer your questions, the amperage rating is your load amperage rating and you need your max plus a little buffer zone so 25 or 30 amp would suffice in your case.
 

Tonyr1084

Joined Sep 24, 2015
4,929
Question: Motor Startup Current?

I did something along the same lines, only, instead of using a relay (which wouldn't have worked in a PWM setup) I used an IGBT (Insulated Gate Bi-polar Transistor) rated for 60 amps. The motor I was powering was 12V 19A running and startup of close to 30A. Also, I had to use a flyback diode sufficient to handle the high current that would be generated when shut down.

So the two things I'm mentioning is "What's the start-up current of the motor?" And consider a solid state relay in place of a mechanical relay. Here's one with 3 to 32 volt control signal and can handle 5 to 60 VDC at 40 amps.

[edit] looking at the reviews. Some are for DC to AC 25A (low reviews) and some for DC to DC 40A (high reviews). The #1 complaint about this attached relay is that they have the polarities wrong. The image is not correct. So read the reviews as well.
[end edit]
 
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Thread Starter

TimGavin

Joined Aug 1, 2020
6
Okay, thanks everyone, there's a lot everyone has said for me to look into and research further, but I think I get the picture. Hopefully I can get this up and running soon! Appreciate your help
 

mmcginty

Joined Feb 4, 2010
46
A quick addendum, startup current may be expressed on the motor spec plate as "LRA", locked rotor amps. That's the current draw when windings are energized but rotor is still. Since the rotor is not physically locked (presumably) it only draws that for an instant at startup. It's the max the motor is capable of drawing (barring shorted windings or other malfunction.)

And I know you haven't quite gotten there yet, but may I make a suggestion for the remote part? Use your mobile's web browser as the remote, and an inexpensive WiFi SoC (system on a chip) like the ESP8266 (~$3) or ESP32 (~$5), set it up as a simple HTTP server with WebAPI entry points that set GPIOs to turn your relay on and off. Sure you can probably make do with something off the shelf (in fact you can buy remote control outlets at the hardware store, ~$35 iirc, that might handle your load) but what fun is that? And what do you do when you later want to fully automate it, and add a current sensor, and log its operations to a web site?
 

DougMI

Joined Aug 13, 2020
2
With relays, the 12v 30A Bosch style relay will work well for you. It is a 4 or 5 terminal relay and you can buy them cheap or go to a wrecking yard or garage and get them for free. They are in abundance. To learn more about relays, visit www.12voltplanet.co.uk and find the section on relays. It shows the different kinds and is a great tutorial.
As for starter relays or magnetic switches, thet are not continuous duty relays. They are for intermittent use. You can however buy continuous duty relays and they do have very different operating characteristics and specifications. Google continuous duty solenoids and you will see what I mean.
To answer your questions, the amperage rating is your load amperage rating and you need your max plus a little buffer zone so 25 or 30 amp would suffice in your case.

BWilliams60, can you contact me when you have time please, have a question about alternators, read your reply regarding 12 to 24 volt conversion and have a current question. I am not sure how messaging works here, I just registered and don't seem to understand how to message you. Thanks
Doug
 

Thread Starter

TimGavin

Joined Aug 1, 2020
6
Okay, I've had some spare time to do more research and work out a plan based on everyone's help.

I've attached a circuit diagram I drew (first one since high school physics so go easy on me!). I'm not super confident about it, I'm hoping someone will be able to have a quick look and let me know if there are any obvious mistakes?

Circuit diagram v1.jpg

I have some other quick questions too.

2) Assuming my diagram is correct, are the only wires that should be the thicker 10 AWG the ones running from the negative terminal on the battery to the motor, the positive motor to pin 87 on the relay, pin 30 to the fuse and the fuse to the battery? Is it ok to make all the other wires a thinner gauge?

3) The RF receiver I bought on ebay isn't labelled like any others I've seen online. Instead of NO, C, NC, it has "OUT L" and "N." Am I correct in assuming "OUT L" is normally open and "N" is normally closed, with the middle terminal being common?

RF receiver2.jpg

Thanks!
 

MisterBill2

Joined Jan 23, 2018
6,785
The connection from relay terminal 85 and the RF module minus terminal to the battery negative is missing. The drawing is quite good, better than many, in fact.
And your #10 wire question you have the places all correct. But add a 1 amp fuse in the other feed from the battery to the RF relay section
 

Thread Starter

TimGavin

Joined Aug 1, 2020
6
The connection from relay terminal 85 and the RF module minus terminal to the battery negative is missing. The drawing is quite good, better than many, in fact.
And your #10 wire question you have the places all correct. But add a 1 amp fuse in the other feed from the battery to the RF relay section
Thanks Bill. Am I correct in assuming that if terminal 85 and the RF module minus terminal are both connected to the battery negative, they don't need to be connected to one another as I have drawn?
 
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