12 volt relay to start a 24 volt motor

[src cornson]

I need an on/off switch to start a 24 volt 42 amp trolling motor. i assume that means using a 12 volt relay to do this. Or, I can just plug / unplut it. I want to put the switch in a wired remote control box with as little wiring as possible leading to the box. Where do I find a 12 volt relay to powere the 24 vdc 42 amp power circuit?

 

[kellys_eye]

Why not use a 24V relay?

https://www.google.co.uk/search?q=2...Gut7VAhUJbVAKHcfyDgQQsxgIJw&biw=1855&bih=1071

 

[duke37]

Why use a 12V relay? What is wrong with a 24V relay or mosfet (if DC).

 

[src cornson]

Thanks, I'll just get a 24 v relay.

 

[duke37]

You could use a couple of 12V automotive relays with coils in series and output contacts in parallel.

Remember to put a flywheel diode across the relay coil and also the motor to protect the contacts during switch off.

 

[kellys_eye]

Paralleling relay contacts isn't a good idea as you can't guarantee them both pulling in at the same time - one will ALWAYS take a greater load, even if only for a very brief period, and wear the contacts in no time.

 

[Externet]

A relay with 12V coil is perfectly useable to power a 24V trolling motor. Just power the relay coil with 12V, which may be tapping only to one 12V battery.

 

[duke37]

Paralleling relay contacts isn't a good idea as you can't guarantee them both pulling in at the same time - one will ALWAYS take a greater load, even if only for a very brief period, and wear the contacts in no time.

I do not think that much heat will be generated in a millisecond or so. Contacts wear due to the generation of an arc at switch off, the diodes will suppress this.

The automotive relays seem to be rated at 40A so a couple should be able to pass 42A without trouble. 12V automotive relays are readily available from car dealers and scrap yards.

 

[kellys_eye]

Why use two when one will do? The 24v relay isn't 'rare' so use the right part for the job - is my motto.

 

[duke37]

Why use two when one will do? The 24v relay isn't 'rare' so use the right part for the job - is my motto.

The reason is that 12V relays are common. I can take a walk over the fields and Rich will give me a couple free, he also lets me use his compressor to blow dust and spiders out of old radios. If I speak to him nicely, he would bring me the relays when he comes down to his mum's for lunch.

24V relays are used on large vehicles but I would need to travel many miles to get to a service depot or order and wait for the post. 24V 100A relays are probably quite common but I have not looked into this.