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12 volt relay to start a 24 volt motor

src cornson

Aug 17, 2017
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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?
 

duke37

Jan 9, 2011
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Why use a 12V relay? What is wrong with a 24V relay or mosfet (if DC).
 

duke37

Jan 9, 2011
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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

Jun 25, 2010
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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

Aug 24, 2009
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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

Jan 9, 2011
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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

Jun 25, 2010
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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

Jan 9, 2011
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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.
 
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