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12 volt relay to start a 24 volt motor

Discussion in 'Sensors and Actuators' started by src cornson, Aug 17, 2017.

  1. src cornson

    src cornson

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    Aug 17, 2017
    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?
     
  2. duke37

    duke37

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    718
    Jan 9, 2011
    Why use a 12V relay? What is wrong with a 24V relay or mosfet (if DC).
     
  3. src cornson

    src cornson

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    0
    Aug 17, 2017
    Thanks, I'll just get a 24 v relay.
     
  4. duke37

    duke37

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    Jan 9, 2011
    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.
     
  5. kellys_eye

    kellys_eye

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    Jun 25, 2010
    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.
     
  6. Externet

    Externet

    705
    142
    Aug 24, 2009
    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.
     
  7. duke37

    duke37

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    Jan 9, 2011
    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.
     
  8. kellys_eye

    kellys_eye

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    Jun 25, 2010
    Why use two when one will do? The 24v relay isn't 'rare' so use the right part for the job - is my motto.
     
  9. duke37

    duke37

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    Jan 9, 2011
    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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