Electrics - relays.

Hi all,
We've heard of two relays being used for drive before - one as a 12v starter circuit and the second as the full power 24v circuit.
However, whenever our electrical team has tried this (they're simulating using Crocodile Clip software) they seem to get a short circuit. They believe that a 3-pole relay is the only solution, but getting one rated to a high enough current to stop it melting is... interesting.
Would someone be kind enough to explain their solution to this problem?


  • edited February 2017
    Hi Sam,

    It should be possible to switch between 12V and 24V using two SPST relays: you will need a SPDT switch to select which voltage to use, as well as an SPST switch to turn the motor on and off. A circuit is shown below demonstrating this


    You'll need to make sure the SPDT switch you use is a "break-before-make" type changeover switch to prevent both the relays activating at once.

    Perhaps a more robust solution would be to use an SPST relay in conjunction with a SPDT relay such as this one (link) this gives the advantages that there will be no possibility of a short circuit and the changeover could be electronically controlled.


    Using a current sensor like ACS758 (L1 in diagram) a microcontroller could be used to set the point in the accelleration at which the voltage should be increased to remove the dependency on the driver remembering to change the voltage.



  • Thanks Dan.
    It all seems so simple when you think about it!
    This should be perfect for our budget...
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