If you are using a spindle with a VFD, there is a 24 volt power supply that will provide the correct power for the V+ and V- on the Mach3 USB. The 12V option is only if you are using the limit switches on that side of the board, but if you are using it to control spindle speed and for the outputs, it's best to use a 24V power supply, or the 24V output on the VFD.
Can i use 12v in the 24v input
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The mach3 USB card requires 24V, how do I supply this?