Help with CNC electronics

Hi all,

I’m building a control box for my CNC (Ooznest ox), it uses Mach3 USB board. This is an upgrade from the smaller CNC I had before, which runs GRBL.

So the issue I’m having is related to the power relay… Both before, using 12V signal from the GRBL board, and now using the 24V signal from the Mach3 board.

I’ve blown 3 cheap mechanical relays already. Now installed a SSR-25 DA solid state relay. It is switching correctly, and works (I can switch the relay on/off from the software, which uses a 24V signal, however it is only measured to be ~17.5VDC at the SSR input).

I’m sure something isn’t right. Because there is a 30VAC passing through the SSR in the open state, and the spindle power indicator connected to the load side of the SSR is pulsing…

I’m unsure what the problem is. Attached a photo of the box here. Don’t mind the mess around the relay/spindle, its temporary

Would anyone be able to help me? I don’t even know what the problem is… happy to bring the box in to the space.

Relay wired like this
images (2)

Hi Jay,
is the spindle motor running on DC? What is the spindle volts and power rating? e.g. 24V/240W

When you say that you’ve blown 3 cheap mechanical relays are you able to provide more detail? E.g. Did the contacts fuse together?


Hang on, just looked more closely at your pic. Are you switching the inputs or the outputs of the spindle motor controller?

Hi Steve,

Thx for looking into this.

The spindle is 500W, running @ 100VDC.

The SSR is switching the 240VAC input to the spindle power module. SSR doesn’t work on DC from what I understand.

I don’t know how to tell if relay is fused together, but afterwards the live terminals on the input /output sides should be off… But it remains on…

Hi Jay,
yeah, contacts sound like they’ve fused.

I am no expert on relays but based on what I can see in the various spec sheets a lot of relays don’t like switching higher DC volts, i.e. > 48V. I imagine that even with a higher rated relay some arcing is likely when disconnecting under full load.

Out of interest, I can see two terminals on the spindle controller connected by a short loop of blue wire. What is that for?

On /off

I think the on/off set of contacts is the right place to start/stop the spindle. Do you have any documentation on the motor controller?

Seems to be some solid copper behind those terminals, I’d like to know what they actually connect to before attaching anything

Problem identified. It’s the SSR.