Spindle Control via Solid State Relay

Hi Folks,
I’ve been having a lot of success with a 40amp Solid State Relay wired into the spindle control pin (and GND obviously), the nice thing about this is that the spindle not only starts when running a program, but shuts off at the end of it.

I say ive been having success … until recently. We returned to our workshop after a weekend of well deserved R&R and found our Dewalt 611 spinning at its set speed. We have no idea how long this had been turned on for but no more than 60hrs. This is obviously concerning for the fact that the spindle could have overheated and caused a fire (we have a new policy in place to prevent this happening again).

My first assumption was that the SSR had somehow failed and as a result, closed the circuit. This was confirmed when I ran it through a PSU for the switch and it was passing voltage regardless of the switch state.

Having now recieved replacements, they dont seem to be recieving a voltage from the Spindle Control pin to activate and set the spindle spinning.

Based on this, it’s my educated guess that there has also been a failure on the board passing voltage from the pin.

Has anyone had any experience with this at all?