I want to use the spindle relay to control my vacuum. Enabled it, changed the default output to "1", enabled Output #1, hooked that to a relay and it works....mostly!  M3 turns on the spindle and the vac.  M5 shuts them both off, but...it also results in a "limit switch activated", requiring a reset, even though none of the axis are anywhere near a limit switch.  If I redo the whole thing using M7/M9, I get the same result.  Looking at the diagnostic page, nothing is showing.  Anyone have a quick answer??

Check for where you wires run parallel to each other.

When the coil in the solenoid deactivates, it'll create a large pulse which is probably inducing current into a limit switch wire.

You can try wrapping the coils wire around a ferrite slug. That can reduce the effect.

I'll investigate that, but wouldn't the surge you are referring to actually be a result of turning ON the dust collector vacuum when there is almost certainly some inrush current above the operating load?  Or at least you would expect one when you turn it on as well as off.

And if that were the case, wouldn't the diagnostic page show one of the limit switches tripped?  Or is that display real-time (meaning the blip wouldn't be noticed)??

When the coil de-energizes there's a pulse generated.  It's like your ignition system, how the spark happens when the points open, not close (yeah, ok, I said points - I'm dating myself!).

Oh yeah,that's not to say that there aren't spikes etc as the motor etc comes on.  It you were saying it only happens as things shut down.

Hi Guys,

It is usual practice to fit a reverse biased diode across a relay coil in order to suppress the high voltage spike (which occurs during the magnetic field collapse) when a relay is switched off. (Just for example;  http://www.hobbyprojects.com/the_diode/diode_protection_circuits.html)

Tweakie.

Okay, back to this problem this morning:

(1) The relay output on the breakout board used to trigger the high voltage (2520vac) load for the dust collector already had a MOV across the terminals, so I didn't go down that path.

(2) Moving the wires around such that there was zero overlap between the coil wires and the limit switch wires resulted in no change.  Wasn't sure what to expect there anyway since the limit switches are hall effect relays.  Not sure what an induced pulse would have caused there.

(3) But...the power supply for the coil is 12vdc unregulated.  The power supply for the limit switches is 5vdc regulated, from the same break out board.  Since I was using USB-SS, I added 5 usec of debounce to the limit switches and bingo, problem solved.  I guess switching off the coil has some impact on the 5vdc supply to the limit switches?? 

Thanks to all who pointed me in this direction!!