Pro 4 PM crashing when switching very light inductive load

Hello,

I have a problem with my Shelly Pro 4. I don't have another to compare it with.

The problem is when switching an external relay. It frequently causes the Pro 4 PM to crash. Sometimes it reboots. Sometimes it just shows a solid white display, and doesn't reboot.

I've tried a snubber connected either at the Shelly end, or at the external relay end. It makes little, if any, difference. [Edit - Please see the 13th post in this thread for an update, and more clarity - what I thought was a snubber is actually a MOV]

I need to install this unit tomorrow. But it needs to be reliable. I've been very impressed with the reliability of Shelly devices, but this unit isn't good.

One thing that occurs to me - could it be caused by the back emf spikes propagating via the power measurement circuitry across the shunt resistors? If so, I would be happy to cut these tracks - I'm only driving relays, so the power measurement is irrelevant.

Thank you for reading. I really hope someone has a solution!

Hello,

Thank you for your reply. It's the latest stable release. Not the beta.

It very much 'feels' like a back EMF issue. It only happens when the external relay switches off.

1st Edit - Out of desperation (in 15 hours I'm supposed to be installing it) I cut the power measurement tracks. Sadly, no improvement (but random figures for power - usually about 7.4W). I'll now try shorting the cut power measurement tracks, on the side not connected to the shunt.

2nd Edit - shorted the power measurement tracks. Still crashing.. But, now reads 0.0W, so that part works.

This is terrible. If the Shelly Pro can't reliably switch a small 230V relay (Finder Type 40.52) then I'm not sure what use it is. At my customer site there is a 25A contactor waiting to be connected tomorrow. That will probably also cause crashes. The reboots aren't so bad - it comes back pretty quickly - but the white screen crashes don't self-rectify - it stays crashed until a power cycle.

Any suggestions as to how I can solve this? I'm pretty desperate now!

Thank you again.

Thank you again for reading.

Another update. Connecting the snubber across L and the Pro 4 PM output (directly - straight into the screw terminals on the Shelly) seems to have fixed it. I've now performed hundreds of switching operations, with no crashes. So, my theory is EMC from the arcing contacts in the Shelly internal relay was causing the crash. [Edit - Please see the 13th post in this thread for an update, and more clarity - what I thought was a snubber is actually a MOV]

I'll think more about it, and do a (hopefully) final update later.