Piezo probe issues and follow up question to avoid repeat



  • hi all, keeping in line with what I seem to be pretty good at these days, I just finished frying another Duet Wifi (not sure whether it's repairable, but I do have lots of fuses just in case now!), but more depressingly, I also simultaneously fried one of Moriquendi's latest 1.21 Piezo Z-Probe boards that I just got. I was attempting to route wires via a more concealed pathway and, then once I thought I had wired it up correctly (I even checked for continuity to ensure that the correct wires were terminating where I had thought they were…), I flipped the power switch on and a miniature fire broke out on the Z-probe board, obliterating the portion furthest from the pin headers.

    The reason for this post is that, seeing as to how I tried to be careful, utilizing my trusty multimeter and such, I am left wondering whether it's possible that the short came via the way I have mounted the piezo disc atop my hotend, and was wondering if one of you smarter guys than I could please tell me where to test (before connecting the second of the two Z-probe boards I recently acquired and probably/possibly burn that one out, too) while the power is off to determine whether there's a short that will lead to another zapped set of componentry, which would be a damn shame -- twice in a single day -- i think not.

    Thanks,

    Jonathan



  • Hi Jonathon,

    You do seem to have a lot of electrical shorts, is it ESD? Do you have static electricity inducing clothing or footwear? Probably not but it might explain the issue.

    Presuming its not that. Ensure the +,Sig and - are the right way around on the piezo board and on the duetwifi. The piezo disc should not be in direct electrical contact with the hotend itself. Place a fibre washer or printed disc between the piezo and the hotend if necessary. Test from your hotend to the PSU negative and then positive DC terminals (not 120v!) measuring DC volts. I read between 2-7 millivolts with heater on, which is probably via the thermistor.

    Use your multimeter from DC positive PSU to the piezo disc measuring volts I read millivolts and microvolts but nothing more on either the positive or negative of the piezo disc.

    If any of these read more than a few micro or millivolts i.e. a few hundred millivolts or any volts then something is amiss, consider heater cartridges, if they short to the hotend they will send 12/24v down the thermistor leads and I am lead to believe blow the VSSA fuse.

    Other than that I'd say be very wary to adjust anything with the power on, or PSU capacitors charged, so disconnect the 120v and allow the caps to drain.


  • administrators

    It's not uncommon to get a short inside the heater cartridge between the element and the case. This puts the hot end metalwork at 12V or 24V, and if that shorts to the thermistor ground lead then the VSSA fuse blows. If the piezo is also in contact with the hot end metalwork it could also account for your recent issue. So I suggest you check the heater cartridge.


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