F1 VSSA Blown - Unknown Cause

  • Hello all,

    Last night my F1 Fuse blew during a print, reset and showed the typical 2000C. I've ordered some self-resetting fuses and plan to do that modification. However before doing so, I would of course like to find the root cause.

    VSSA to GND is open, and the F1 fuse is too.
    I've visually inspected all related wiring and cannot see any melted sleeving or frayed wires etc. And I've also done some resistance measuring and everything is within spec, results are as follows;

    Hotend Thermistor - 123k
    24V 50W Heater - 10.8
    Heater lead to outer casing - 3M
    Heater block to Thermistor - Open
    Bed Thermistor - 129.1K
    Bed Heater to Bed Thermistor - Open

    I'm not sure what else I can check while waiting on the new fuses? And if I can't find the cause, what order of operations would you recommend for systematically testing? Attaching both thermistors and one other component at a time, and running, repeating until something trips? And if it doesn't, could this just be component degradation, or are their other failure modes that are not described in https://duet3d.dozuki.com/Wiki/Connector_and_spare_part_numbers#VSSA_fuse ?

    Thanks for reading, and I hope that we can get to the bottom of this.

  • administrators

    It could be that the hot end thermistor is shorting to the heater (possibly via the hot end metalwork) but only when the hot end is hot.

  • I've just ran a couple heater cartridges on a bench top power supply, one 24V and one 12V, once powered on and at ~300C they both have about 1.5V of leakage to the outer sleeve (as measured from the positive lead to the casing). Is this enough to blow the VSSA? However at the time I was only printing at 210C, and the leakage at that temperature is only around 0.85V...

    I'm starting to suspect my mains silicone heated bed with integrated thermistor, as its a recent addition. It does only move in Z, so cable movement is minimal, but I suppose it could still be breaking down (insulation) at elevated temperatures. I'll have to devise a way of testing that too.
    Had another look at my bed, and it appears that the thermistor is stuck to the outside of the mat, so is isolated from the element, and couldn't short without serious overheating of the silicone.

  • administrators

    AFAIR the ruse is rated at 200mA, so it needs a lot of leakage to blow it.

    Also check that the thermistor wires can't come into contact with the bed during printing, or with parts of the frame. One of my printers turned out to have the temperature sensor leads drooping slightly so that they touched the bed or the print. I don't use that printer a lot, but it the insulation on both wires had worn through and the wires were shorting together.

  • Well, I'm still none the wiser.

    I pulled out all the wires going to the hotend assembly, visually inspected them, to see nothing amiss and put them back in place.
    Added the reset-able fuse mod to the motherboard, and have successfully completed a couple prints (totalling ~3.5hrs) with no issue.

    I guess I'll just wait and see what happens.

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