The speed controller of a lathe I have has exploded. I have located a replacement, but while that is shipping, and before I install it, I want to understand why the speed controller exploded. Here is a partial schematic, the most relevant bits, deduced from the circuit board. Visual inspection shows a rectifier diode used its anode leg as a fuse, and one small surface-mount capacitor disintegrated itself.

Partial schematic of speed controller

What I do not understand is why the controller failed in this way. (I am not an EE engineer, but I can understand the basic idea behind this controller.) I can understand the rectifier failing if the motor shorted out. (Its rated 8A while the gated diodes in the bridge are rated 15A) However, so far the motor checks out, no obvious shorting, but its still being tested. And I do not understand why C6 failed, or how it could have caused the rectifier to fail.

Any enlightenment or suggestions would be appreciated.

  • \$\begingroup\$ Your varistor may have blown on the previous indirect-lightning surge then along comes the next one... \$\endgroup\$
    – Andy aka
    Aug 28, 2021 at 8:22
  • \$\begingroup\$ Cracks in surface mount ceramic caps can happen spontaneously and they create a short circuit with very high amperage rating equivalent . The disintegrated ceramic cap is probably your smoking gun assuming it is indeed ceramic (MLCC). \$\endgroup\$
    – crasic
    Aug 28, 2021 at 8:29
  • \$\begingroup\$ Here is a relevant answer I wrote on another question electronics.stackexchange.com/a/467023/1729 \$\endgroup\$
    – crasic
    Aug 28, 2021 at 8:31
  • \$\begingroup\$ Rather a bad and dangerous controller you have. The gate driver is galvanically isolated, but all other isn't. Also the schematics is weird, in my opinion it is fake. \$\endgroup\$ Aug 28, 2021 at 9:26


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