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I made a 450v coil gun. For control over thyristors I use PC817 optocouplers with ESP32. For gate voltage on thyristors I use 1.5V AA battery. I don't know why but my PC817 keep violently blowing up(picture). The circuit is very simple. What could cause that?enter image description here

P.S. The always blew up while they are closed!!!

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    \$\begingroup\$ Please show a schematic. \$\endgroup\$
    – qrk
    Nov 21, 2023 at 21:55
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    \$\begingroup\$ My guess is the gate current required for your thyristors is well in excess of what the little pc817 can switch. We can only guess as no schematic has been provided. A pulse transformer might be a better choice of isolated triggering. \$\endgroup\$
    – Kartman
    Nov 21, 2023 at 22:11

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I would guess that you have a sneak or accidental path (perhaps to ground from your supply) that causes high voltage at considerable fault current to appear across either the phototransistor (less likely, imo, but could also be the LED).

Simply applying a few volts to the phototransistor won't (can't) cause that kind of drama even with a dead short.

Something like a grounded oscilloscope probe, a loose wire, a wrong connection or a programming connector could do it.

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