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Qidi 'engineering' strikes again: https://www.reddit.com/r/3Dprinting/comments/1jnuju1/rqiditech3d_permanently_banned_me_for_warning/
tl;dr - a Qidi Plus4's SSR (which we have been warning about since day freaking 1) finally blew and caught fire, with serious consequences.
I've said it before and I'll say it again: do not buy Qidi printers, don't suggest others buy Qidi printers. This isn't just sloppy engineering, this is outright dangerous.
@koz @3dprinting I want to implement an analog safety for this case (heater mosfet/ssr fails closed):
Crowbar circuit that's essentially an xor gate on heater mosfet gate and output. If they don't match, it closes another chonky power mosfet shorting the psu and blows the fuse.
@koz @3dprinting (For mains heater, this would blow the circuit breaker at the panel, but would need secondary mains rated ssr for the crowbar.)
@koz @3dprinting Crowbar circuits in general are an awesome underappreciated family of safeties I love.
Instead of trying to shut off current, which won't always be possible, provide a sudden alternate zero resistance path for the current to take and let nature take its course.