Teaching tech put out a video about using an AC powered silicone bed heater recently. There is a huge glaring issue missing from the video. To use any AC powered device you need to use an SSR (solid state relay) to do the switching. The problem with SSRs is that when they fail, they very often fail in the “on” state, meaning that even when you aren’t sending the signal to allow power, they’re doing it anyway. This means your thermal runaway protection does nothing unless it’s killing power to the whole printer and removing that AC feed. To reiterate, when they fail, they fail in a state that just allows full power 100% of the time to the device they are controlling.There are more cheap knock off dangerous SSRs on the market than good ones. And when you mount them without a heatsink as teaching tech shows, they are very likely to fail. Heat is the enemy of electronics and SSRs need a heatsink for any decent lifespan expectations. There are tons of cheap SSRs that have grossly over rating markings and cheap dangerous internals. A good reliable one is often an order of magnitude more expensive than these cheap ones. On top of that there are a huge number of knock offs labeled as the real deal, FOTEKs are often knockoffs.He also completely ignores proper grounding. Every metal part of the printer should be connected to a real earth ground run back to your wall outlet. If something goes wrong and AC power leaks to the frame it can no joke kill you. The printer should be plugged into a GFCI outlet as well.Please, if you use SSRs with live wall AC on any part of your printer, be safe and have a way for your printer to kill that power should they fail. Live AC power safety is no joke. My experience with them comes from the home brewing world where I’ve built full electric setups and the controllers that use SSRs and AC powered heaters are often quite intricate to make them safe. https://ift.tt/2mh5nQ0