Here’s a sure red flag the home contractor you’re considering is untrustworthy | Lrd12 Overload Relay

The MOT cores are welded to the head, so they’re not coming loose without shearing. The sole purpose of the epoxy would be to keep the coils mounted to the cores (as well as keeping them free of damaging debris that would short the coils.) The only force the epoxy sees is from the eddy currents trying to push the coils away from the core (or vice-versa.) That energy then becomes the electromagnetic force that gets “transferred” to the ferrous cores – that are welded to the head. Since the force from the eddy current is fairly weak, there’s an low extremely low risk for such a failure. Not impossible, but it would take a catastrophic failure.

Not sure where did you get “…A quick looks shows BT137 will burn 20W to switch a 5A load…”. BT137 has the following listed: On State voltage (Vt) @It=10A is 1.3V, max 1.65, so the worst case is 16.5W, more realistic 13W. Not to laugh at, but still not close to 20W

It’s *mostly* read-only. They usually have a LED for power-on indication. You could stick something to the front to detect that, if you needed a response. Tho for things like lights, usually the user will press the button again if they don’t come on the first time.

The gate resistor will eventually short to the trace after so many cycles. It may take a while, but it will eventually happen. We had that problem years ago with a product. Resistor has to be lifted up above the trace. Traces and such seem fine for 120VAC for low current loads. Wouldn’t pass muster overseas.

http://www.aliexpress.com/store/product/110v-220v-Silicone-Rubber-Heater-Bed-600MM-x-600MM-24-X24-For-3D-Printer-With-100k/709519_1847472742.html

Remember that, across the same resistance, power goes up with the square of voltage; 120v RMS can deliver 100x the wattage of a 12v supply, and 240v delivers 400x.

And for example the max continuous current one is “rated” for is 20A, and when I calculated the values without any heat sink, I should be able to run about 700mA. A lot of people forget to do this step before de-rating.

Does matter since he runs it on DC, even so the lamination are already welded at the factory, its how the transformer is assembled.

I’ve spent four to eight years in apprentice school learning how to plan, install, inspect, and repair electrical products. Don’t insult my intelligence by implying otherwise.

15A is more like 1,500 times what’s required to kill you, depending on what part of your anatomy it finds to traverse.

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The rest of the circuit would be built on the backside, with judicious use of surface mount electronics rated for miltemp operation (to 125C). Standard connectors from Samtec or Molex can handle the current drive easily, with no problems from the heat. Add an acrylic conformal coat on the backside, or perhaps potting the electronics, and you can make the entire module one single brick module.