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The skin depth varies with the magnetic permeability of the material. Iron for example reacts at very low frequencies, so much so that it becomes useless for wires at 50 Hz.

2. It’s very likely that this device will need to be used with extension cords in order to find two GFCI-free outlets in different phases. That, fundamentally, is why this project is – generally speaking – a bad idea. Still, extension cords can be used safely if you don’t draw too much current through them. The use notes in the article speak to the need to derate circuits appropriately. But that said, the derating doesn’t change just because you pull 240V instead of 120V.

It should be possible. I used to have a heated tray for keeping my home brew beer and wine warm while fermenting. It was probably only about 30 degrees C but mains voltage.

I’ve never used a 3D printer but I imagine the printer nozzle is conductive – a small misalignment, the nozzle breaks through the solder-mask, your whole printer then becomes live, blowing up the PC connected to it at the same time.

Yeah really though. It’s kind of embarrassing. I’m an engineer and work in big industry (usually minerals, bulk material handling, oil and gas, doing a big robotics project right now for a wood products plant, etc.) and this would be an afterthought. No one would even question it. I mean really a no brainer. People use electric blankets on their bed. I really don’t get it. It really shows how amateur hour most of the people who comment on this site are.

Reading at least the related article and a datasheet before making comments about something is quite a heavy task to ask this days…

You are right, FR-4 has a glass transition temperature of around 120°C, so if you heat up your traces above that, and take into account the mechanical stress from expanding copper traces, delamination is something waiting to happen sooner or later (especially if you design corners in your traces). Add to that the fact that you usually don’t measure the temperature directly on the traces to regulate input power, you could go way beyond 120°C in your copper traces. I don’t think a lot of DIY guys actually know that there would be high-TG materials on the market better suited for PCB based heater applications…

Sources: licensed mechanic-level electrician Joe Amato; licensed master electrician Johnny Goudie, owner of John Goudie Electric in White Plains, Maryland; master electrician and electrical contractor Al Hildenbrand, owner of Al’s Electric works in Minneapolis, Minnesota; Dmitri Kara, senior executive of London-based Fantastic Handyman

If you had the choice between two identical-looking boards, with the same pictures, and review ratings, would you spend an extra 20¢ on the more expensive one? That twenty cents really adds up, you know- if I was buying 1,000 ESP-12 boards, for example, that would be two hundred dollars saved, and you would be pretty happy with yourself. That is, if it works. Otherwise, you feel really dumb.

I think that’s technically just thin graphite. Graphene is 1 molecule thick sheets. People were gobsmacked when they discovered you could make it with sellotape.

Actual 220V is present elsewhere, and is a convenient global average of 240V (UK,) 240/208V (NA), 230V (EU), 200V (Japan, though like NA it’s spit to 100V at normal receptacles). So if a manufacturer designs for 220V +/- 10%, it’s probably usable worldwide.

FR4 is actually quite a good insulator. It’s only real failure is when you have impurities on the surface of the PCB in very high humidity conditions. The impurities will cause the surface tension of the water molecules in air to lower and as this happens the water will condense onto the PCB. The water on the PCB reduces the effective creepage clearance and can cause a flash over or arc.