A Linux-powered microwave oven (2016)
lwn.net
lwn.net
Yes, microwave ovens are ubiquitous and are in just about every home so they seem a harmless enough appliance - and they are if one uses them as intended.
Inside however lurks great dangers. Transformers in, say, 1kW MW ovens are truly lethal devices. They supply well over 1kV at a current in excess of one amp! Given that one only needs about 70mA across one's chest to call it curtains, simple math says that these transformers have much more than ten times that capacity.
With such a high voltage one doesn't even have to contact a terminal to be electrocuted, a small arc onto one's hand is all that it takes.
I hate giving lectures like this because I sound like a lecturing PIA schoolteacher but I speak from personal experience. I've had several nasty encounters with electrons where I've definitely come off second best.
Certainly the worst was when I accidentally came in contact with the secondary winding of a transformer that supplied 850V at 125mA at full load - that's far short of what a MW oven's transformer is capable of delivering.
It was a momentary contact and my muscles recoiled rather than gripped (as they so often do in these circumstances) - had they gripped then you wouldn't be reading this now.
I was in the middle of the room and was thrown back against one wall and the transformer hit the opposite wall with sufficient force to put a dent in the brickwork. I couldn't move - essentially paralyzed - for about a half hour and no one was around to help me at the time. I was very lucky to survive.
Again, remember your microwave oven transformer is considerably bigger and more powerful than the one that zapped me.
As I've said on previous occasions, MW oven transformers would make good substitutes for those used in electric chairs, they'd be equally as effective.
Horrible, gory thought but true.
This doesn't happen under normal operation, because the chassis allows for a short and/or a ground fault. But when you take that chassis off, and especially when you take the transformer out, you're putting yourself into a life threatening situation.
TL,DR: Microwaves cook things, including you.
Much of my learning was on vacuum tube technology (often of the HV transmitting kind) and almost invariably anode supplies etc. are isolated from the mains so relying on ground-fault detection is, as you say, a waste of time. Actually, a microwave oven is a high voltage vacuum tube (magnetron) transmitter and should be treated as such.
My experience has been that it's easy to become careless and blasé through familiarity and that's when nasty things happen.
Every time I've been zapped (and it's been many) it's been through carelessness. Fortunately, most of those shocks were of the 250-400V DC variety inside a chassis where it was only my hand that was between the 'hot' point and earth.
Nevertheless, I've also had a few very close shaves and I shudder even now whenever I think about them. One was on top of a microwave tower in a rainstorm whilst adjusting a klystron that had slipped outside its AFC/S-curve range. It was fed with a -1500V (on the heater circuit which one usually thinks of as being at ground potential) and the other supply was +800V at the collector. Water was everywhere. To make matters worse, at the time I was standing on wooden 2x4s outside the tower which also supported the link. It was over 150' to the cement below. Right, I must have lost my presence of mind.
I don't mention that just for dramatic effect (after all, it's quite embarrassing that I could be that stupid), rather it's to show how easy it is for this 'blasé' effect to take hold when one's wrapped up in the minutiae of what one is doing.
One thing I've learned is that once one's equipment hits 400/500V and up things become very different and much more dangerous. For starters, component layouts often change due to large components and HV wiring often isn't located in just one part of the circuit but rather it can amble widely. Moreover, manufacturers aren't consistent in labeling the HV parts of the circuit. Where possible, one should always try to keep one of one's hands in one's pocket. ;-)
In some ways the problem is worse nowadays as everyone is now used to working on low voltage solid state devices, thus most will never encounter very high voltage electronics until they open their MW ovens and come across its magnetron's HV power supply. It's a worrying problem as many such ovens have inadequate or no HV warnings inside the case.
Incidentally, not long after posting my comment I noticed a few people down-voting it without comment. Sometimes I think I should not interfere and let evolution follow its normal course (as it nearly did with me).
Often you'll catch downvotes for playing devil's advocate, even when it's very justified like in this circumstance.
I'm one of those low voltage DC noobs and I was still shocked when I saw the title... I was even more shocked by the wood burning article - people are dying playing with this stuff. If anything that Linux-powered microwave article should have a huge warning at the top outlining the risks.
I appreciate you taking the time to educate on the dangers + sharing your anecdotes.
Perhaps I'm just naïve and need more education in the matter of the human condition.
The last time I raised this matter I also mentioned neon light transformers which are also used for creating Lichtenberg figures. It's likely that these transformers are even more popular for that purpose.
They're also very popular for producing Jacob's ladder arcs, there's any number of videos on YouTube about them.
Neon light transformers are also extremely dangerous as they can easily deliver several hundred mA across one's chest. At that current one's pretty much history.
I was load testing it and whilst it was rated at 850V at 125mA it was very conservatively rated, it could handle well over 200mA without any significant voltage drop (as one would expect from a transformer made to mil specs).
Source: I was in the room for the original conference talk.
The article shows a microwave oven with its cover off and being modified without any adequate safety warnings and that's unacceptable and irresponsible. QED!
In case that's not clear, the conference organizers/authors of the article were careless and irresponsible for not discussing safety at the outset. You need to aware that microwave ovens and the misuse of their transformers is a major cause of electrocutions.
BTW, I'm no whingeing safety touter or campaigner by a long shot, and I'm not timid or scared of electricity but I'm respectfull of it. In the past I've had to climb FM and television masts with the transmitters still running and power to the antennae - many 10s of kW of RF energy radiating. There was so much RF energy that it used to arc off the tower and onto my knees and burn holes in my jeans and turn the LCD in my watch a black color. Nevertheless, that work was far less dangerous than working on a microwave oven without taking adequate precautions.
Incidentally, my training and certification requires me to bring such dangerous situations to the attention of those involved.
I can't be the only one?
https://www.lg.com/us/cooking-appliances/lg-LMC0975ST-counte...