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I remember having seen microwave ovens without the earth connection.

My first question is : Is it correct that if you touch the MOT transformer chassis, but leave the other terminal floating in air without touching anything, you won't get a shock? Since there is no path for the current to flow back to the other terminal.

My understanding is, before touching the secondary winding, the whole secondary is floating, so the whole secondary winding is like a capacitor in air. Now when you touch one terminal of it, the current in the whole secondary winding has no other source, so only a tiny amount of charge flows through you body to bring the terminal you touch on the secondary to ground voltage, 0V. So, in conclusion, it's safe to touch a floating HV transformer as long as the other terminal is well insulated.

But that doesn't seem right...

Edit: In reality though, perhaps the secondary winding is not totally insulated from the core, so part of the HV in the second winding can arc to the core then arc to the mains, which flows through the ground and complete the circuit, which gives a shock?

My second question is : Suppose for safety reasons I grounded one terminal of the MOT transformer. How do I check if the ground wire really works (i.e. it is really connected to the metal plumbing below where I stand?) My friends says to connect the chassis to a water puddle on the floor through a resistor and use the multimeter to measure the voltage between the resistor. Is that the usual way it's done?

Edit: I want to construct a sputtering chamber, the chassis should be grounded, while inside the chamber there will be a high voltage electrode. Using a flyback transformer or a MOT, the high voltage can be generated, but I'm still not comfortable connecting one terminal to the chassis.

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    \$\begingroup\$ Don't do any of that, buy another Microwave. \$\endgroup\$
    – KyranF
    Oct 21, 2014 at 18:09
  • \$\begingroup\$ I want to learn how it works so I can determine myself which way is safe and which way is not. \$\endgroup\$
    – seilgu
    Oct 21, 2014 at 18:09
  • \$\begingroup\$ Perhaps dealing with high voltage should be left to professionals. Also, electronics stack exchange is not the place for asking about consumer goods repair! The fact that you could die horribly is good cause not to place around with things you do not understand - and asking questions on a forum is a little better than asking your friend, but still... \$\endgroup\$
    – KyranF
    Oct 21, 2014 at 18:12
  • \$\begingroup\$ I'm not asking about consumer goods repair. I'm constructing a sputter chamber and want to learn about high voltages. \$\endgroup\$
    – seilgu
    Oct 21, 2014 at 18:12
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    \$\begingroup\$ Re: on hold. Huh, As far as I can tell he's not even going to use the microwave component. This is a question about HV grounding and safety. I'm far from an expert, but it seems worthy of a question and answers. (+1...) \$\endgroup\$ Oct 22, 2014 at 0:54

1 Answer 1

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I'm not a HV guy.
I'd never touch part of a HV circuit while it was powered. (at least on purpose, we all make mistakes.. 5kV across my finger once.) Disconnect the power and then short all the nodes to ground with a grounding rod.

This says that one of the secondary taps is already connected to the core.

RE: Checking ground connections. Sticking the transformer is a puddle of water sounds like a crazy idea. (Are these people really your friends?) I certainly wouldn't do it with the tranny under power. If measuring the resistance with a DMM is not good enough for you. Then I might get a low voltage DC bench supply, push current to ground and measure the voltage drop.

Above all be safe.

Edit: Just an addition,

I was thinking about some current limit for your circuit. (A big inductor as in link above, or an active thing.) How much current does DC sputtering need? (Do I understand that you are bending the current around with magnets?)

And if you wanted to check for HV before you touch something you could make a little probe with an led and resistor. (100k or so?)

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  • \$\begingroup\$ I don't want to touch it also, but since the chassis of the chamber is one part of the electrode, I will have to touch it to turn on valves. To test the grounding, I thought about using DMM with one terminal on the chassis, one terminal on my hand, but suppose there really is kV voltage difference between me and the chassis, either the DMM gets destroyed or I get killed. So measuring with a resistor seems a good idea, but how to connect the resistor to the floor? You can't solder it on the floor, that's why he suggested water.. \$\endgroup\$
    – seilgu
    Oct 21, 2014 at 19:00
  • \$\begingroup\$ Can you elaborate on how to use the DC bench supply to test the grounding? One terminal to ground on wall socket, the other terminal to? \$\endgroup\$
    – seilgu
    Oct 21, 2014 at 19:05
  • \$\begingroup\$ OK a series resistor is a good idea when checking HV. (I blew up a good DMM with HV.) Re: checking ground. I was picturing one wire into the wall outlet ground, and the other connected to the chassis. (But I have no idea is this is an "approved" method for testing ground.) \$\endgroup\$ Oct 21, 2014 at 19:22
  • \$\begingroup\$ Look up 'high voltage probe' and ask why all the fins? Because the energy finds the path of least resistance. So a 100Mohm resistor is a good idea, unless there is any oil moisture or dirt on the surface, making a lower resistance pathway. The ceramic fins are there to increase the length of any unwanted outer surface pathway. \$\endgroup\$
    – MarkU
    Oct 21, 2014 at 20:31
  • \$\begingroup\$ With all microwave oven transformers (MOTs) the magnet wire on the secondary will always be connected to the MOT. The reason why there is not a earth connection on the MOT is due to the breakers, switches, and, or any form of disconnect having a very very very large possibility of constantly arcing across the disconnect. And damaging other devices. There is a ground that goes into the microwave but it goes to the fan. The MOT is made and insulated for if something goes wrong. And always do research before messing with microwaves. And always take it as slow as needed to be completely safe. \$\endgroup\$ Jan 21, 2019 at 19:57

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