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I've been trying to understand how plasma lamps (or plasma globes, according to Wikipedia) work and why you don't die a horrible fiery death when you touch them. What I've got so far is that the center electrode is fed a high voltage and high frequency input. Due to the high voltages, the gas ionizes and you get the plasma filament thingies. All this makes sense, but I have some questions I can't answer (or find the answer to online). Firstly, related to how current kills:

  1. What current is needed to harm you? I've heard of the milliamp current that causes fibrillation, but I'm talking about actual burns and people on fire. Human bodies are resistors typically on the MOhm range (as confirmed by my multimeter and me). Even with main voltages (220 where I live), that's a ridiculously small current, not even milliamps. Also, the power you dissipate is like 0.1W, which shouldn't be terribly harmful. Can you be severely burnt (in dry conditions) with mains voltage?

    Now onto the Plasma Ball part:

  2. What is a good equivalent circuit for the ball? Maybe if someone could supply an equivalent circuit, I could understand it better.

  3. Why does it need to be high-frequency?? Wouldn't a few kV ionize the gas just the same?

  4. Which parts of the lamp are grounded? It does seem to make a difference whether you're grounded or not when you touch it. Also, why does the filament concentrate on your finger when you touch it? It seems as though you're closing a circuit with ground, but this seems weird because of question 1.

  5. Why isn't this extremely dangerous?!?! Under right, but not too unlikely circumstances, if you're standing on the ground and offer a low resistance, the center electrode on the kV range would output a current on the amps range or even more (assuming the reference of the center electrode is actually grounded). This should at least fry you right where you stand, if it doesn't cause fibrillation before. I understand it would be pretty stupid to touch one of these lamps standing bare foot on the ground and wet, but you wouldn't get just shocked, a few amps would make for a gruesome spectacle, right? How can you sell such a thing to the public? I've heard people say it's not that harmful because of the high frequency but how can that be the case? Does the human body stop behaving like a resistor at high frequencies?

  6. On a similar note, if I had the lamp on a metal table and someone were to tip it over, a few kilovolts short circuited to ground have got to create some fireworks. Isn't this really dangerous? I know that probably the power supply inside won't be able to supply the amperage and the voltage will drop, causing a smaller current, but again, isn't this really dangerous anyway?

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closed as too broad by Leon Heller, Daniel Grillo, Joe Hass, Chetan Bhargava, Matt Young May 16 '14 at 0:49

Please edit the question to limit it to a specific problem with enough detail to identify an adequate answer. Avoid asking multiple distinct questions at once. See the How to Ask page for help clarifying this question. If this question can be reworded to fit the rules in the help center, please edit the question.

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    \$\begingroup\$ Too many questions. \$\endgroup\$ – Leon Heller May 15 '14 at 19:48
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ANSWER FOR QUESTION 1

Your multimeter is wrong method to measure body resistance. Human body is few kiloohms! When human is electrified - effective impedance is between 1000-5000 ohm (not kilo, not mega!).

IEC says (text from Wikipedia):

The International Electrotechnical Commission gives the following
values for the total body impedance of a hand to hand circuit for dry
skin, large contact areas, 50 Hz AC currents (the columns contain the
distribution of the impedance in the population percentile; for
example at 100 V 50% of the population had an impedance of 1875Ω or
less)

Voltage  5%          50%          95%
25 V     1,750 Ω     3,250 Ω    6,100 Ω
100 V    1,200 Ω     1,875 Ω    3,200 Ω
220 V    1,000 Ω     1,350 Ω    2,125 Ω
1000 V   700 Ω       1,050 Ω    1,500 Ω

See also: Electric Shock on Wikipedia

Human body is not a resistor!!! It's not linear. It contains:

  • electrolytes and they behave very diffrent at diffrent voltages or currents
  • cells (biological), some of them act like small capacitors and they conduct high frequency better than just a resistor

Power able to stop human heart or cause ventricular fibrillation may be less than you think.


ANSWER FOR QUESTION 2

Im not sure if this is what you want. This is very simplified equivalent circuit for plasma ball and human:

enter image description here

(sorry for bad image quality, I have only paint on this notebook)

If you calculate voltage on resistors - you will realize that guy with a mustache gets very small voltage. Most of it is on resistor representig glass resistance. Im not sure how big glass resistance may be (depends on glass type, thickness etc), but im almost sure thats more than 1Gohm. If someone think I'm wrong - please leave comment, I will correct that.

At 30kV, 1Gohm glass resistance and 10k body resistance - you get only ~3 volts and current is ~3uA.

However human is not a resistor, and glass works as capacitor. For high frequency that changes impedance a lot, and glass impedance will be much less for higher frequencies.

On my schematic capacitor is completely unknown, because it's very hard to determine it. It depends on how much skin surface is touching glass.

Capacitor impedance is inversely proportional to frequency.


ANSWER FOR QUESTION 3

I think it must be frequency high enough to pass current thru glass (as I mentioned earlier - it acts like capacitor). Plasma is one electrode, glass is isolator and your finger is another capacitor electrode.

Capacitors have low impedance at high frequency, high impedance at low frequency.


SHORT ANSWER FOR QUESTION 4

Grounded part is shown on schematic (this is simplified!)


SHORT ANSWER FOR QUESTION 5

This is relatively safe because there is a glass between you and 30kV source. As I mentioned before - it's a capacitor and very high value resistor conected in parallel. You can calculate current or voltage on human body if that "glass resistor" was 10kOhm instead of 1GOhm.


SHORT ANSWER FOR QUESTION 6

Playing with kilovolts is always dangerous.

However 15-30kV at 1-5kHz from very low power transformer should not hurt you if there is glass thick enough between your body and plasma/electric arc.

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  • \$\begingroup\$ great answer and even better drawing lol, thank you for taking the time to do this! \$\endgroup\$ – freejuices May 16 '14 at 3:33
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The high frequency feed is to create the "plasma filament thingies" instead of just lightning.(I am not sure what the proper term for the plasma formations is, so I just used yours.) At high frequencies, you don't get a shock proper; most of the time all you will feel is heat. Your body does not respond the same way at higher frequencies, partly because electricity doesn't work the same way at higher frequencies. I am not sure about the safety issues, as I haven't done any research on plasma globes (I have done a good bit of work with plasma, just not with commercially manufactured products).

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  • \$\begingroup\$ I think you are not completely correct about why high frequency is needed for plasma ball to work. It's not just for better visual effect. I think it may not work at all at low frequency, because glass capacitance plays big role here. See my answer. \$\endgroup\$ – Kamil May 15 '14 at 21:56
  • \$\begingroup\$ That could be. The kind of plasma I work with is generated at 1 atm and is concentrated electromagnetically. It behaves very differently from anything else I have run into. Very interesting stuff. \$\endgroup\$ – Delta1X May 16 '14 at 0:58

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