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Re: 20 joules at 100 bps vs 4 joules at 500 bps

Original poster: Jim Lux <jimlux@xxxxxxxxxxxxx>

At 12:28 PM 8/2/2005, you wrote: 
Original poster: "Dmitry (father dest)" <dest@xxxxxxxxxxx>

Hello Jim.

Tl> Original poster: Jim Lux <jimlux@xxxxxxxxxxxxx>

>>where did you find that? It`s 3MV/m for DC, but we have HF AC, so this
>>value can be several times smaller, 1MV/m - i like it much more :-)


Tl> I don't know that the AC breakdown voltage is all that much different
Tl> than the DC breakdown voltage, certainly not factors of 2 or 3 different.

ac voltage causes more heavy dielectric ionisation than dc. for
example - maximum ac voltage amplitude for lots of film caps even at
low frequencies more than 2 times smaller their dc rated voltage. in
fact, ac peak-to-peak voltage couldn`t exceed rated dc voltage.


One should be careful about using capacitor failure behavior as a model for sparks in air. The systems are not very similar, and one thing about HV breakdown is that scaling and similarity are tough to use.

In capacitors, one also has to do with heating effects and mechanical dielectric stress from the reversing field, not to mention any weird transmission line effects or charge deposition on insulator surfaces.

In air gaps, though, you don't have these issues (although you do have the charge deposition on insulators problem... why the creepage distance needs to be 3 times the free air distance).

For very high (e.g. microwave) frequencies, the breakdown can also change, because the field reverses quickly enough to prevent the ionization avalanche from propagating towards the other electrode.