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Dr. Shark

Tue May 30 2006, 01:36PM

Registered Member #75 Joined: Thu Feb 09 2006, 09:30AM
Location: Montana, USA
Posts: 711

I've got a silly question too: How much voltage can one realistically expect to come out of a flyback? My experiments have stalled, since I killed the two units I had, and I really don't understand how they could have overvolted when giving me less than an inch of sparks. When people talk here about striking arcs at 3", this equates to more than 200kV assuming the 3kv/mm breakdown strengh of air that is generally assumed, and still at least 100kV when high frequency and pointy electrodes are factored in with a generous 50% decrease in dielectric stregth of air. IIRC, flybacks are designed to run something like 20kV, so this is pushing the limits by 500%.

So, do I have to worry that my flybacks die at an estimated 50kV (2cm sparks between 1cm radius sherical electrodes)? Am I missing something really stupid here?

Marko

Tue May 30 2006, 01:48PM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

I don't know where you dug out arc-starting 3kV/mm for air, isn't it 1,1kV/mm?
At least for smaller distances, it seems that this gets pretty non-linear at megavolt ranges.

Steve Conner

Tue May 30 2006, 03:48PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

There is a very strong resonance between the primary-to-secondary leakage inductance and the self-capacitance of the secondary. Much the same as in a SSTC. I found that if you hit the resonant frequency, the thing goes crazy with corona crawling all over the plastic casing.

Starting an arc between spherical electrodes puts a great stress on the insulation. People who claim long arcs from flybacks tend to start the arc short between pointed electrodes and pull it out, which needs a lot less voltage.

vasil

Tue May 30 2006, 09:09PM

Registered Member #229 Joined: Tue Feb 21 2006, 07:33PM
Location: Romania
Posts: 506

30 kV/cm is breakdown voltages between two spheres with 3 cm radius...and it stands for DC voltages. In RF, the sparks can strike at greater distance because the high frequency oscillations of the free electrons in the RF field.

Dr. Shark

Thu Jun 01 2006, 11:10AM

Registered Member #75 Joined: Thu Feb 09 2006, 09:30AM
Location: Montana, USA
Posts: 711

The 3kV/mm figure is what you get if you just ask google for the dielectic strengh of air. It is very hard to find any values for pointy electrodes and for high frequencys, presumably because it depends very much on a lot of factors. Still I suppose some of the more experienced members could give a typical value for flyback frequencys and hookup wire electrodes? I don't know many people who have the means to do these measurements (I don't think measuring 50kV at 100kH is an easy task), but somebody must have done it at some point!?!

cbfull

Fri Jun 02 2006, 02:56PM

Registered Member #187 Joined: Thu Feb 16 2006, 02:54PM
Location: Central Ohio
Posts: 140

joe wrote ...

...When people talk here about striking arcs at 3", this equates to more than 200kV assuming the 3kv/mm breakdown strengh of air that is generally assumed, and still at least 100kV when high frequency and pointy electrodes are factored in with a generous 50% decrease in dielectric stregth of air. IIRC, flybacks are designed to run something like 20kV, so this is pushing the limits by 500%.

So, do I have to worry that my flybacks die at an estimated 50kV (2cm sparks between 1cm radius sherical electrodes)? Am I missing something really stupid here?

No you do not have to worry. I think that when people say 3 inch arcs, the arcs are not struck at that distance, they are struck at a much shorter distance and the excessive current allows the arc to be stretched out to 3" before it extinguishes.

You are right, a flyback pushed to a voltage high enough to strike an arc at that distance would likely die way before you ever got to that point. Most of us push excessive currents through our flybacks just because we can. The secondary tends to fry if we operate them at that current for any appreciable length of time.

Dr. Dark Current

Fri Jun 02 2006, 08:42PM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

Another question (not related only to flybacks): when I draw an arc, which is purple, and then try to capture it with a digital camera, it comes out as blue. why?

Wolfram

Fri Jun 02 2006, 08:50PM

Registered Member #33 Joined: Sat Feb 04 2006, 01:31PM
Location: Norway
Posts: 971

I would think it has something to do with the UV from the arc affecting the image sensor in the camera. Maybe the bright arc or the UV is fooling some automatic exposure/white balance electronics in the camera.

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