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alan sailer

Tue Jun 14 2016, 03:03AM

Registered Member #59110 Joined: Mon Apr 11 2016, 04:35PM
Location: Camarillo, California
Posts: 74

This is just a quick question that I'm hoping someone can educate me about.

I'm working with a HV AC circuit that runs at about 25kHz. Based on the arcs that I can draw the output voltage is below 20kHz. (i don't have an AC HV probe). On the output of the supply I am using good quality silicone insulated HV wire that is rated to 40kV. I have used this same wire with a DC HV power supply at 20-25kV for many years and have never noticed any leakage.

However with the 25kHz HV I can touch the wire and get an alarming amount of what looks like corona (and ozone). My guess is that the AC is coupling across the dielectric and creating a surface charge which looks and behaves like corona.

Is this a crap theory? If so, does anyone have a better explanation for what I am seeing?

Cheers.

Mads Barnkob

Tue Jun 14 2016, 06:07AM

Registered Member #1403 Joined: Tue Mar 18 2008, 06:05PM
Location: Denmark, Odense C
Posts: 1968

Material properties changes with high frequency high voltage, things that was insulators before are suddenly acting like capacitors.

I can not remember the correct term to search up more information on the subject, someone else must know what I am talking about :)

alan sailer

Tue Jun 14 2016, 06:41PM

Registered Member #59110 Joined: Mon Apr 11 2016, 04:35PM
Location: Camarillo, California
Posts: 74

Mads,

Yes I'm sure that you are right. The more I think about it the more I am inclined to think that it is a capacitance issue.

The reason I am trying to understand it better is because my project will require passing this ~20kV/25kHz inside enclosures and I need to know what I need to pay attention to for either safety or performance.

Cheers.

Proud Mary

Tue Jun 14 2016, 08:24PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

That is exactly the effect you get when bringing a screw driver blade near the EHT line in a colour television set. This is normal, and is due to capacitance.

If you fed the same HV line with the same high voltage, but at 50 Hz, the effect would disappear because of the increased capacitive reactance Xc.

I use the standard 25 kV red TV EHT wire, but thread it inside close-fitting automotive silicone hose, for added protection. I don't know what the breakdown voltage of the EHT cable inside the silicone hose combination is, but I have run it at 40 kV for ten minutes or so (the highest voltage I can measure accurately), without seeing any corona tell tales when viewed in darkness, or hearing that frighteningl hiss which means it's just biding its time till it can burst out and get you.

alan sailer

Tue Jun 14 2016, 09:16PM

Registered Member #59110 Joined: Mon Apr 11 2016, 04:35PM
Location: Camarillo, California
Posts: 74

Proud,

Thanks.

I guess the best way to deal with the issue is to do as you say ie add more insulation which would decrease the leakage capacitance and thus the coupling to external ground.

What is a TV flyback driving frequency? I'm guessing it would be above the audio range to keep from driving people crazy...

Cheers.

Proud Mary

Tue Jun 14 2016, 09:43PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

alan sailer wrote ...

What is a TV flyback driving frequency? I'm guessing it would be above the audio range to keep from driving people crazy...

The horizontal line rate of US NTSC analog colour TVs was 15.734 kHz, which is above most people's hearing range.

The British 405-line TV system, which was finally closed down in 1985, had a horizontal line frequency of 10.125 kHz, very much within the hearing range of children. I remember it as a high pitched constant howl which you just had to ignore and accept if you wanted to watch TV.

Shrad

Wed Jun 15 2016, 10:51AM

Registered Member #3215 Joined: Sun Sept 19 2010, 08:42PM
Location:
Posts: 780

I'm 30 and can still hear/feel up to around 15khz
I feel continuous HF as if it would be a kind of line feeling between ears and larynx, without any better description
When bats are flying in the garden at night, I can feel their chirps as a modulated tickling... closest feeling I could think of would be when you drive on the top of a slope and feel your guts tickling

I remember the sony trinitron of my parents made this slight line feeling between ears and larynx, but the first TV set we had was a barco set from the 80's and I could hear the hiss from two rooms away

Ash Small

Wed Jun 15 2016, 01:34PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

Copper cored auto ignition leads are used by a lot of people. An extra layer of silicon tubing is always a good idea. Un-suppressed ruber spark plug caps can also br useful.

alan sailer

Wed Jun 15 2016, 04:05PM

Registered Member #59110 Joined: Mon Apr 11 2016, 04:35PM
Location: Camarillo, California
Posts: 74

I plan to look into the extra insulation suggestion.

Thanks.

Patrick

Wed Jun 15 2016, 07:46PM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

Mads Barnkob wrote ...

Material properties changes with high frequency high voltage, things that was insulators before are suddenly acting like capacitors.

I can not remember the correct term to search up more information on the subject, someone else must know what I am talking about :)

In my high voltage oil insulation and oscilloscope posts, i found a PhD paper that explained in terms of molar quantity and Coulombs the interaction of molecules at HF that start getting their weird dance on at high frequency.

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