How to measure high voltage with low inductance means

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3l
Posts: 1866
Joined: Sat Jul 07, 2001 3:51 pm
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How to measure high voltage with low inductance means

Post by 3l »

Hi Folks:

In working with very high power systems, I have found this probe
to be very usefull.
It does not use resistive dividers which can burn at high amperage... or require insulation voodoo to work.
But it uses the extremely old tech trick of voltage division by capacitors. We are talking Volta and Edison here .
To design it is simple as pie, simply take the desired voltage to be measured and divide by ten. Thats all there is to it.
We don't really care about capacity at all (Perhaps only as a discharge hazzard!!), the smaller the better. Only the caps rated voltage counts here.
The divider I have in the powerpoint pict is a probe that I will be using later this month. it is made with (you guessed it )10-470 pf @ 30 kv ceramic caps. Please note the whole gismo is in oil.
Oh yeah you take measurements at the last capacitor nearest ground. I use a B&K HV Probe good for 30 kv for readout.

To get the "real" voltage multiply the meter probe's reading by 10.. is that simple enough?
For no extra charge (pardon my pun) the unit will retain that voltage until the divider is discharged....sample and hold especially for a high speed pulse work. (with proper design)

NOTE:
Please remember to put a safety discharge with the probe..
or it will bite you with the applied voltage...Yikes.

Happy Fusoring!
Larry Leins
Fusor Tech
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DaveC
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Re: How to measure high voltage with low inductance means

Post by DaveC »

Larry -

Depending on how high you are trying to measure, the Jennings Vacuum AC Hv dividers are quite good. Typically you just use a quite small HV capacitor (pF) as the equivalent to the very high resistance resistor in a DC divider, and a rather large(nF or uF), low loss capacitor at the low side. Quick voltage division ratios come from the ratio of small cap to large cap.. Thus a 10 pF HV cap in series with a 1nF low end cap will yield a 100:1 step down.

For the high side capacitor is it best to use the smallest possible capacitance, otherwise (especially true for pulse conditions) you will load the output. Of course if your output is GW then you dont care, but normally you do.

Dave Cooper
3l
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Joined: Sat Jul 07, 2001 3:51 pm
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Re: How to measure high voltage with low inductance means

Post by 3l »

Hi Dave:

At this time 100kv is my working voltage.
I just had the ten...470 ceramic caps left over from my half wave cascade. The most my cascade can crank out is 105 kv @ .1ma
at 60 hz but I get 40 ma at 20 khz.
The wakefield stuff starts at 50 kv and goes up to a reported 300kv !
Humm... saw a bunch of fairly cheap vacuum caps at
Nonesuch on Ebay when I bought my 5kv @ 10 amp rectifier
GGM-1 tube. Great idea Dave!

Keep on Fusing!
Larry Leins
Fusor Tech
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