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Re: High Voltage Probe Instability



Original poster: Bert Hickman <bert.hickman@xxxxxxxxxx>

Or a bit of resistor dR/dV effect...

-- Bert --

Tesla list wrote:
Original poster: Sparktron01@xxxxxxxxxxx
Steve, David
Error term on my OHVM oil immersed divider network with increasing voltage followed a square law relationship. Absolute error terms (below 25kV rms) was < 0.4%. At 50kV rms (70kV BIL pk), error term was ~ 1.2%. Dielectric polarization of oil was my take on root cause of error term, more plausible with error term square law relationship with applied voltage observed...
Regards
Dave Sharpe, TCBOR/HEAS
Chesterfield, VA. USA


 > Original poster: "S&JY" <youngsters@xxxxxxxxxxxxxxx>
 >
 > David,
 >
 > I had a similar problem with a high voltage divider.  Very mysterous.  I
 > finally decided the cause was corona that was ionizing the air around the
 > string of resistors and in effect adding some parallel resistance, thus
 > making the reading creep up.  I put the string of resistors in some vinyl
 > tubing and that cured the problem, at least for me.  You could try taking
 > the cover off your probe so that air circulates around the divider
 > components and see if that reduces the problem.  If so, then suspect some
 > sort of ionization is happening.  Perhaps your probe is supposed to be
 > insulated with oil, and the oil is no longer there??
 > --Steve
 > ----- Original Message -----
 > From: "Tesla list" <tesla@xxxxxxxxxx>
 > To: <tesla@xxxxxxxxxx>
 > Sent: Monday, February 07, 2005 8:29 AM
 > Subject: High Voltage Probe Instability
 >
 >
 >  > Original poster: "David Rieben" <drieben@xxxxxxxxxxxxxxx>
 >  >
 >  > Hi all,
 >  >
 >  >
 >  > I have a question that I was hoping that some of you would have
 >  > some insight on. I have been trying to measure the output voltage
 >  > of my x-ray transformers with 1000:1 HV probe that I have. I mea-
 >  > sure the input voltage while carefully controlling it with a variac
 >  > and, at the same time, measure the output of one of the HV term-
 >  > inals to ground and thereby obtain a ratio for the output/input
 >  > voltage so I can have a pretty close approximaton of the output
 >  > of the transformer. Sounds simple enough but the problem that I
 >  > am having is that the high output voltage measured through the
 >  > HV probe keeps slowly but steadily climbing while the input re-
 >  > mains relatively constant. For instance, with an 80 volt input,
 >  > the HV output initially reads around 28,400 volts but after around
 >  > 10 minutes of constant measurement, the HV output has crept
 >  > up past 30,000 volts, at least according to my HV probe/DVM
 >  > combo. Of course, all measurements are taking with no load
 >  > on the transformer. Is my measurement equipment defective or
 >  > is this a normal phenomena? Should I wait for the output mea-
 >  > surement to reach as high as it will go and go with that as an
 >  > accurate measure or should I take the first initial measurement
 >  > as the true measure of the voltage? Could the resistor of the
 >  > 1000:1 probe be heating up and causing the instabiltiy of the
 >  > reading? Any comments are welcome.
 >  >
 >  > David Rieben
 >  >
 >  >
 >  >
 >  >
 >
 >