High Voltage Probe Instability

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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