Measuring EHT, high voltages

[N_Cook]

I usually use a 1000:1 divider with 300M ohm loading but I notice my other,
bigger Tektronix one, is 100 Meg. Is there a standard impedance for
monitoring the output of self-contained generators that may be used in
anything requiring high voltages.

The 40KHz one in front of me is 33V dc supply and label says 5.55KV , 0.3mA
output, but to measure that and state for general purpose use, there must
surely be some standard load for the monitoring.


--
Diverse Devices, Southampton, England
electronic hints and repair briefs , schematics/manuals list on
http://home.graffiti.net/diverse:graffiti.net/

 

[Wild_Bill]

Meter input impedances and internal components in HV probes can be a bit
confusing, at least to me.

The resistance values used in various HV probes varies widely, as you've
commented. I have one old HV probe that was originally filled with Freon,
and the resistor is (maybe) 99M ohm IIRC.

There is a section in the SER FAQ regarding HV measurements, and probe
fabrication.

FWIW, typical input impedances can vary widely also.. osilloscope inputs are
often 1M, but meter inputs are all over the place.
A DMM input could be 10M, some Sencore equipment inputs are 15M, some
Beckman meters are 22M.

My SWAG would be that many HV probes are made to specifications for some
fairly specific measurements, such as CRT HV measurement, and if the probe
is used to check other types of HV sources, a certain degree of error is
introduced. If the error is limited to a couple of percent of the total
measurement, it could be close enough for some test purposes, as tolerances
in the internal components can probably introduce a small measurement error.

All HV probes that I've used have had specification sheets or certain specs
listed in the user manuals.

I don't know how one would go about certifying an accurate measurement of a
HV output, but if the voltage accuracy is critical, the equipment
manufacturer (equipment being tested) will often specify the measurement
device (a certain model of HP test equipment, for example), and test
conditions.. ambient temperature, etc.

--
Cheers,
WB
..............


"N_Cook" <diverse@tcp.co.uk> wrote in message
news:h1akb6$qfq$1@news.eternal-september.org...

I usually use a 1000:1 divider with 300M ohm loading but I notice my other,
bigger Tektronix one, is 100 Meg. Is there a standard impedance for
monitoring the output of self-contained generators that may be used in
anything requiring high voltages.

The 40KHz one in front of me is 33V dc supply and label says 5.55KV ,
0.3mA
output, but to measure that and state for general purpose use, there must
surely be some standard load for the monitoring.


--
Diverse Devices, Southampton, England
electronic hints and repair briefs , schematics/manuals list on
http://home.graffiti.net/diverse:graffiti.net/

 

[Shaun]

"N_Cook" <diverse@tcp.co.uk> wrote in message
news:h1akb6$qfq$1@news.eternal-september.org...

I usually use a 1000:1 divider with 300M ohm loading but I notice my other,
bigger Tektronix one, is 100 Meg. Is there a standard impedance for
monitoring the output of self-contained generators that may be used in
anything requiring high voltages.

The 40KHz one in front of me is 33V dc supply and label says 5.55KV ,
0.3mA
output, but to measure that and state for general purpose use, there must
surely be some standard load for the monitoring.


--
Diverse Devices, Southampton, England
electronic hints and repair briefs , schematics/manuals list on
http://home.graffiti.net/diverse:graffiti.net/

The frequency of the generator will probably be the determining factor for
what probe you can use, Many probes available are rated for 60Hz and the
accuracy goes way down as the frequency increases, it has to do with self
capacitance and capacitance to ground. This in assuming it's an AC supply,
if it's DC filtered supply then you can disregard my above comments and a
probe made for measureing DC high voltage will be fine. The 100 Mohm or 300
Mohm will not matter in your case, your probe will not LOAD down the
generator. 300 Mohms is only going to load it to 18.5 microamps and 100
Mohms will load 55.5 microamps; you supply is 0.3 mA or 300 microamps.

Shaun
Electronic Technologist