Re: funky voltage readings

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "by way of Terry Fritz <teslalist-at-qwest-dot-net>" <dhmccauley-at-spacecatlighting-dot-com>


Are you connecting this HV probe to your oscilloscope?  The scope is
typically 1 Meg input impedance while your HV probe has an output impedance
of 10 Meg ohms.
This mismatch is going to screw up the readings a bit.  You need to attach a
shunt resistance across the input to the oscilloscope so that the HV probe
matches correctly.
Typically, inline shunt impedance matchers are used.  They come in a variety
of sizes ranging from 50 ohms up to 50Meg ohms from what've seen.

The Captain


 > I was playing around with a HV prove and meter and noticed I get odd
 > readings. I'm using an Agilent 34136 AC/DC probe which is just 1Gohm
1000:1
 > divider for meters with a 10Mohm impedance (which I'm using.)
 >
 > DC voltages read ok for DC as far as I can tell so far.
 >
 > For AC, digital meter and scope measures the same voltages, which are
3.4kV
 > for the 7.5kV winding of a 15kV neon sign transformer, which seems wrong.
 > High voltage probe goes to one HV lead on the transformer, and ground lead
 > of the probe goes to the ground on the transformer. I get the same with
two
 > different transformers.
 >
 > The waveform is almost a clean sine wave according to scope, so I'm not
sure
 > where this number is coming from. I'd expect 7500 volts /1000 for 7.5
volts
 > on the meter, but I read 3.4
 >
 > 120 volts from an outlet give the expected readings with the HV probe in
AC
 > mode.
 >
 > Maybe soembody can explain this, or share what they get with a close
setup.
 > The probe is rated for 60Hz AC use, and should be good to 2%.
 >
 > KEN
 >
 >
 >
 >
 >