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RE: HV meter



Original poster: Vardan <vardan01@xxxxxxxxxxxxxxxxxxxxxxx>

Hi Gary,

At 07:29 PM 6/24/2006, you wrote:
Hi Terry:

I have a cheap eBay HV probe that I bought years ago and never used.
I've long since lost the instructions that came with it so I don't know
if it states that it may be used for AC measurement.  Basically it's a
two terminal device, with just a 990 Meg HV resistor between the
terminals.  So if one plugs it into a meter with a 10 Meg input
impedance, it works out to a 100:1 division ratio.

All the ones I have seen are 1000:1 and have a third ground wire and a ~1.11 M ohm resistor across the meter leads.


I tried it tonight on both DC and AC measurements.  With a 10kV DC
source, it read a stable 94.6V DC on my Fluke 27 DMM.  Multiplying by
the 100X puts it clearly close to 10kV.  I don't know how accurate the
meter on my HV supply is, but it's clearly in the ballpark.

But things were far less stable with AC measurements.  I used a 4kV NST
for the source.  When I put my 120.3VAC line voltage on the secondary
winding, I measured 2.91VAC on the primary, so that suggests that when
hooked up normally, I'd get 4973VAC from the secondary.  It does seem a
bit high for a 4kV NST...

Does it have any resistance between the meter leads or is it a "pure" series resistor? If it is just a resistor then it can't have much AC compensation since the signals have to travel down the leads and such where any stray AC signal will mess them up.


When I hooked the normal output from the secondary to my eBay HV probe
and my DMM, I got readings between 56-263(!) VAC, depending on which NST
lead the HV probe went to and the routing of the probe wires.  With such
an unstable reading, it doesn't seem to be of much value.

That would be the case of just a series resistor.


I noticed in the Fluke probe you cited, it forms a 3-terminal divider.
But since it also claims a 1000 Meg input impedance and it's also made
to be used with a 10 Meg input impedance meter, I don't see how the
resistor that shunts the meter can be anything but an open circuit.
Maybe having both meter leads being part of the probe assembly results
in a stable and "appropriate" capacitance between them?

The shunt resistor immediately terminates the probe's capacitance. If they want to, they can add a terminating capacitor there too to help AC response.

These probes often "come apart" if you unscrew the red long tip from the black handle.

http://hot-streamer.com/temp/HVprobeInside.JPG

In many cases, you can take out the resistive elements too. Mine is epoxy filled, but the handle has two burden resistors and a ten turn pot for calibration. The cable to the meter is shielded coax. They say they use "special thick film resistors". Fluke will sell you a replacement resistor set (Fluke PN939335).

I tested mine at 528 V on a direct meter and it read 503 V. Barely inside the 500V spec. You do have to keep the read tip part away from objects and noise since it is sensitive to capacitive pickup there. I did not worry about that in my test. I just set it on the floor since I don't like to be "holding" it ;-))

I only use it for 60Hz AC and it has always worked perfectly. Maybe you should get a "not cheap" eBay HV probe and throw the one you have away ;-))

Cheers,

        Terry



Regards, Gary Lau
MA, USA

> Original poster: Vardan <vardan01@xxxxxxxxxxxxxxxxxxxxxxx>
>
> Hi,
>
> At 05:51 AM 6/22/2006, you wrote:
> >The 40kV HV probes often seen on eBay are usually rated only for DC
> >measurements.  I've not tried them on AC, and would be concerned that
> >their very high resistance is not compensated for AC use, and may
> >deliver misleading results.  Has anyone else tried these probes on
60Hz
> >AC?
>
> They are used for AC all the time.  The usual accuracy is 5% for AC.
>
> http://hot-streamer.com/temp/80k40___iseng0900.pdf
>
> The HP 34300A is rated DC to 150Hz.
>
> Cheers,
>
>          Terry
>