Re: HV Measurement - Back to Basics

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

Original poster: "Jim Lux by way of Terry Fritz <teslalist-at-qwest-dot-net>" <jimlux-at-earthlink-dot-net>

No, the low voltage end... but, again, it doesn't have to be a precision
rectifier... all you care about is that the current goes through the meter
coil winding the right way. A cheap bridge made of 1N914s would work
admirably, as would 1N4001s . The small forward voltage drop of the diodes
(which changes with current) isn't really a problem in a system like this,
where most of the voltage drop is in the resistor string.  Likewise, if you
don't have well characterized diodes, it makes no real difference.  The HV
and the big resistor essentially act like a current source, so small (<100
V) voltage drops in and around the sensor are immaterial

If you were trying to accurately measure something like a 5V rms AC signal,
then the few tenths of a volt drop across the diodes does become an issue,
and there, precision rectifiers are important.  These days, one would use
some form of circuit using an op-amp to get rid of the forward voltage
issue.  Of course, you might also need to worry about thermoelectric
potentials, etc.

Now, with the meter that he has, which has a dropping resistor inside, it's
a bit trickier.. you'd have to use diodes with a voltage rating suitable to
the application, but a 200PIV bridge would work admirably.  Again, the meter
will read what the current is, and that current is almost entirely
determined by the series resistor from the HV.

----- Original Message -----
From: "Tesla list" <tesla-at-pupman-dot-com>
To: <tesla-at-pupman-dot-com>
Sent: Sunday, January 05, 2003 11:22 AM
Subject: Re: HV Measurement - Back to Basics


 > Original poster: "David Speck by way of Terry Fritz <teslalist-at-qwest-dot-net>"
<dave-at-davidspeckmd-dot-org>
 >
 > Jim,
 > If you were not going to use a precision rectifier on the low voltage end
 > of the chain, then you would have to use a high voltage rectifier before
 > the dropping resistors.  I guess it's about a wash in terms of difficulty
 > -- building a 15 kV full wave bridge for the high end vs. a precision
 > rectifier at the low end of the divider network.  I guess that if you had
a
 > set of HV diodes available, then it wouldn't be too hard to tackle the HV
 > end approach.
 > Dave
 >
 > Tesla list wrote:
 >
 > >Original poster: "Jim Lux by way of Terry Fritz <teslalist-at-qwest-dot-net>"
 > ><jimlux-at-earthlink-dot-net>
 > >
 > > > tool, and make it the resistor closest to the meter, and hence,
ground.
 > >Of
 > > > course, I am assuming that you are measuring a DC voltage.  If you are
 > > > measuring unrectified AC MOT output, you will have to add a precision
 > > > rectifier circuit, a much more complicated issue.
 > > >
 > >
 > >Don't need a precision rectifier... The few tenths drop in the diode
bridge
 > >is insignificant compared to the huge drop in the resistor.  You
calibrate
 > >it empirically anyway, so the RMS/Average/Peak issue doesn't arise (that
is,
 > >you put in 15 kV ACRMS, adjust a trim pot til the meter reads FS, and
you're
 > >done).  If you were trying to measure a few volts, or make average
voltage
 > >measurements with varying waveforms, then a precision rectifier might be
 > >needed.
 > >
 >