Re: HV Meter

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

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


----- Original Message -----
From: "Tesla list" <tesla-at-pupman-dot-com>
To: <tesla-at-pupman-dot-com>
Sent: Friday, November 08, 2002 7:31 PM
Subject: Re: HV Meter


 > Original poster: "Matthew Smith by way of Terry Fritz
<twftesla-at-qwest-dot-net>" <matt-at-kbc-dot-net.au>
 >
 > Hi Dan/All
 >
 > >Neither.  I believe half-rectified with large filter capacitor will read
:
 > >Vout = Vpeak / 3.14
 > Is that Pi to 2dp or another 3.14 constant?
 >
 > >Basically you need to take the absolute value of the signal, square that
 > >signal, and simulatenously integrate that signal over one period.
 > Ay Caramba!  No wonder true RMS instruments are so pricey - wouldn't fancy
 > doing that myself ;-)

The "easy" broadband way is to use the input signal to heat a resistor, then
measure how much heat there is. (typically, it's done by actually using two
resistors, each with a temperature sensor, in a bridge. The unknown signal
goes to one, DC goes to the other, and a feedback amplifier adjusts the DC
to match the temperatures. cf. HP 3400A RMS voltmeter, for instance..)

Another approach is to use an analog multiplier chip to do the squaring.
cf. old Motorola application notes, and also some stuff from Nat Semi (I
think Bob Pease had a thing on this quite recently in a column).

Or, use semiconductor junctions to do log/antilog and square, etc.

Or, digitize the signal at a high enough rate and precision, and do it
numerically.

Or, if all you're interested in is relative measurements, and the waveform
is always the same, use any old AC meter.. the non-sinusoidal waveform
effect is a multiplicative constant..


 >
 > >Of course, you could always use just a single diode and capacitor and
make
 > >a peak detector.  The output voltage of this type of circuit would be the
 > >peak voltage - forward diode voltage drop (plus any ripple on there)
 > Aha!  Nice and simple and also the figure that I'm really interested in.
 >
 > Anyone out there know Vf of the SK HV1X diode off the top of their head?
 > (It's the one that seems to haunt 85% of the microwave ovens I've taken
apart.)
 >
 > Thanks for your help, Dan.
 >
 > Cheers
 >
 > M
 >
 > --
 > Matthew Smith            | Business: http://www.kbc-dot-net.au
 > IT Consultant            | PGP Key:  http://gpg.mss.cx
 > Kadina, South Australia  | * Tivis Project * Community Connect *
 >
 >
 >
 >
 >