Re: HV Meter

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

Original poster: "Ed Phillips by way of Terry Fritz <twftesla-at-qwest-dot-net>" <evp-at-pacbell-dot-net>

Tesla list wrote:
 >
 > Original poster: "Mccauley, Daniel H by way of Terry Fritz 
<twftesla-at-qwest-dot-net>" <daniel.h.mccauley-at-lmco-dot-com>
 >
 > The meter is labelled as being electrostatic:
 > 1) Am I correct in assuming that this means that it's DC only?
 > 2) If (1), would an MO diode and capacitor make adequate rectification for
 > it?
 >
 > Yes, any typical rectifier configuration should work as long as your
 > components are rated as such.
 >
 > 3) Would I be reading peak or RMS with such a half-wave arrangement?
 >
 > Neither.  I believe half-rectified with large filter capacitor will read :
 >
 > Vout = Vpeak / 3.14
 >
 > For full wave rectification with large filter capacitor will read :
 >
 > Vout = (Vpeak * 2) / 3.14
 >
 > As you can see, it can be very difficult to implement an analog circuit
 > capable of getting
 > true RMS measurements.
 >
 > Basically you need to take the absolute value of the signal, square that
 > signal, and simulatenously integrate
 > that signal over one period.
 >
 > 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)
 >
 > Dan

1. The DC value of a rectified sine wave is about Vrms * 1.4 (square
root of 2).  If there is no load on the circuit it doesn't matter
whether a half-wave or full-wave rectifier is used.

2. The force-balance electrostatic voltmeters I have used respond to AC
as well as DC; since it's a panel meter I suspect that is what you
have.  I don't know whether they read average or rms voltage in ac
measurements, but wouldn't be surprised if it were the latter.

3. In looking on the web I find that there are other gadgets called
electrostatic voltmeters which uses principles other than the classic
force balance (field mills, for example) and suspect they have peculiar
responses to AC.

Ed