Re: Reliable Moderate HV testing

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

Original poster: "Antonio Carlos M. de Queiroz by way of Terry Fritz <twftesla-at-qwest-dot-net>" <acmq-at-compuland-dot-com.br>

Tesla list wrote:
> 
> Original poster: "Jonathan Peakall by way of Terry Fritz
<twftesla-at-qwest-dot-net>" <jpeakall-at-madlabs.info>

> I am having trouble measuring the output of my flyback system. The
measurements
> vary widely. I am using a 100:1 HV probe marked " X100 For 10mohm inputs
model
> 1MA-100-10" and a Metex M-4650CR DMM, rated to 200kHz. When I hold the
probe, I
> get much lower readings when measuring AC off the flyback. I understand
that I
> am capacitively coupled to the probe and grounding it, but which reading is
> "real" me hanging on to it or not? Which represents the "working" voltage of
> the system?

None. The flyback generates short pulses, and a DC meter will only
measure 
the average value. In AC mode you will see a scaled version of the 
average value of the rectified output. Even a true RMS meter will only
show a low measurement. Conventional meters usually don't have a "peak 
value" mode, that is what you want. 
 
> Also, while hanging on to the probe, if I put the flyback output through
a full
> wave bridge, I get much less than 1/2 of what I read on the AC side.

Probably the high voltage destroyed the diode bridge...
 
> Further, when reading the AC output of the system, if I disconnect the ground
> lead, sparks will jump a good distance from the lead to the ground. This
makes
> me nervous. I tried using my mains ground, which should be isolated from
the RF
> ground of the system, and the same thing happens. What if I put a HV probe on
> the ground side? Is my DMM at serious risk?

Certainly, YES. Keep the high-voltage system and the meter grounded
while making measurements. Disconnect always the high voltage
connections
first (or better, turn everything off) before disconnecting the ground.
 
> So what can I do to get at least consistent if not accurate readings?

Use a spark gap to evaluate voltages by spark length. Or, rectify the
output of the flyback with a high-voltage diode (if it hasn't already
one),
connect a high-voltage capacitor across the rectified output, and
measure
the DC voltage in the capacitor, that will be close to the peak output
voltage, if the probe is not loading excessively. Probably you are
already approximating this, using you as the ground plate of the
capacitor.

Antonio Carlos M. de Queiroz