Re: funky voltage readings [solved]

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

Original poster: "Crow Leader by way of Terry Fritz <teslalist-at-qwest-dot-net>" <tesla-at-lists.symmetric-dot-net>

Sent: Saturday, June 14, 2003 12:18 PM
Subject: Re: funky voltage readings


 > Original poster: "Terry Fritz" <teslalist-at-qwest-dot-net>
 >
 > Hi Ken,
 >
 > I have done that test many many times with similar probes (mine is the B&K
 > version) without problem.  It sounds like you have done all the tricks
like
 > check it against the 120 VAC line and tried two different NSTs.  I can't
 > think of anything that would cause this.  But, It really "should" work
just
 > fine...
 >
 > Those "same" probes are also sold by Fluke, B&K, Tektronix...  They are
 > well proven and free from funkyness for tests like this.

I found a manual for the Fluke version of this probe:

http://energybeam-dot-com/resources/Fluke80K40.pdf

It seems Fluke may be the one who makes this probe for everybody else. The
math on that sheet is right on with my readings that were about 50% of what
they were (1Mohm meter impedance vs. 10M as in DC, which any DMM can do.)

The schematic shows the probe is just two resistors, and unless you have
10Mohm meter impedance, you're going to get off readings unless you apply
some corrective math.

I tried the probe again with only a scope connected, and the voltage reading
was around 7V for about 7000 on the tranformer, probably higher because the
scope probe is more than 10Mohm. Amusingly, a hamfest special Fluke 73
series II meter (inexpensive and older) had direct readings of 7 to 7.3
volts. When I connected the 34401 benchtop DMM, voltages dropped to about
3.4 again.

Lessons learned:

- An expensive digital meter can be worthless for AC readings. I'm glad I
leaned then Agilent 34401 meter has a 1Mohm input impedance in the AC
ranges, which is poor if not disturbing.

- a $20 hamfest DMM can have superior AC input impedance. Play around, check
specs.

- Be careful when using big stick high voltage probes, you can can easily
get trash readings that may be way low. It seems the Agilent site says the
probe I'm using is only for use with the 34401 meter, and can be used for
AC, but there is no info about how direct readings in the AC range are
wrong.

- instead of CE compliance and other nonsense stickers on the probe, there
should be a warning aside from "use the ground lead."

Maybe somebody has a HP 400 series AC meter, or Leader millivolt meter
that's for sale. Ebay is a great way to waste a week looking at the HP
meters before some fool pays too much at the last 15 seconds.

KEN