Re: HV Probe and DMM Input Impedance

Michael A. Terrell
 

The resistor should be smooth and clean. A goo builds up on those , from
contaminants in damp air. More expensive versions encased the resistor in a
seald, glass tube to keep it clean. Back in my early days in electronics
(>50 years ago) failed 66Meg resistors in the focus circuit of color TVs
failed from humidity, nicotine and HV across their bodies. The HV attracted
the particles from smoking, and the moisture turned it into goo. Spring and
fall were busy times, replacing HV parts in tube color TVs from homes where
people smoked. We rendered to those times as 'It's Flyback Season!' in a
Daffy Duck imitation. ;-)

On Mon, Oct 28, 2019 at 10:59 AM n4buq <n4buq@...> wrote:

I do have older VTVMs (HP 410B, 410C, etc.) so I could use it with those
as well. I have an older HP probe (459A) designed to be used with those
(has a different resistor) but it's missing the spring-loaded end-cap as
well as has a different resistor value. I took a look at that one
yesterday and noticed the resistor has a rather sticky film on it. I
wonder if that was originally there to reduce the effect of arcing?

If the resistor in my "new" probe didn't have fingerprints on it before,
it does now. I'll be sure to wipe down. Thanks for the tip!

Thanks,
Barry - N4BUQ

----- Original Message -----
From: "Michael A. Terrell" <@michaelaterrell>
To: TekScopes@groups.io
Sent: Monday, October 28, 2019 9:42:01 AM
Subject: Re: [TekScopes] HV Probe and DMM Input Impedance

That probe was made for a common, TV shop grade VTVM, with a 10M input
impedance, for AC measurements. In DC mode, a 1M resistor at the probe
tip
gave a total of 11M impedance. This was done to reduce the input
capacitance which would detune RF stages, or quench an oscillator. This
was
critical while servicing the high impedances encountered on tube based
electonics.

Add 1090M to 10M, and you get 1,100M wich is 100 times the impedance with
the HV probe. Be sure there are no fingerprints on the body of that
resistor, or it may arc over under HV use.

On Mon, Oct 28, 2019 at 9:36 AM n4buq <n4buq@...> wrote:

This last weekend, I found an EICO HV probe in very nice condition at
an
estate sale. Opening it up, it contiains a 1.09G ohm resistor in
series
with the tip and cable. I connected it to the input of my Fluke 27
DMM and
measured a low voltage source (all I had handy at that moment). From
what
I could tell, the probe gives me a 100x scale factor (e.g 10VDC
measured
0.1VDC). While I may need to measure some higher voltages to confirm
whether this is really accurate, it appears to be at least somewhat
accurate.

I have a question, though, regarding the theory of the way this works.
The Fluke has a 10M ohm input resistance which, if I'm thinking about
this
correctly, makes the measuring circuit 1100M ohms of which 10M ohm is
the
meter and the remaining resistance in the voltage divider network is
the
probe's resistor; however, I'm having trouble with the math.

Intuitively, (for me, at least), to obtain a 1/100 divider, I would
think
that ideally the probe resistance should be 0.990M ohms with the meter
providing the remainin 10M ohms. But I find it odd that the resistor
has
that odd value which makes it seem like it was almost intended to work
with
a 10M device.

If I'm not mistaken, those probes were intended to be used with a
particular device (meter) that provided the proper readings but not
sure
about that either (not finding a lot of info on this probe).

Am I off base here? I know that some of the HV probes designed to work
with the Fluke are designed to connect differently and I think the
meter is
used in mA mode with them but not sure about that.

Is it a false expectation that the meter give me a 1/100 reading when
used
with the probe in that manner? Is it also possible that the 1090M ohm
is
giving me a "close enough" with that low voltage test and the
difference
would become more measurable with higher voltages?

Sorry - this should be simple but, for some reason, I can't make it
make
sense to me at the moment.

Thanks,
Barry - N4BUQ







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