Re: HV Probe and DMM Input Impedance

Bruce Griffiths

Likely intended for use with a Heathkit VTVM or similar which had an 11M input R (!M in standard probe tip) 10M in internal divider. Once the HV probe is substituted for the standard probe rge scaling is effectively 100x compared to when standard peobe is used.


On 29 October 2019 at 02:47 n4buq <n4buq@...> wrote:

BTW, this is what the probe looks like:

Not sure, but it appears the one pictured has a scale factor of 106 (as stated on the home-made label when used with a 10M ohm meter) so maybe that's more the case (e.g. not exactly a 100:1 divider when used that way)?

Barry - N4BUQ

----- Original Message -----
From: "n4buq" <n4buq@...>
Sent: Monday, October 28, 2019 8:36:37 AM
Subject: [TekScopes] HV Probe and DMM Input Impedance

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

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.

Barry - N4BUQ

Join to automatically receive all group messages.