Re: HV Probe and DMM Input Impedance


On Mon, Oct 28, 2019 at 08:36 AM, 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.

Barry - N4BUQ
Barry, the Eico (and other manufacturers as well) made those probes so that the resistor could be changed to accommodate the particular VTVM or multimeter you needed to use them with. In the case of the 1090 Meg resistor, it was to be used with a meter having 11 Meg input resistance. That would calculate to a 1000:1 divider.
So, to use your HV probe with a meter having a 10 Meg input, you would need to add an additional 1 Meg resistor into the probe (or wherever you could make it fit) so that the probe works into 11 Meg.

Dave M

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