How to measure 2KVac using a DMM

if both of the same type probe read low on your meter, then the input impedance of your meter is lower than the probes are designed for. you have 4 choices:
1) get the meter the probes are for (i know flukes are pricey, but they're worth every penny)
2) get the proper probe for your meter (better price wise, but there may not be a high voltage probe made for your meter)
3) build a resistor divider (a bit of work, and must be checked for accuracy, and the R2 shunt resistor adjusted for an accurate reading)
4) work out the math for the scaling factor of the probe you have. not the best choice, but if you can verify that the scaling error is consistent, you can figure out a conversion factor from your meter reading to the actual voltage. it's not the best solution, but math is free.

Dean Huster said:

3. Also, as I recall, unlike the HV probes of the old vacuum tube voltmeters, the Fluke probe has a built-in termination resistor so that it doesn't depend upon the meter load being exactly 10M ohms as the second resistor of the voltage divider scheme.


Dean

Click to expand...

Dean has a point here. Plug the banana plugs of the HV probe into your 187 (leave the tip of the HV probe and its ground clip unconnected to anything) and set the meter to the ohms range. What does it say?

If it's substantially below 10 megohms then the input resistance of the meter you plug it into won't matter much.

This is a link to the Fluke 80K-40 probe. Back in the beginning of this thread I actually measured the probe output impedance and read 1.1 MΩ. That would be about right.

The 80K-40 represents a 1000 MΩ load to the circuit being
measured, or 1 uA per 1 kV.

Click to expand...

Measuring for example 10KV the probe output should be about 10 Volts with 10 uA current flow in the probe. That would lead one to believe the output impedance of the probe to be about 1 MΩ. As to the 10 M input impedance of the meter used it should be 10 M +/- 1% for the probe to work within its rated (AC +/- 5%) uncertainty.

As to the ground lead and its placement:

Notice that the divider depends upon a
ground lead to complete the low side of the circuit path.
Therefore, this connection must always be secure before
attempting a voltage measurement. Otherwise, instrument
damage or a shock hazard will result.

Click to expand...

Ron

Fluke is actually depending upon the meter impedance in parallel with the termination resistor to provide the exact divider ratio for best accuracy. Differences in meter input resistance won't be as major of an effect as it would be in an unterminated probe. I was thinking it might be a safety issue, but that 1000M ohm resistor will limit current to a safe value when unconnected to a meter and a termination resistor wouldn't make much difference even with the ground connected to a grounded chassis.

Dean

The Electrician said:

Dean has a point here. Plug the banana plugs of the HV probe into your 187 (leave the tip of the HV probe and its ground clip unconnected to anything) and set the meter to the ohms range. What does it say?

If it's substantially below 10 megohms then the input resistance of the meter you plug it into won't matter much.

Click to expand...

Hi,

I leave the probe unconnected and it reads 1Mohms