How to you measure the voltage

[wbp]

Aha! Found this on Fluke's user forums:

the HV probe manual states that:
"* The input impedance of Autoranging Fluke handheld digital
multimeters varies as a function of range. The only range that
deviates significantly from 10 MW is the 3V (Models 21, 23, 25,
27, 70, 73, 75, 77) or 4V (Models 10, 11, 12, 29, 79, 83, 85, 86,
87, 88) range where the impedance is 11.11 MW. To enhance the
measurement accuracy when using this range, apply a correction
factor of 0.99, i.e. multiply the displayed reading by .99."

This doesn't exactly apply to the 117 or 289, but it does indicate that the meter is designed this way and Fluke just left this little detail out of the manual for the meter.

[mightyohm]

I'm pretty sure that the Fluke 87 manual includes this information. Not sure why Fluke decided to leave it out of the 117 manual...

[wbp]

If my calculations are right, for models 117 and 289 (the two that I have) the lowest range is 10.6 M and the others are 10 M ohm.

William

[mightyohm]

Wondering what effect swapping R5/R6 has on this?

Yeah, this is the issue. I swapped R5/R6 at some point in testing (due to the reversed silkscreen) and then optimized R8 for the swapped resistors. So if you want VR1 to cover the right range you need to make the resistors match the photo (the resistor labeled R6 is 330, R5 is 1k).

I'm doing more testing but I really don't think the swapped resistors cause any problems. They might cause the HV supply to vary more with battery voltage, but on the flip side they make the circuit more tolerant of low battery voltages. I'm going to play with the circuit a bit more but since every kit that has been assembled to date has been assembled with the "wrong" values and there have been no problems, I'm not too worried about it.

[wbp]

I'm doing more testing but I really don't think the swapped resistors cause any problems. They might cause the HV supply to vary more with battery voltage, but on the flip side they make the circuit more tolerant of low battery voltages. I'm going to play with the circuit a bit more but since every kit that has been assembled to date has been assembled with the "wrong" values and there have been no problems, I'm not too worried about it.

I replaced the 10 ohm pot with a 20 and that gives a nice range. This is with the r5,r6 as shown in the schematic.

[mightyohm]

I replaced the 10 ohm pot with a 20 and that gives a nice range. This is with the r5,r6 as shown in the schematic.

I didn't realize 20 ohm pots were even available - thanks for pointing this out.