On eBay, I bought a Beckman HV-211-22 EHT probe for a low price.
The only problem with it was designed for 22M meters, and it's a bit hard to find voltmeters with a 22M impedance
. Most of the new voltmeters from most electronics shops have an impedance of 10M (but not all have the same impedance on all ranges, until you test them 
).
Inside was a large 954M resistor with a 1M0 divider resistor. I developed a voltage divider probe calculator (attached) and calculated that I need to change the divider resistor to a 1.055777M.
I then added a 100K 25-turn trimpot in series with the 1M0 divider resistor, so the probe can be calibrated
.
First, I made a 10:1 voltage divider probe with nine 10M 1% resistors to confirm that the voltmeter impedance is the same for all voltage ranges.
It required a lot of patience to calibrate the EHT probe when attached to a stable 600V supply (no matter what the resistive load). On some high voltage power supplies, even a 10M load cause the voltage to drop significantly, even if it is a volt or two).
Now I have an EHT probe that now works perfectly.
Attached is the calculator for the probes, which includes calculations for a probe from Silicon Chip magazine (800M for R2).
One of the precautions mentioned in the magazine article mentioned that you should insert and remove the probe with power off if possible, making absolutely sure that the ground clip is attached to the grounded (!) metalwork on the chassis.
The only problem with it was designed for 22M meters, and it's a bit hard to find voltmeters with a 22M impedance
. Most of the new voltmeters from most electronics shops have an impedance of 10M (but not all have the same impedance on all ranges, until you test them ).Inside was a large 954M resistor with a 1M0 divider resistor. I developed a voltage divider probe calculator (attached) and calculated that I need to change the divider resistor to a 1.055777M.
I then added a 100K 25-turn trimpot in series with the 1M0 divider resistor, so the probe can be calibrated
.First, I made a 10:1 voltage divider probe with nine 10M 1% resistors to confirm that the voltmeter impedance is the same for all voltage ranges.
It required a lot of patience to calibrate the EHT probe when attached to a stable 600V supply (no matter what the resistive load). On some high voltage power supplies, even a 10M load cause the voltage to drop significantly, even if it is a volt or two).
Now I have an EHT probe that now works perfectly
.Attached is the calculator for the probes, which includes calculations for a probe from Silicon Chip magazine (800M for R2).
One of the precautions mentioned in the magazine article mentioned that you should insert and remove the probe with power off if possible, making absolutely sure that the ground clip is attached to the grounded (!) metalwork on the chassis.
