I needed a high voltage probe for measuring the voltage from my HVG10 high voltage power supply. It puts out DC voltage using a Cockroft Walton voltage multiplier (so DC with ripple). So my probe is intended to work with DC only. To do this I followed the very helpful instructions on the Basics of High Voltage Probe Design page. I also drew inspiration from the amazing probe shown on the High Voltage Probe (> 50kV) page. Special thanks to Zoltan Losonc for all his valuable help. The calculations I made were for 50kV and 150kV. I say it is a > 40kV probe because the probes you commonly find in electronic shops usually top out at 40kV. In the end, safety measures we added which will prevent it from going over 250kV. How good it is up to 250kV is unknown.

The divider network for my probe consists of 25 resistors of 200M ohms +/- 1% each from Caddock. The part number is MX440-200M 1% and they were $9.56 US each. My DMM has an input impedance of 10M ohm so I use a 10M owm resistor for measuring across. This also provides an alternate path for arcing instead of my DMM. This combination gives me a dividing factor of 1000. So 50kV would read as 50V on my DMM. In hindsight I should have bought higher ohm resistors so that my probe could be shorter.

2 corona gaps were put in place at 2 different points along the length of the probe. My DMM has a maximum voltage rating of 1000V and I don't want to exceed it. However, before 1000V would be reached at the DMM the resistors would long since have broken down. To prevent this a maximum output voltage of 250V will be allowed. So the corona gaps are designed to create an ion path (corona) when 250V would be reached on the DMM. The calculations for where to position the gaps and the method for how to determine the gap size were worked out by Zoltan Losonc and can be found here.

Schematic for the probe.

The final probe. From the end of the connectors to the tip of the probe it is 1.27 meters (50.25 inches).

The connectors.

The probe tip.

Construction

• July 12, 2003 - Only up to 46kV
• July 13, 2003 - Maxed out on variac without arcing (62kV)
• July 17, 2003 - Solved the battery measurement problem
• July 18, 2003 - Testing power line AC
• July 20, 2003 - First high voltage measurements with oscilloscope
• July 26, 2003 - Soldered resistor chain into final form
• August 4, 2003 - Corona gap progress
• August 11, 2003 - Corona gaps finished
• August 11, 2003 - Stand-offs attached
• August 23, 2003 - Waxing the connections
• August 24, 2003 - PVC tube construction and chain insertion
• August 24, 2003 - Testing

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