DIY 500x High Voltage Passive Oscilloscope Probe

[leblanc]

** I will not responsible for any injuries or for any damage this may cause to your equipment. Use the following design at your own risk and use caution when dealing with high voltages! Always review your connections before applying power to the system. **

I needed to measure high voltage (a few kV) with an oscilloscope; we only had standard 10x passive probes (and some other low voltage active probes). Instead of spending a few thousand dollars on a high voltage probe, I built my own 500x (500:1 attenuation) high voltage passive oscilloscope probe for about 25USD. It's really easy to build. I thought I'd share the design.

I didn't care really care about AC performance above a few hundred Hertz, so I omitted the probe tip capacitor and the adjustable compensation capacitor. This design can certainly be improved upon.

Diagram (I also printed this and taped it to the probe):


Place high voltage resistor and coax cable on a piece of perfboard (make sure you get the kind with no pads or plating, otherwise there will be far less creepage).


Solder on the resistor, coaxial cable, and mini-grabbers. Apply a generous amount of hot glue to reduce stress on solder joints. If you want, you can conformally coat the board. Cut the perfboard to size. I find tin snips usually work well on FR4, but the perfboard I got was particularly brittle, so I just scored it with a knife and broke it with my hands.


Place heatshrink and apply label. Depending on your heatshrink diameter, you may want to do this before attaching the minigrabbers.


BOM:
1x coax cable (TPI 58-072-1M), cut in half, I used one to make two probes
1x 500M high voltage resistor (Ohmite SM108035006FE)
1x perfboard (i.e. Keystone 3396), with no pads or plating
2x minigrabbers (i.e. Pomona 4176-02, this is a pair of black+red)
A few inches of high voltage wire, i used some 22AWG high voltage wire we had in the lab
A few inches of high voltage heatshrink, large diameter

Usage:
Your oscilloscope channel should be configured in 1Mohm mode (like you would set it for an ordinary 10x passive probe), not in 50ohm mode. If your oscilloscope allows, you set the gain to 500x (actually 501x would be more accurate, since we have a voltage divider of 500Mohm with the scope's internal 1Mohm). If you don't have the option to set this, just remember to mentally scale your measurements (i.e. multiply by 50 if your setting is 10x).
The mini-grabbers are obviously not rated for high voltages; treat them as live conductors when the system is powered.
Do not touch the high voltage side of the probe when the system is powered. Instead, secure the probe clips with the system powered off, then turn on the power.
Keep in mind that the ground clip on passive oscilloscope probes is connected to earth ground.