Circuit: DIY High-Voltage Probe

Measuring tiny differences in high-voltage circuits usually requires specialized differential test probes, which are often expensive. Fortunately, it’s possible to build your own high-voltage probe using affordable components while learning valuable hands-on lessons in the process.

A Smart, Cost-Effective Approach

High voltages appear in many places: switch-mode power supplies, vacuum tube circuits, and increasingly in hybrid and electric vehicles, where high battery voltages and currents help minimize losses and cable sizes. While a standard multimeter is a good solution for measuring absolute high voltages, it falls short when you need to observe small fluctuations or differences between two high-voltage levels. Think of situations like comparing anode voltages in a push-pull amplifier, or analyzing switching nodes in a full-bridge transformer circuit. Both demand precise measurements of relative differences, not just raw voltage values. Alfred Hesener’s 2010 DIY high-voltage probe project offers an accessible way to take these measurements without spending a fortune on commercial equipment. Along the way, you’ll not only build a useful tool for your lab, but also deepen your understanding of high-voltage circuit behavior, safety considerations, and measurement techniques.

High-Voltage Probe Circuit

Refer to the complete high-voltage probe circuit (apart from the two 10-MΩ input resistors). Hesener explained:

“On the left-hand side the two extra ceramic capacitors can be seen in parallel with the trimmers, also the second divider resistor. The diodes at the inputs (D11/12 and D21/22) are for over-voltage protection; they operate much faster than Zener diodes and their capacity is far lower. The high input resistors (10 MΩ) reliably clip any over-voltages from the diodes plus any current leakage from the power supply. For good frequency response it is vital to minimize parasitic capacitance at nodes in the layout.” The finished high-voltage probe circuitry was constructed on a PCB and enclosed in a diecast aluminum box. The input stage (left) had copper foil laid below it and shielded from the main differential amplifier on the right, so as to minimize capacitive coupling. The copper strips were soldered to the PCB using grounding pins. Hesener took care around the inputs to provide adequate insulation and space between the high-voltage conductors. The 10-MΩ input resistors and their connecting leads were made safe with heat-shrink tubing.

The Original Probe Project

The original article, “High-Voltage Probe,’” appeared in Elektor September 2010. You can access the article for free during the two-week period following the publication of this post. If you create a similar circuit, consider sharing your project on the Elektor Labs platform!

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Editor's Note: This article first appeared in 2010. Given the project’s age, some components or products might not be available, and the key design techniques might seem antiquated. However, we believe the project will inspire you to start new designs in the future.