Capturing fast-changing currents on an oscilloscope is tricky, but this upgraded differential current probe makes it practical and cost-effective. It delivers wide bandwidth and a gain you can tweak by changing just one resistor — an improved follow-up to the earlier design.

Current Probe for Real-World Measurements

Measuring the dynamic behavior of current in a circuit is notoriously difficult, especially when you don’t have access to an expensive differential current probe. This project tackles that challenge head-on with a compact, high-performance differential current probe designed specifically for use with standard oscilloscopes.

Developed by Alfred Rosenkränzer and refined in a 2020 collaboration with the Elektor Labs, this updated “Current Probe 2.0” builds on an Elektor design published previously. The original probe was limited to about 130 kHz bandwidth; the new version dramatically improves performance, achieving bandwidths of up to 8 MHz in practice, while offering a simple, robust, and affordable solution for everyday lab work.
 
The prototype built into its enclosure with soldered cable and SIL socket
The prototype built into its enclosure with soldered cable and SIL socket.

The Circuit

At the heart of the design is a precision differential amplifier that converts tiny voltage drops across a shunt resistor into a clean, single-ended output referenced to oscilloscope ground. This allows accurate current measurements anywhere in a circuit without disturbing operation or risking ground loops and accidental shorts. The probe’s default gain of two is set by a single resistor and can be easily adjusted to suit different measurement needs — up to very high gains while still maintaining impressive bandwidth.
 
Scope current probe circuit
Complete circuit diagram of version 2.0 of the current probe. Note the close attention paid to the supply rails.
Thoughtful details round out the design: input filtering for protection and flat frequency response, proper output impedance matching for coaxial cables, onboard voltage regulation with EMI filtering, and clear LED status indicators. While the improved bandwidth comes with a reduced common-mode input range, the trade-off is well worth it for many low- to mid-voltage applications.

For engineers, makers, and students who want to see how current really behaves in their circuits — without investing in specialized probes — this project offers a practical, elegant, and thoroughly modern solution.
 
For ‘test leads’ the designers used two lengths of thin enamelled copper wire
For "test leads," two lengths of thin enamelled copper wire soldered to a three-pin SIL socket.

The Current Probe Project

The original article, “Differential Oscilloscope Current Probe 2.0,” appeared in Elektor November/December 2020. Check out the article.
Editor's Note: This article first appeared in a 2020 edition of Elektor. Given the project’s age, the components might not be available. Nevertheless, we think the design will inspire you to start a similar project at your workbench.

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