Takes a look inside the Tektronix P6042 current probe, a 1969 classic.
Tektronix introduced the P6042 current probe in 1969, priced at $625 ($4100 in 2016 dollars). It has a probe you can clamp onto a wire carrying current. The output of the P6042 goes to an oscilloscope. The scope will show the dc and ac current in the wire, up to a bandwidth of 50 MHz. This allows you to understand and troubleshoot reactive circuits like switching power supplies, where the voltage and current waveforms are not in phase or proportional.
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The architecture of the probe is clever. The powdered-iron core clamps around the wire-under-test. This closes a magnetic circuit. There is a Hall-effect sensor installed into a gap in the core. The Hall sensor will work at dc. However, it is not very fast, in the range of a few kHz. The differential output of this Hall sensor is amplified by a discrete transistor circuit, and that output goes to one side of a coil on the iron core that responds to fast ac signals. The output of that coil, now a combination of the ac and dc components of the measured current, goes to a second discrete transistor amplifier. The 50-Ω output of that amp goes to the BNC connector on the front panel. The dc output to the coil bucks the dc magnetic field it is measuring. This feedback improves the dc range of the probe and linearizes the output. The range attenuator circuit is not shown, nor is the degaussing oscillator and probe-open circuits. The complete P6042 manual is available online, including calibration, schematics, and theory of operation.
Read the complete article, Teardown: The Tektronix P6042 current probe is a classic on EE Times' sister publication, EDN.