DIY Hall current probe, 100 mV/A +-12A, DC to 30 MHz, basic components

Hall element is inserted into a cut and grinded toroidal ferrite core, which is glued to a lever. That way the core can be opened using such lever to allow a non invasive measurement of current value and waveform in circuit.

Wideband amplifier has two sections, first is a differential common emiter pair of BC550C NPN low noise transistors (low noise and class C beta type is recommended) driven directly from the output of the hall element, common emitor current defines the gain. Its differential current output is lead into a differential common base pair of BC557B PNP general purpouse transistors (signal is lead into the emitor section of a current mirror) which converts the differential current into a single ended one, which drives the output. This section also handles the zero adjustment.

Both pairs of transistors are thermally interfaced by thermal paste and heatshrink. This is required for low thermal drift of the zero value.

Power is supplied by a floating 5V voltage source and the power draw is around 50 mA (250 mW total power draw). Floating supply is required due to the simulated symetric power in the design, provided by 1N4148 diodes 3 of which are in series for the V+ and 2 are in series for the V-. This arangement provides a simple voltage regulation as well as thermal compensation of the gain (about -0,5% change per 1 °C) and thermal compensation of the zero adjustment.

STL files of used 3D printed parts are in the project attachments.