The principle of oscilloscope current probe in testing DC and low-frequency AC

The oscilloscope current probe is crucial to the accuracy and correctness of the measurement results. It is an electronic component that connects the circuit under test to the input of the oscilloscope. A simple probe is a wire that connects the circuit under test to the input of the electronic oscilloscope. A complex probe is composed of resistors and capacitors and active devices. Simple probes are not shielded and are easily interfered by external electromagnetic fields. In addition, their equivalent capacitance is large, which increases the load of the circuit under test and distorts the measured signal.

When the current clamp is closed and surrounds a conductor with current flowing through it, a magnetic field will appear in response. These magnetic fields deflect the electrons in the Hall sensor and generate an electromotive force at the output of the Hall sensor. The current probe generates a reverse (compensation) current based on this electromotive force and sends it to the coil of the current probe to make the magnetic field in the current clamp zero to prevent saturation. The current probe measures the actual current value based on the reverse current. In this way, large currents, including mixed AC and DC currents, can be measured very linearly.

Working principle of current probe when testing high frequency

As the frequency of the measured current increases, the Hall effect gradually weakens. When measuring a high-frequency AC current without a DC component, most of it is directly induced to the coil of the current probe through the strength of the magnetic field. At this time, the probe is like a current transformer. The current probe directly measures the induced current, not the compensation current. The output of the power amplifier provides a low-impedance ground loop for the coil.

How current probes work in the crossover region

When the current probe operates in the high-low frequency crossover region of 20KHz, part of the measurement is achieved through the Hall sensor and the other part is achieved through the coil.