How to measure current with a current probe

The current probe converts the magnetic field generated by the movement of electrons in the conductor into a linear voltage output within the measuring range. By measuring the voltage value, the current value in the conductor is indirectly measured.

Lift the pin at one end of the component (usually an inductor or resistor), and then connect the lifted pin of the component and the pad with a coil of N turns. If the measured current is I, the actual current measurement value is I/N. If the number of turns is too small, the oscilloscope measurement value is small and the measurement may not be accurate enough. If the number of turns is too large, because the coil itself has DC or AC resistance, it will cause errors in the measurement results.

If the current being measured is a DC current, set the oscilloscope to DC coupling and the input impedance to 1MΩ. If the signal being measured is a high-frequency signal, use a passive probe with a 10:1 attenuation, which has a smaller parasitic capacitance. Because the cutoff frequency in the RC filter network is 1/(2π(RC)), the smaller the parasitic capacitance of the oscilloscope, the higher the cutoff frequency, and the smaller the attenuation of the high-frequency signal.

If the tested signal is a low-frequency signal (less than 20MHz), in order to observe a clearer and more accurate waveform, you need to set the oscilloscope to "bandwidth limit". If the tested signal is a high-frequency signal (greater than 20MHz), there is no need to set the bandwidth limit.

If the measured signal is a small non-periodic signal, it is easily disturbed by noise, and it is not easy to measure accurately with cursors. To solve this problem, high-resolution sampling mode can be used, that is, the average value of a number of consecutive sampling points is calculated, and the high-frequency components are averaged, so this operation is equivalent to passing through a low-pass filter.

In order to observe the details of the signal more clearly, the storage depth can be maximized. When the time base and the number of horizontal grids are constant, the time the signal is displayed on the oscilloscope is constant. If the storage speed is higher, it means that more signal points can be collected during this period of time, that is, the sampling rate of the signal is higher, so the collected signal waveform is clearer.

According to the size of the signal, set the appropriate vertical coordinate, and it is best if the signal can cover 3 grids.

Before using the current probe, you need to perform a zero calibration, as shown in Figure 1, "ZEROADJ". If the current probe is marked as 0.1V/A, the probe on the oscilloscope should also be set to 0.1V/A.

Figure 1 Current probe interface

Hook the coil with the hook of the current probe and tighten the current probe.