Abstract: The accurate measurement of the current is very important to the test results, but to measure the current accurately, it is necessary to select the appropriate current probe and the correct test method. This article tells you about current measurement, and takes you to understand the test solution of ZLG Zhiyuan Electronics.
In daily measurement and test work, current is a very important measurement value. Compared with measuring the current by using the sampling resistor to generate the voltage drop, the current probe can realize the current test by simply wrapping the wire completely around the probe core. This method does not need to disconnect the power supply to connect, and the current flowing through the wire can be measured without damaging the wire, which is very convenient to use. In addition, the use of current probes in conjunction with voltage probes can test data such as power, phase, etc., which is very meaningful for electrical design.
1. The principle of current probe Commonly used current probes are two types: Hall sensor and measuring current magnetic field.
Hall-effect sensor is a transducer that outputs voltage according to magnetic field changes, and its current probe generally measures DC or low-frequency signals. This type of current probe is measured using the principle of compensation, and the measurement range can be changed by changing the transfer impedance with the aid of a compensation amplifier.
Current probes of the current transformer type can only be used to measure AC current and are often used for high frequency measurements. The alternating current in the core of the transformer creates a magnetic field in the core, which then draws the current in the second winding circuit and is fed to the meter. The induced voltage in the secondary winding will be proportional to the primary winding current. The current snap ring is to wind the coil winding on a magnetic material with high permeability. When the measured current is large, there is a problem of magnetic saturation. For accurate measurements, it is necessary to degauss the probe occasionally and compensate for any DC offset remaining on the probe after degaussing.
A common technology today is a hybrid AC/DC current probe that combines Hall-effect sensor elements for measuring DC and low frequencies and a current transformer for measuring AC in one probe.
Figure 1 Hall Effect Sensor Types

Figure 2 Types of current transformers

Figure 3 Hybrid Hall Effect Sensor and Current Transformer Types

2. ZCP0030-50 Current Probe The ZCP0030-50 current probe developed by our company is a hybrid AC/DC type, which can measure DC/AC current, measure the analog bandwidth of 50MHz, and the measurement accuracy can reach 1%. This probe has two ranges of 30A and 5A, and the current transfer ratio is 0.1V/A and 1V/A respectively. It is mainly used to measure the instantaneous current of motors, switching power supplies, inverters, controllers, sensors, amplifiers, etc.
Figure 4 Appearance of ZCP0030-50

3. ZCP0030-50 current probe performance test The test results are shown in the table below.
Figure 5 ZCP0030-50 test result table

1. The DC accuracy test calibration source outputs different measured current values, and uses a six-and-a-half-digit multimeter to read the response voltage value of the current probe. It can be seen from the test data that the DC accuracy of ZCP0030-50 within the range is ±1%±1mV.
Figure 6 DC Accuracy Test Table

2. Rise time test The signal generator outputs a square wave with a rise time of 5ns, and the oscilloscope measures the rise waveform time after passing through the current probe. The test result is shown as 8.5ns, this test time includes the rise time of the signal generator itself and the rise time of the oscilloscope probe. The actual rise time of the current probe is calculated using the following formula.
Figure 7ZCP0030-50 rise time

3. Square wave response test The square wave response is mainly used to test the amplitude-frequency response characteristics of the current probe. The current probe can completely reproduce the square wave waveform without distortion, indicating that the probe has a flat amplitude-frequency response.
The signal generator outputs a square wave with a peak-to-peak value of 100mA and a frequency of 100KHz and 1MHz respectively, and the oscilloscope displays the corresponding square wave waveform. As can be seen from the figure, when testing a 1MHz current square wave, the overall square wave response waveform of the probe is basically not distorted, the waveform is relatively flat, and there is no slope.
Figure 8 100kHz square wave response test

Are you measuring current the right way?
Figure 91MHz square wave response test

4. Noise test Noise is an important parameter of the current probe. The size of the noise directly affects the resolution of the probe. The smaller the noise, the higher the resolution. Use an oscilloscope to test the actual noise of the current probe, the peak-to-peak value of the test result is less than 10mA, and the AC RMS value is less than 1mArms.
Are you measuring current the right way?
Figure 10 Current probe noise test

Fourth, the correct use of the current probe 1, the current probe maximum range The current probe has two maximum rated current: continuous current, peak current. Test currents in excess of either will saturate the current probe and magnetize the probe resulting in erroneous measurements.
It should be noted that the maximum current will decrease as the frequency increases, and continuous testing of the maximum current exceeding the corresponding frequency will cause damage to the probe. Our ZCP0030-50 current probe can measure the maximum continuous current of 30Arms and 2Arms at the test bandwidth of 50MHz.
Figure 9 Relationship between test frequency and maximum rated current

2. Test discontinuous current When testing discontinuous current, pay attention to the maximum safety test time of the current probe, and consider the following factors: maximum peak current, ambient temperature, and test bandwidth. Note: The probe head will heat up when the test current is equal to or close to the peak current.
3. On-line degaussing of the probe Due to the magnetic saturation problem of the current transformer, the degaussing of the probe can effectively compensate the offset voltage caused by the residual DC magnetic field. In-line degaussing is possible in most cases when a wire with no current flowing is embedded in the jaws.
4. Improve measurement sensitivity The accuracy of using a current probe to test small currents is related to the vertical noise of the oscilloscope. The oscilloscope can be set to reduce vertical noise: turn on the bandwidth limit filter, set the high resolution acquisition mode or average mode.
Additionally, measurement sensitivity is increased by wrapping multiple turns of the conductor under test around the probe. At this time, the strength of the signal will be multiplied according to the number of turns of the conductor under test wrapped around the probe. Because winding more than a few turns will increase the insertion impedance of the probe, the bandwidth of the probe will also be reduced, this method is suitable for measuring DC signals or low-frequency AC signals with small amplitudes.
5. Summary In metrology testing and electronic design, the role of current probes cannot be replaced by other probes. Using current probes and oscilloscopes can be very easy to measure current. Correct and proper use of current probes is very important for measurement accuracy. In addition, the vertical noise of oscilloscopes can also prevent accurate low-level current measurements.