Oscilloscope Probe Series - Current Probe Basics

Friends who have learned about oscilloscope probes in previous issues should already be familiar with the most commonly used accessories on oscilloscopes, or standard accessories - passive probes, and active probes, such as high-voltage active differential probes and high-bandwidth active differential probes. Probe. In addition, there is another major category of oscilloscope probes, which is the current probe .

In fact, the most direct reason is that there are many occasions where we not only need to measure voltage signals, but also need to observe current signals. For example: when we design a switching power supply circuit, we need to pay attention to the voltage signal on the one hand, and also need to observe the current signal. For another example, if we pay attention to the power consumption of the circuit, we will probably also pay attention to the trend and way that the current consumption changes with time, etc. . This is where we need the help of a current probe to detect changes in current in the circuit.

Current probes utilize Hall sensors and induction coils to measure DC and AC currents. The current probe converts the current signal into a voltage signal, and the oscilloscope collects the voltage signal and displays it as a current signal. The current probe can test currents from tens of milliamperes to hundreds of amps. When using it, you need to lead out the current wire (the current probe clamps the wire in the middle for testing and will not affect the circuit under test).

When the current clamp is closed and surrounds a conductor carrying current at the center, a magnetic field will appear in response. These magnetic fields deflect the electrons in the Hall sensor, producing 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, so that the magnetic field in the current clamp is zero to prevent saturation. The current probe measures the actual current value based on the reverse current. Using this method, large currents, including mixed AC and DC currents, can be measured very linearly.

As the frequency of the measured current increases, the Hall effect gradually weakens. When measuring a high-frequency AC current that does not contain a DC component, most of the current is directly induced to the coil of the current probe through the strength of the magnetic field. At this time, the probe acts 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.

This type of probe is the most common and widely used. The characteristics are: supporting a wide frequency band, covering a bandwidth range from DC to tens of MHz or even hundreds of MHz. The detection current range is wide, from milliamp level current to ampere level, or even hundreds of amps, with one probe. It is often used by engineer friends. For example, N7026A N2893A can provide bandwidths up to 150M and 100M Hz, and the current detection range can also range from milliamps to 30A (DC).

Rogowski Coil is also called Rogowski coil, Rogowski coil or air-core coil. Rogowski coil is an AC current sensor. It is a hollow ring-shaped coil, available in two types: flexible and rigid. It can be directly placed on the conductor to be measured to measure AC current. Rogowski coils are suitable for measuring AC current in a wide frequency range. They have no special requirements for conductors and sizes. They have fast instantaneous response capabilities and are widely used in situations where traditional current measuring devices such as current transformers cannot be used. Suitable for current measurement, especially high frequency and large current measurement. For example: N7040A series can measure AC current up to 3000A.

This type of current probe is specially designed for weak current measurement. It was originally developed to help test and evaluate the power consumption of consumer products. It is later used by a variety of power sensitive circuits, sensor circuits, and intrusive measurement circuits (requiring interruption). The design of this type of probe (open current is connected in series to the sampling resistor) is very unique. Although the frequency range is not high, only about 3MHz, it can provide a current resolution of tens of microamps. This indicator allows us to measure the standby power. Consumption brings hope and solutions. Because the resolution of ordinary AC/DC current probes is mostly above 1mA, and the higher the resolution, the higher the price, which discourages many engineers. In addition, considering that customers may need a period of continuous measurement and observation of the current consumption process while paying attention to the details of current changes, the N2820A designed by Keysight also supports the second channel partial amplification function, that is, a probe , providing two observation channels. One is a normal observation channel, and the second channel provides 100 times magnified observation, specifically used to observe details.

For more detailed information, you are welcome to visit our Probe Resource Center. The resource center contains data sheets, user manuals, compatibility information, application documents, video demonstrations, selection and configuration, SPICE models, etc. for various probes.

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