How to Choose a Current Probe

Overview

An oscilloscope current probe enables an oscilloscope to measure not only voltage but also current, expanding its application range. Simply put, a current probe senses the current in a conductor and converts it into a voltage signal that can be observed and measured on the oscilloscope. Currently, the most commonly used current measurement technique detects current-carrying conductors through magnetic field sensing. However, many types of current probes are available on the market, and each type has its optimal application scenario. As long as it is used correctly for its intended design purpose, optimal measurement results can be achieved.

This application note introduces the common types of current probes, their basic principles, the advantages and limitations of each current probe type, and practical precautions for using current probes in oscilloscope applications so as to fully utilize their capabilities.

Current Probe Types

Currently, probes are widely used for current measurement of power devices or power supplies and have become indispensable tools for accurate current measurement with oscilloscopes. To meet these current measurement needs, although multiple current measurement techniques exist, the techniques most commonly used with oscilloscopes include:

1. Resistive or current-shunt type: based on Ohm's law

2. Clamp-on current probe: AC transformer or hybrid Hall-effect sensor/AC transformer

3. Rogowski coil: a convenient high-current AC measurement probe

Sense resistor or shunt resistor

A common method for directly measuring the current of the device under test (DUT) is to insert a shunt resistor into the current path, measure the voltage drop across the resistor, and then use the Ohm's law formula (i.e.,I=V/R) to convert the voltage into current. This method is an invasive measurement because the current probe/shunt resistor is electrically connected directly to the voltage measurement circuit or probe, and all are part of the device under test. Therefore, several key factors require special attention.

Sense Resistor Selection

When selecting the resistance value, accuracy, temperature coefficient, and physical dimensions, the magnitude and characteristics of the measured current must be comprehensively considered. The larger the resistance value, the higher the measurement accuracy and the signal-to-noise ratio (SNR）is also better. However, increasing the resistance value increases power loss, which produces an unnecessary voltage drop called “load voltage.” In addition, the selection of the sense resistor value must also balance measurement noise, sensitivity, bandwidth, and other metrics. To reduce the effect of load voltage, users may tend to choose the smallest sense resistor value, but too small a resistance value has negative effects on measurement—the larger the resistance value, the larger the voltage drop across the sense resistor, resulting in insufficient load voltage and thus affecting system performance and efficiency. This is like a game requiring precise balance.

Input common-mode voltage

This parameter defines the common-mode input voltage of the probe or sensing device (amplifier) relative to ground.

High side/Low-side testing

When measuring load current, the sense resistor can be placed between the supply voltage and the load (high side) or between the load and ground (low side). Low-side sensing is more ideal and simpler to implement because the common-mode voltage is nearly at ground. The advantage of high-side sensing is that it can directly monitor the power supply output current, thereby effectively detecting load short circuits.

Four-terminal Kelvin measurement configuration

This effectively eliminates the load's lead resistance and temperature coefficient. Kelvin connection is essential for accurate current sensing and is particularly suitable for high-current applications.

Advantages

• Depending on the system implementation, high-sensitivity and high-bandwidth measurements can be achieved.

• Compact and inexpensive.

Limitations

• There is a trade-off between load voltage and measurement accuracy (noise, sensitivity, and bandwidth).

• To achieve more accurate measurement, increasing the sense resistor value increases the voltage drop across the sense resistor while reducing the load voltage, thereby affecting system performance and efficiency.

• This method is an invasive measurement because the sensing/The shunt resistor is electrically connected to the voltage measurement circuit or probe and is part of the device under test (DUT).

Clamp-onCurrent probe

Another common type of current probe is the magnetic-core current probe (also called a clamp-on current probe). This indirect current-sensing technique achieves non-contact current measurement by clamping the probe around a current-carrying wire or conductor. The voltage signal output by the probe is proportional to the amplitude of the measured current, enabling non-invasive or isolated measurement——In this case, the probe does not require an electrical connection to the device under test (DUT) .

Clamp-on current probes are available in AC and AC/DC versions and offer multiple current conversion ratios. These probes detect the electromagnetic field strength around a conductor and convert it into a voltage measurable by an oscilloscope./Clamp-on current probes mainly use two sensor technologies. The first is the Hall-effect sensor, used to measure DC or low-frequency signals. This sensor responds to magnetic field changes by changing its output voltage and is a typical magnetoresistive device. The second common technology is the current-transformer method. When AC current flows through the transformer core, it generates a magnetic field in the core, which induces a current in the secondary winding circuit that is fed to the oscilloscope. The induced voltage in the secondary winding is proportional to the current in the primary winding; this technology is only suitable for AC current measurement.

Clamp-on current probes mainly use two sensor technologies. The first is a Hall-effect sensor, which is used to measure DC or low-frequency signals. This sensor responds to changes in the magnetic field by changing its output voltage, and is a typical magnetoresistor. The second commonly used technique is the current transformer method. When an AC current flows through the transformer core, it creates a magnetic field in the core, which in turn induces a current in the secondary winding circuit that is input to the oscilloscope. The induced voltage produced in the secondary winding is proportional to the primary winding current; this technique is suitable only for measuring AC current.

DC current probe, which integrates a Hall-effect sensor element for measuring DC and low-frequency components and a current transformer for measuring AC into a single probe./Advantages

Electrical isolation between the probe and the device under test

• No need to break the circuit; they can be installed anywhere in the current path

• Low insertion impedance

• Limitations

Demagnetization and offset correction

• To ensure measurement accuracy, the probe must be periodically demagnetized, and the DC offset remaining after demagnetization must be compensated——Higher cost: Hall-effect sensors are among the more expensive types of current sensors

• Rogowski coil

If AC currents above tens of amperes must be measured and flexible current measurement capability is required, a Rogowski current probe is recommended.

If AC currents above several tens of amperes need to be measured and flexible current measurement capability is required, a Rogowski current probe is recommended.

A Rogowski coil is an electronic sensor used to measure AC current; it can detect high-speed transient currents, pulsed currents in power equipment, and50or60hertz sinusoidal currents in power lines. The sensor uses a flexible clip-around coil design and can be easily wrapped around a current-carrying conductor for measurement; even when the current reaches thousands of amperes, the sensor size remains unchanged.

RogowskiHow does the coil work?

The operating principle of the Rogowski coil is based on Faraday's law, which states that the total electromotive force induced in a closed loop is proportional to the total rate of change of magnetic flux through the loop.

The principle of the Rogowski coil is similar to that of an AC current transformer: both induce a voltage proportional to the current flowing through an isolated conductor via a secondary coil. The key difference is that the Rogowski coil uses an air-core structure, whereas a current transformer relies on a high-permeability steel core and secondary winding for magnetic coupling. This air-core design has lower insertion impedance, enabling faster signal response, and the voltage signal remains highly linear.

      When an air-core coil is placed around a current-carrying conductor in a toroidal manner, the magnetic field generated by the AC current induces a voltage in the coil. The voltage generated by the Rogowski coil is proportional to the rate of change (derivative) of the current in the coil loop. By integrating the coil voltage, the probe can output a voltage signal proportional to the input current signal.

Figure 1 The voltage generated by the Rogowski coil is proportional to the rate of change (derivative) of the current in the coil loop.

Advantages

Rogowski coil current probes have significant advantages over other current sensors or detection technologies.

• When measuring high currents,magneticcore will not saturate

A Rogowski coil, with its unique non-magnetic“air”core design, can measure a wide current range from milliamperes to thousands of amperes without causing core saturation. Its upper current measurement limit is mainly constrained by the maximum input voltage of the measuring instrument and the breakdown voltage of the coil or integrator circuit components. Unlike other current transformers——other products increase in size and weight when the measurement range expands——Rogowski coils maintain a compact coil size regardless of the current magnitude being measured. This design makes them the preferred tool for measuring high AC currents of hundreds or even thousands of amperes.

• Very flexible in use

This lightweight clip-around sensor coil is flexible and easy to wrap, can be easily clamped around live conductors, and can be easily inserted into hard-to-reach components in circuits. Most Rogowski coils are thin enough to be installed inT0-220orTO-247the lead spacing of power semiconductor packages, allowing a current probe to be connected without additional winding. This design offers an advantage in achieving high-signal-integrity measurements.

• Bandwidth up to30MHzand above

This design enables Rogowski coils to accurately capture rapidly changing current signals (e.g., thousands of amperes/at the microsecond level). With high-bandwidth characteristics, it can both analyze high-order harmonics in high-frequency switching systems and monitor in real time switching waveforms with extremely short rise/and fall times, thereby significantly improving the accuracy of signal integrity testing.

• Non-invasive or non-destructive measurement

Because the insertion impedance is extremely low,Rogowskithe coil draws current from the device under test (DUT)， and the drawn current is extremely small. The impedance injected by the probe into the device under test is only a few picohenries, which makes the signal response faster and keeps the signal voltage highly linear.

• Low cost

Compared with Hall-effect sensors/Compared with current probes, Rogowski coils are usually lower cost.

Limitations

AC only

• Rogowski coilCannot handle DC; supports AC only

Sensitivity

• Because they do not have a high-permeability magnetic core, Rogowski coils have lower sensitivity than current transformers

Key questions to ask when selecting a current probe

•  Determine whether you are measuring AC, DC, or AC superimposed on DC。

•  What is the maximum current you need to measure?

•  What is the minimum current you need to measure?

•  What is the common-mode voltage of the current signal you are measuring?

•  What is the frequency of the target current signal?

• DUTWhat are its dimensions?

•  How many current probes or voltage probes are used simultaneously?

•  What is the maximum voltage of the conductor under test?

•  Which type of range do you intend to use for this?

•  What is your budget?

Comparison

The figure below compares the main characteristics of sense resistors, clamp-on current probes, and Rogowski coil current probes. When selecting a current probe for an application, refer to this figure.

Conclusion

There are many methods for measuring current, and each has its advantages and limitations; different current probes also have their own optimal application scenarios. If used correctly as intended, they can achieve the desired results. You should now have a deeper understanding of various current probes, their basic principles, and the comparison of advantages and disadvantages among different probe types.

Reprinted from Keysight Technologies