This basic non-contact transducer is often used to measure AC currents.
Accurate measurement of static and dynamic current flow in a conductor is a common requirement in many circuits and systems. Designers have several transducer options for this, including sensing the voltage across a known, in-circuit sense resistor, using a non-contact Hall-effect sensor, or employing an inductive coil as a sensor. Each offers capability, installation, and performance tradeoffs.
The Rogowski coil is a lesser-known but widely used current-sensing transducer. This FAQ will explore the basics of this versatile, rugged passive component used in power monitoring, energy management systems, and industrial applications where accurate current measurement over a wide range is critical.
Q: First things first: what is a Rogowski coil?
A: Conceptually, it’s straightforward. A Rogowski coil is an air-core toroidal coil placed around a conductor, as shown in Figure 1. The alternating magnetic field produced by the current induces a voltage in the coil, which is proportional to the rate of change of current. The output is independent of frequency over an extensive range of a few hertz to around one megahertz (typical) for standard coils. The first parameters cited for the performance are minimum/maximum sensing range (in amps) and coil sensitivity in volts (output) per amp sensed (V/A).

The coils have accurate phase response and can measure complex current waveforms and high-speed transients, both important attributes for today’s power-switching circuits. They can be placed around conductors of various shapes and sizes, making them a good choice for installations in tight spaces or around irregularly shaped conductors.
Q: Is the output of the coil used directly to represent current?
A: The coil constitutes a closed loop with no discontinuities, so the voltage Vout induced in the coil is proportional to the rate of change of the encircled current I (Faraday’s Law). The direct output from the coil is given by Vout = M dI/dt, where M is the mutual inductance of the coil, and dI/dt is the rate of change in current.
Therefore, the voltage is integrated electronically via a simple analog circuit to obtain the actual current value. The integrator’s output is a voltage that accurately reproduces the current waveform. (Note that the integration is increasingly done digitally by amplifying the coil output, digitizing it, and then using digital circuitry or a processor and firmware instead of all-analog circuitry, but the principle is the same.
Q: What’s the application difference between a coreless (air core) Rogowski coil and a current transformer (CT) as a transducer?
A: A conventional current transformer has an iron core to enhance sensitivity. However, traditional CTs are prone to saturation, which limits their accuracy and effectiveness at high current levels. Rogowski coils, on the other hand, do not have an iron core, eliminating the risk of core saturation. This allows for more accurate and reliable current measurements, especially in high-current applications.
As a result, Rogowski coils can measure a much wider range of current levels than traditional CTs, which makes them a good fit for various applications that involve varying current loads, from very low to extremely high currents. For instance, a single appropriately chosen Rogowski coil can handle currents from as low as 5 amps to as high as 5,000 amps, representing a 1000:1 dynamic range.
Q: Is that their only advantage?
A: No. Air coils have a minimal loss, mostly due to the current flowing through the resistance of the windings, as the absence of ferromagnetic cores means there is no energy loss due to eddy currents and hysteresis loops. Also, they do not have the typical nonlinear relationship between a magnetic field and magnetic flux density.
In contrast, iron-core CTs have magnetic losses due to the core, which wastes power and causes them to get hot, affecting the magnetics; even if they can tolerate the heat, it still causes a dissipation issue.
Q: Does the current-carrying conductor under measurement have to be disconnected at one end to be slipped through the Rogowski coil? Is this a problem if it has to be done in the field?
A: Yes and no. The coil must surround the conductor, but some vendors make special coils that can be “snapped” around the conductor even while that condition is tied down at both ends.
Q: What fabrication details affect coil performance?
A: It is critical that the coil winding is as uniform as possible. A non-uniform winding makes the coil susceptible to magnetic pickup from adjacent conductors or other sources of magnetic fields. Also, the coil must be wound with constant turns per unit length on a former uniform cross-section.
Q: What are some of the other positive attributes of this coil?
A: Rogowski coils are flexible, lightweight, and inherently linear (the electronics may add some nonlinearities, but that’s a separate issue). The coil has no magnetic core to become saturated, so the output increases linearly in proportion to the current with the operating limit determined by voltage breakdown.
Linearity makes Rogowski coils easy to calibrate because a transducer can be calibrated at any convenient current level, and the calibration will be accurate for all currents, including very large ones. Also, because of their linearity, the transducers have an extensive dynamic range and excellent transient response.
Q: Are there any other notable benefits?
A: There are many. First, they inherently offer galvanic (ohmic) isolation, which is essential in many circuits and systems for performance, safety, and regulatory approval. This isolation eliminates the need for a separate isolation circuit or components where it is required. They are intrinsically safe, with no open contacts or points where energy (sparks) could occur and initiate an explosion in a hazardous atmosphere. They are capable of handling very large current overloads without damage.
Q: Can the Rogowski coil measure DC currents?
A: No, they are for AC only. However, this is also a benefit, as they can measure small AC currents in the presence of a large DC current component.
Q: Are there bandwidth limitations or issues?
A: Very few. They function over a very wide frequency range, from a few hertz to hundreds of kilohertz and higher.
Q: Is the Rogowski coil a relatively new development?
A: No, not at all. A.P. Chattock’s 1887 paper “On a Magnetic Potentiometer” is the first to describe a winding on a long, flexible former for making magnetic measurements. This theoretical paper became the foundation for the practical implementation, which we know as the Rogowski coil. It uses Ampere’s Law (“the value of the line integral to the magnetic field along a loop which completely encircles a current is equal to the current”) for calibration.
The detailed description of the Rogowski coil was provided by German physicists Walter Rogowski and W. Steinhaus in a 10-page paper, “Die Messung der magnetischen Spannung,” published in 1912 in Archiv für Elektrotechnik; it is not clear why Steinhaus is not associated with the coil. Their paper describes several ingenious experiments to test that their coil provided reliable measurements, including using it to measure electric currents.
Q: Are these coils available as standard items from vendors?
A: Absolutely. Among the many sources are Aim Dynamics, Power Electronic Measurements Ltd. (UK), Rocoil Ltd (UK), and Pulse Electronics/Yageo Group.
Q: Can you give an example?
A: Pulse Electronics has a new family with three physical sizes and electrical ratings, shown in Figure 2. The coil wires are encased in epoxy for ruggedness and protection and to ensure they do not accidentally touch anything live or conductive.
- RC01: 100 MV/KA, 200 MV/KA, and 300 MV/KA versions weigh 7 grams, designed for PCB mounting;
- RC03: 200 MV/KA, 400 MV/KA, and 600 MV/KA versions, 23 grams;
- RC05: 150 MV/KA, 300 MV/KA, and 450 MV/KA versions, 48 grams.

Q: Are all Rogowski coils rigid?
A: No. Flexible coils can also surround a conductor and bend with it. These require a more complicated and costly manufacturing and testing process.
The electronics also have a role

Q: We have discussed the coil thus far, but what are the associated electronics?
A: The traditional analog integrator of Figure 3 is a standard circuit building block. It is easily built from discrete op-amps, optimized for the required current and voltage characteristics, and low offset, but more highly integrated ICs are also available. If a discrete op-amp is used, it is followed by a buffer/amplifier to prevent the integrator’s load from affecting the integrator itself.
Q: What components are needed in a basic integrator?
A: In its simplest form, the op-amp integrator has an input resistor Rsh and feedback capacitor C. Therefore, the overall transducer gain is Vout=RshI, where Rsh= H/CR is the transducer sensitivity (V/A).
Q: Seems pretty simple, is that all?
A: Of course not; it never is. At low frequencies, the integrator gain increases and, in theory, will become infinite as the frequency approaches zero. This would result in unacceptable DC drift and low-frequency noise, so the integrator gain must be limited at low frequencies. This is done by placing a low-pass filter parallel to the integrating capacitor. The low pass filter sets the low-frequency bandwidth fL; typically, this is less than 1Hz.
Q: What about higher-frequency performance?
A: Due to the distributed inductance and capacitance of the Rogowski coil, there is a high-frequency bandwidth fH (generally 1 MHz or greater) above which the measurement is attenuated and significant phase delay occurs, as seen in Figure 4. The bandwidth of the electronic integrator and the length of cable connecting the integrator to the coil also influence this limit. Note that the usual -3 dB attenuation threshold defines bandwidth.

Q: Do you have to build your own Rogowski coil-interface circuit?
A: Not at all, as coil vendors generally offer basic interface circuits that include the integrator and buffer.
Q: What if your application requires a specialized output or must be integrated into a power measurement/management product?
A: IC vendors offer application notes and evaluation designs to ease the process. For example, Texas Instruments uses their ADS131M04, a four-channel, 24-bit, 64-kSPS, simultaneous-sampling, delta-sigma ADC, to build a split-phase energy measurement front-end covering 0.05 A to 100 A, as seen in the schematic diagram of Figure 5. The ADCs sample the outputs of the Rogowski coils and voltage dividers to measure the current and voltage in each leg of the AC mains.

Summary
The Rogowski coil is a widely used, effective, isolated sensor for AC current. It operates over a wide dynamic range and can support high current values. It is electrically and mechanically rugged and provides high accuracy at a modest cost. Integrating its output signal requires some basic electronic circuitry, which can be done using basic analog ICs or digitization and firmware.
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External references
What is a Rogowski Coil?, Power Electronic Measurements Ltd
Why Buy a Rogowski Coil Instead of a Regular Current Transformer?, AIM Dynamic
The Rogowski coil principle, Rocoil Ltd
Rogowski coils, Rocoil Ltd
Articles and Papers, Rocoil Ltd
Rogowski Coil, Wikipedia
Two-Phase Rogowski Coil Based Electricity Meter Analog Front-End Circuit, SBAA385a, Texas Instruments
Multi-Phase Rogowski-Based E-Meter, SLAA580, Texas Instruments
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