Rogowski-Relief – An Open Source Rogowski Coil Based Current Probe

The Rogowski-Relief current probe, utilizing a Rogowski coil, has proven to be an invaluable tool for measuring high-frequency AC currents in confined spaces. Its design facilitates non-intrusive current measurements, making it highly versatile and applicable in various scenarios. The probe comprises two main components: the Rogowski coil and the integrator/amplifier, working in tandem to deliver accurate and reliable current readings.

One of the significant advantages of the Rogowski coil lies in its ability to measure high-frequency AC currents without disrupting the circuit under test. The sensing coil’s optimized size allows it to fit between the leads of a TO-220 transistor or slip around the leads of through-hole power components, making it suitable for tight spaces.

The underlying principle behind the Rogowski coil’s operation is based on Ampere’s law, wherein the coil generates a voltage proportional to the change in the magnetic field induced by the current-carrying conductor encircled by the coil. This unique characteristic allows the Rogowski coil to produce a voltage signal corresponding to the rate of change of current (dI/dt), making it ill-suited for measuring DC currents.

To process the signal from the Rogowski coil, an op-amp-based integrator is employed. The integrator consists of a passive RC integrator for high-frequency integration and an op-amp integrator for low frequencies. The passive RC integrator, co-located with the coil, matches the coil’s characteristic impedance and incorporates the capacitance of the cable between the coil and the integrator, preventing bandwidth limitations.

To further optimize the performance of the probe, a servo high-pass filter with a second op-amp is integrated into the design. The filter effectively reduces gain below the cutoff frequency, mitigating noise and nullifying the amplifier’s DC offset, which could otherwise restrict the output signal range.

An additional op-amp acts as an output buffer, allowing for adjustable gain to normalize the output to a fixed V/A ratio and compensating for losses in the Rogowski coil, particularly at high frequencies.

To power the integrator, a USB-B connector is utilized. Power is supplied through a specialized isolated DC/DC converter module, providing bipolar outputs of +/-9V, which are then regulated down to +/-6V with linear regulators. This choice of regulators significantly reduces switching noise that could otherwise interfere with the output signal.

While the Rogowski-Relief current probe has proven effective for general-purpose lab use, the most recent version of the integrator encountered issues with interference from wireless signals like WiFi and LTE. To address this problem, design updates have been made to the PCB layout, allowing the circuitry to be shielded by a metal can, thereby minimizing the impact of external signals on the probe’s performance. However, these updates remain untested at the time of this summary.

Safety considerations are of utmost importance when working with high-voltage conductors. The Rogowski-Relief’s potential contact with high-voltage conductors necessitates implementing safety precautions. To mitigate the risks of arc-over events, the PCB layout incorporates a PCB trace fuse on both connections from the coil. In the event of an arc-over, the fuse is designed to blow, isolating the coil and preventing further damage. Additionally, a gas discharge tube is included on the output to provide further protection to any connected devices in case of an arc-over.

It is essential to be aware that no specific voltage rating is implied or tested for the Rogowski coil itself. The recommended heat-shrink and magnetic wire possess certain dielectric strengths, but factors such as mechanical wear and high dv/dt signals can still impact the coil’s voltage rating. Proper care and adherence to safety guidelines are necessary to ensure the safe and reliable operation of the Rogowski-Relief current probe.

In summary, the Rogowski-Relief current probe, featuring a Rogowski coil and an integrator/amplifier, is a cutting-edge solution for measuring high-frequency AC currents in tight spaces. With its non-intrusive design and accurate current readings, it serves as a versatile tool for various lab applications. Recent design improvements aimed at addressing interference issues enhance its overall usability. Nevertheless, users must prioritize safety precautions, especially when working with high-voltage conductors. By incorporating features like PCB trace fuses and gas discharge tubes, potential hazards can be mitigated. Awareness of the Rogowski coil’s voltage limitations further contributes to the probe’s safe and effective use.

For more information about the project, visit the GitHub page

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