Tekbox TBCP5-150K400 RF Current Monitoring Probe: A Useful Tool for High-Power Electronics Systems

As EMC and power electronics engineers here in Mach One Design, we often need to measure RF currents in high-power electronics systems.

Traditionally, RF current measurements are performed by clamping a current probe onto AC cables or DC bus bars. However, as system power levels continue to increase, this approach becomes increasingly challenging. Commonly used probes with a 32 mm inner diameter are often too small for modern high-power applications.

Figure 1 An RF current probe with a 32 mm inner diameter can only just accommodate two screened cables.

This is just one of several challenges. High RF current levels can easily drive probes into saturation—something you may only notice when the probe begins to heat up. In addition, measuring differential-mode currents on individual conductors is particularly difficult, as these currents can reach hundreds of amps. Until recently, there were limited practical solutions available.

The Tekbox TBCP5-150K400 RF Current Monitoring Probe addresses many of these limitations. As an RF current monitor probe, the flat transfer impedance of approximately 11 dBΩ from 200 kHz to 300 MHz. Its usable frequency range is from tens of kHz up to 400 MHz.

The unique feature which makes this probe special for us is listed below:

·      Large 46 mm inner diameter, suitable for high-current cables and bus bars

·      High saturation performance due to large diameter and low-permeability core material

·      Capable of handling up to 200 A (DC–400 Hz) without affecting transfer impedance

·      Primary RF current rating of 2 A

These characteristics make it well suited for high-power EMC measurements, addressing both saturation concerns and physical size constraints.

Practical Applications

We have since used this probe in several projects and share some representative results and practical insights below.

The purpose is to help engineers in the field avoid making expensive mistakes.

1. Measurement on Individual Phase Conductors

In a three-phase system carrying approximately 60 A per phase, the probe was clamped onto individual conductors (Note the photo shows the clamp was placed on L1&2&3, but we did measure individual wire later). RF current measurements were obtained without issue. For comparison, results were also correlated with LISN-based measurements.

By measuring RF currents on individual conductors and comparing the results with measurements taken on the entire cable bundle, we can distinguish between differential-mode and common-mode noise. This provides a clear advantage when designing the filter stage.

For example, based on the results below, we can conclude that the noise between 150 kHz and 1 MHz is predominantly differential-mode, whereas the resonance around 1 MHz exhibits common-mode behaviour.

Figure 2 Using the TBCP5-150K400 probe on individual wire: Current probe result vs LISN result

The reader may ask: why use an RF current probe on individual conductors? Why not use a Hall-effect current probe instead?

In practice, Hall-effect current probes are typically limited to bandwidths of around 30 MHz, with higher-bandwidth models becoming significantly more expensive. More importantly, such probes are often susceptible to picking up electric field (E-field) noise, which can distort the measurement results.

This becomes a major limitation in large power electronics systems, where stray E-fields inside the cabinet can be substantial. In these environments, a Hall-effect probe is more likely to detect E-field interference rather than the true RF current flowing in the conductors. Higher current-rated Hall-effect probes are also considerably more expensive, making them less practical for this type of measurement.

2. Measurement on DC Bus Bars

When measuring RF currents on DC bus bars, we used the probe together with a 30 dB attenuator before connecting to an oscilloscope. This is essential to protect the 50 Ω input of the instrument, as RF currents—and therefore induced voltages—can be significant (in this case, the peak to peak current is about 13Amp).

Figure 3 Using the current probe on DC bus bars, a 30dB attenuator was used in this case, the peak to peak current is calculated to be about 10-13 Amp

Important Practical Consideration

The most critical point when using this probe is related to connection timing:

·      Always power up the system first and allow it to stabilise before connecting the probe to the measurement instrument

·      Before powering down, disconnect the probe from the instrument

This precaution is necessary to avoid high voltage spikes caused by V = L·di/dt effects.

To demonstrate this, we conducted a simple test:

• The probe was placed on a live wire of a switched-mode power supply drawing only 2–3 A in steady state

• During power-up, the probe output produced a voltage spike exceeding 10 V peak-to-peak together with lots of ringing and resonance

Figure 4 Powering the DUT up while the current probe is connected to an oscilloscope

This is significant. If the probe were used on higher-current systems (e.g. 50 A or more), and connected to a spectrum analyser or receiver during power-up, the resulting voltage spike could easily damage the instrument front end.

Conclusion

The Tekbox TBCP5-150K400 RF Current Monitoring Probe is a highly effective tool for EMC measurements in high-power systems, particularly where conventional probes fall short.

However, like all powerful tools, it must be used with care. Understanding its behaviour—especially during system transients—is essential to avoid costly measurement errors or equipment damage.