Current Measurements Made Easy: A Current Probe for RF

In many cases, current measurements without a DC component are useful. The most common is the case of CTs (current transformers) for AC mains. This article is about the design of current transformers for medium to high frequencies, which are really straightforward to build. The formulas presented are valid also for AC mains units.

The probe in Figure 1 is designed to measure up to 50 A peak in a frequency range from 7 kHz to tens of MHz. The schematic in Figure 2 is quite simple: the wire whose current must be measured is passed through the toroid, which is an ordinary Amidon FT 82-43 core that performs well up to at least 50 MHz.

The secondary winding consists of ten turns of wire evenly distributed over the core. If available, use a medium-gauge stranded wire, but this is not mandatory. Due to the ratio of 1:10 turns, the maximum current in the secondary is 5 Ap.The secondary side is loaded with 0.2 Ω, which was realized by a parallel connection of five 1 Ω resistors. At a peak current of 5 Ap, the peak voltage across these resistors is 1 Vp, which is very convenient for measurements with an oscilloscope. For a sinusoidal current, the average power dissipation at the resistors is R·I2 = R·Ip2 / 2 = 2.5 W or 0.5 W per resistor. A continuous sinusoidal current of 50 Ap can be measured only with 0.5 W or larger resistors. But, if the waveforms are pulsed or very short measurements are made, ¼ W resistors will do. That was my choice because I wanted to keep the design compact for better RF performance. OK, I also have to admit that these were the resistors I had on hand.

Figure 3 shows typical usage with a scope probe, using a BNC adapter for scopes. The device can also be used with a direct coaxial cable connection to the scope input, because 1 Vp is ideal for scope 1× operation: In this case, using a short cable is recommended to avoid reflections in the band of interest, because the coax will be mismatched on both sides. Even better, the coaxial cable can be terminated at its characteristic impedance on the scope side: Many modern scopes offer the ability to set the input impedance to 50 Ω, so this is particularly easy. In this specific case, one has to remember that the measurement will be slightly off-scaled, due to the parallel of the 50 Ω load with the 0.2 Ω incorporated in the probe (total resistance becomes 0.1992 Ω, giving a scaling factor of 50.2 A/V).

What must be avoided is attaching the scope probe directly to resistors using the clips and skipping the BNC connector, because when measuring high RF currents, even the minimum unshielded loop in the probes will add artifacts to the measurements.

The design of the current transformer is not complicated, but some electromagnetic formulas are necessary. First, about the load resistor RL, which should be as small as practically possible in order to minimize the power loss introduced, because the circuit under measurement will "see" at least R·n2, where 1:n is the turns ratio (1:10) and R is the sum of RL (0.2 Ω) and of the secondary wire resistance (some mΩ). As already said, it is very important that the secondary side is evenly wound, as otherwise the circuit under test will present some stray inductance in series. On the other extreme, if we choose too low a value for RL, we’ll also have a very small voltage to measure, causing noise on the traces. Finally, RL should be greater than the secondary wire resistance.In my case, I chose 0.2 Ω so that I could get 1 V at 5 A (50 A on the primary), which adds 2 mΩ to the circuit under test. The number of secondary turns, n, determines the current ratio. In case of a high-frequency CT, this number must be kept low to avoid self-resonance caused by stray capacitance together with high inductance. In the case of mains CTs, the frequency is quite low (50 or 60 Hz), and therefore n = 1000 is a common value. Powers of 10 are common, so that the current conversion ratio is simple, but other values are possible.The highest usable frequency for a toroidal ferrite CT depends on the:

A design such as mine can easily work for up to several tens of MHz if a suitable ferrite, such as material 43 from Amidon/Fair-Rite, is used. High permeability cores used for EMI suppression can be used as well, but only up to much lower frequencies. Low permeability cores used for power chokes and high-Q inductors are not recommended, because their inductance per turn is too low, which affects the following point.The choice of ferrite core also has an impact on the lowest usable frequency, for two reasons:

Capacitive coupling between primary and secondary windings can disturb measurements at the highest useful frequencies, or even at moderate frequencies if the primary conductor is subject to a high RF voltage.The design can be improved by adding an electrostatic shield that avoids this capacitive coupling: In practice, the primary wire is passed inside a small piece of metal tube (typically copper or brass) connected to the output GND of the secondary, as shown in Figure 4. This doesn't alter the magnetic linkage, but acts as an electric field blocker.

This example of an RF current transformer together with the most important design criteria proves that this matter is less complex than it initially seems. I hope that the considerations and formulas presented here are useful in simplifying the handling of toroidal cores, as well as serving as a basis for your own developments.

Roberto Visentin is a recently retired electronic engineer who worked on electronics and control systems for marine applications and underwater robotics. Still working as a freelance consultant, he enjoys finding more time to develop hobby projects in his home electronic laboratory.

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This article originally appeared in Elektor May & June 2023. Become an Elektor member today!