Current tongs are instruments for measuring alternating currents in cables. Their great advantage over most other methods is that the circuit need not be broken. Their action depends on the same principle as that of a transformer: after the tongs (transformer core) have been placed around one or more current-carrying conductors (primary winding), magnetic fields arise in the core material. Because of the alternating magnetic field components, a voltage is induced in the coil wound around the core (secondary winding). In accordance with Faraday’s law of induction, this voltage is a measure of the vector sum of the currents through the wires in the cable.
A frequent requirement is the measurement of an alternating current in the supply lines on a printed circuit board. In that case, technicians are often more interested in the waveform than in the r.m.s. value. Traditional current tongs can, of course, not be used then. It is, however, not too difficult to construct current tongs. or rather a current probe, for this purpose. All that is needed is a short length of ferrite material, a few meters (yards) of thin enameled copper wire, a BNC plug, and a meter (yard) of 50-0 coaxial cable, for instance, Type RG58/U.
Ferrite has a low magnetic resistance and draws the fields around the track towards itself. To ensure optimum coupling, the ferrite must be placed as close as possible to the track. The most convenient shape of the ferrite is a bead cut in half lengthwise: the hollow inside can then envelop the track at an angle of 180 (degrees) see diagram.
About 40 turns of the copper as are wound around the half bead as shown. The terminals of the winding are soldered to the coax cable. Solder one terminal of the coil to the conductor of the cable, insulate the connection with tape and then fold the earth braid back over the conductor. Then solder the second terminal of the winding to the braid.
To give the probe some mechanical rigidity. push a narrow tube (such as an empty ballpen) over the cable on to the winding. When that is done, fit the tube to the sensor with superglue or epoxy resin. Terminate the other end of the coaxial cable into the BNC plug.
The self-inductance of the sensor winding is some 24-30 μH, which gives a reactance (= 2ΠfL) in the wanted frequency range that is much greater than 50 Ω. The voltage measured with a spectrum analyzer or oscilloscope is thus virtually independent of the signal frequency.
The core of the prototype shown in the photograph is 4 mm thick and 15 mm long. The self-inductance of the sensor winding is 80 μH. which makes the probe suitable for frequencies >500 kHz. As long as the Core material is not saturated. the sensitivity of the probe is about 0.2 V A-1.
