A current probe generates a voltage proportional to a current in the circuit being measured; as the proportionality constant is known, instruments that respond to voltage can be calibrated to indicate current. Current probes can be used both by measuring instruments and oscilloscopes.
The classic current probe is a low valued resistor (a "sampling resistor" or "current shunt") inserted in the current's path. The current is determined by measuring the voltage drop across the resistor and using Ohm's law. (Wedlock & Roberge 1969, p. 152.) The sampling resistance needs to be small enough not to affect circuit operation significantly, but large enough to provide a good reading. The method is valid for both AC and DC measurements. A disadvantage of this method is the need to break the circuit to introduce the shunt. Another problem is measuring the voltage across the shunt when common-mode voltages are present; a differential voltage measurement is needed.
Alternating current probes
Alternating currents are relatively easy to measure as transformers can be used. A current transformer is commonly used to measure alternating currents. The current to be measured is forced through the primary winding (often a single turn) and the current through the secondary winding is found by measuring the voltage across a current-sense resistor (or "burden resistor"). The secondary winding has a burden resistor to set the current scale. The properties of a transformer offer many advantages. The current transformer rejects common mode voltages, so an accurate single-ended voltage measurement can be made on a grounded secondary. The effective series resistance of the primary winding is set by the burden resistor on the secondary winding and the transformer turns ratio , where: . The core of some current transformers is split and hinged; it is opened and clipped around the wire to be sensed, then closed, making it unnecessary to free one end of the conductor and thread it through the core. Another clip-on design is the Rogowski coil. It is a magnetically balanced coil that measures current by electronically evaluating the line integral around a current. High-frequency, small-signal, passive current probes typically have a frequency range of several kilohertz to over 100 MHz. The Tektronix P6022 has a range from 935 Hz to 200 MHz. (Tektronix 1983, p. 435)
Transformers cannot be used to probe direct currents (DC). Some DC probe designs use the nonlinear properties of a magnetic material to measure DC. Other current probes use Hall effect sensors to measure the magnetic field around a wire produced by an electric current through the wire without the need to interrupt the circuit to fit the probe. They are available for both voltmeters and oscilloscopes. Most current probes are self-contained, drawing power from a battery or the instrument, but a few require the use of an external amplifier unit. (See also: Clamp meter
Hybrid AC/DC current probes
More advanced current probes combine a Hall effect sensor with a current transformer. The Hall effect sensor measures the DC and low frequency components of the signal and the current transformer measures the high frequency components. These signals are combined in the amplifier circuit to yield a wide band signal extending from DC to over 50 MHz. (Wedlock & Roberge 1969, p. 154) The Tektronix A6302 current probe and AM503 amplifier combination is an example of such a system. (Tektronix 1983, p. 375) (Tektronix 1999, p. 571)
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