Current measurement is needed for devices such as the thermocouple-gauge pressure sensor and the ionization gauge that have an output in the form of a varying electrical current.
Analogue meters are useful in applications where there is a need to display the measured value on a control panel. Moving coil instruments are used as panel meters to measure d.c. current in the milliamp range up to one ampere. Moving iron meters can measure both d.c. and a.c. up to several hundred amps directly.
Shunt resistance is inserted in the electromechanical meter to measure the voltage drop across it. Current transformers provide an alternative method of measuring high-magnitude currents, which avoids the difficulty of designing a suitable shunt. Different versions of these exist for transforming both d.c. and a.c. currents. A d.c. current transformer:
The central DC conductor in the instrument is threaded through two magnetic cores carrying two high impedance windings connected in opposite in series. It can be shown that the current flows in the windings when it is excited with an a.c. The voltage is proportional to the d.c. Current in the central conductor. This output current is commonly rectified and then measured by a d.c. voltmeter.
An a.c. current transformer typically has a primary winding consisting of only a few copper turns wound on a rectangular or ring-shaped core. Secondary winding, however, would normally have several hundred turns according to the current step-down ratio required. The output of secondary winding is measured by any suitable current-measuring instrument.
The design of the current transformers is substantially different from that of the voltage transformers. The rigidity of its mechanical construction must be sufficient to withstand the large forces arising from short-circuiting currents, and special attention must be paid to the insulation between its windings for similar reasons. A low loss core material is used, and the flow densities are kept as small as possible to reduce losses. In the case of very high currents, the primary winding often consists of a single copper rod that behaves like a single-turn winding.