High-input-impedance clipon a.c. voltmeter

The paper describes a fully portable clipon a.c. voltmeter, whereby the voltage of an insulated cable can be indicated on a conventional a.c. voltmeter which has one of its terminals connected to earth. The operation relies on the capacitance between a conducting cylinder, which is clipped around the cable, and the inner conductor of the cable. A transistor amplifier is employed in a feedback arrangement which makes the reading sensibly independent of variations in this capacitance; e.g. in the instrument described, a 3:1 variation in capacitance results in a 5% change in reading. By virtue of the feedback, a very high input impedance (~4×1010 Ω) is achieved, and a wide bandwidth is possible (about one fifth of the amplifier-gain-bandwidth product). An important use of the instrument could be in the checking of mains-power circuits, and, in the device described, a transformer is employed to extend the voltage range to 260V r.m.s. at 50c/s; further extension is possible. The instrument can be used in conjunction with a conventional clipon ammeter to obtain direct indication of power, apparent power and power factor, without physical contact.