Electrical indicating and test instruments

6.2.5 Analogue multimeter

The analogue multimeter is a multi-function instrument that can measure current and resistance as well as d.c. and a.c. voltage signals. Basically, the instrument consists of a moving-coil meter with a switchable bridge rectifier to allow it to measure a.c. signals,

 as shown in Figure 6.6. A set of rotary switches allows the selection of various series and shunt resistors, which make the instrument capable of measuring both voltage and current over a number of ranges. An internal power source is also provided to allow it to measure resistances as well. Whilst this instrument is very useful for giving an indi[1]cation of voltage levels, the compromises in its design that enable it to measure so many different quantities necessarily mean that its accuracy is not as good as instruments that are purpose designed to measure just one quantity over a single measuring range.

6.2.6 Measuring high-frequency signals

One major limitation in using analogue meters for a.c. voltage measurement is that the maximum frequency measurable directly is low, 2 kHz for the dynamometer voltmeter

and only 100 Hz in the case of the moving-iron instrument. A partial solution to this limitation is to rectify the voltage signal and then apply it to a moving-coil meter, as shown in Figure 6.7. This extends the upper measurable frequency limit to 20 kHz. However, the inclusion of the bridge rectifier makes the measurement system particu[1]larly sensitive to environmental temperature changes, and non-linearities significantly affect measurement accuracy for voltages that are small relative to the full-scale value.

 An alternative solution to the upper frequency limitation is provided by the thermo[1]couple meter (see below).

6.2.7 Thermocouple meter

The principle of operation of the thermocouple meter is shown in Figure 6.8. The measured a.c. voltage signal is applied to a small element. This heats up and the resulting temperature rise is measured by a thermocouple. The d.c. voltage generated in the thermocouple is applied to a moving-coil meter. The output meter reading is an r.m.s. quantity that varies in a non-linear fashion with the magnitude of the measured voltage. Very high-frequency voltage signals up to 50 MHz can be measured by this method.