Electrical Engineering Technology EE306 Lecture 5 Northrop : Chapter 8 – pgs 501 – 503 Hughes : Chapter 47 – 47.1, 47.2,

Measurements of Voltage, Current and Resistance – DC Measurement Although most measuring instruments are now manufactured with digital electronic displays, most measuring devices still are based on analogue principles. The analogue measuring devices are to be found for a variety of reasons: They are cheaper. People still often prefer an analogue display rather than a digital one. The additional cost of digital instruments can only be justified when accuracy is essential.

Analogue Measuring Instruments Analogue measuring instruments have three essential features :- A deflecting device whereby a mechanical force is produced by the electric current, voltage or power. A controlling device whereby the value of the deflection is dependent upon the magnitude of the quantity being measured. A damping device to prevent oscillation of the moving system and enable the latter to reach its final position quickly.

Permanent magnet moving coil ammeters and voltmeters

Due to the delicate nature of the moving system, this type of instrument is only suitable for measuring currents up to about 50 mA directly. When a larger current has to be measured, a shunt S (Fig. 47.6), having a low resistance, is connected in parallel with the moving coil MC, and the instrument scale may be calibrated to read directly the total current I. The moving-coil instrument can be made into a voltmeter by connecting a resistor R in series, as in Fig. 47.7.

The main advantages of the moving-coil instrument are: high sensitivity; uniform scale; well shielded from any stray magnetic field. Its main disadvantage is that it is only suitable for direct currents and voltages.

Multi-range Moving coil Meter

Loading Error All measuring instruments draw energy from the source of the instrument – this is called the loading effect of the instrument Ideal ammeter has zero internal resistance and no voltage across it. Ideal voltmeter has infinite internal resistance and draws no current from the circuit. A practical ammeter can represented by an ideal ammeter with added series resistance that represent the meter resistance. A practical voltmeter can represented by an ideal voltmeter in parallel with the meter resistance.

A A V V Rm Rm True value – ideal value Ideal Ammeter - Practical Ammeter - Rm V Ideal Voltmeter - V Practical Voltmeter - Rm

Loading error in ammeter Example to measure current in a resistor :- Rt RL Vt Rm A

Loading error in voltmeter Example to measure voltage acrossa resistor :- Rt Vt RL Rm V

Problems on moving coil meter problems on loading errors