Fundamental Electrical and Electronic Principles, Third Edition

212 Fundamental Electrical and Electronic Principles6.11 Rectifier Moving Coil MeterThe circuit arrangement for such a meter is illustrated in Fig. 6.16 . Thesymbol, with the letter M in it, represents the moving coil movement.The current through the coil will therefore be a series of half-sinewavepulses, as in Fig. 6.15 .a.c. to bemeasuredMFig. 6.16Due to the inertia of the meter movement, the coil will respond tothe average value of this current. Thus, the pointer would indicate theaverage value of the a.c. waveform being measured. However, thenormal requirement is for the meter to indicate the r.m.s. value.Thus, some form of ‘ correction factor ’ is required.The majority of a.c. quantities to be measured are sinewaves. Forthis reason, the meter is calibrated to indicate the r.m.s. values ofsinewaves. Now, the ratio between r.m.s. and average values is theform factor. For the sinewave, the form factor has a value of 1.11. Thevalues chosen for shunts and multipliers, used on the a.c. ranges, aretherefore modified by this factor. Thus, although the pointer positioncorresponds to the average value, the scale indication will be ther.m.s. value.Worked Example 6.8QA moving coil meter has a figure of merit of 10 k Ω / V. The coil has a resistance of 50 Ω . Calculate thevalue of multiplier required for (a) the 10 V d.c. range, (b) the 10 V a.c. range.AR c 5 0 Ω ; figure of merit 10 kΩ /V; V 10 V(a)I fsd 1/10 000 10 0 μ Atotal meter resistance, R VI fsd10ohm1 0 4so, R100k Ω; and since R R R, then:Rm100 00050 99. 95 k Ω Ansmc