SMOOTHED RECTIFICATION 



When a direct voltage is to be read, all the switches are in position 1 and the 

 circuit is by way of the appropriate multiplier resistance, straight to the 

 moving coil unit, which has the resistance R^ connected across it. When the 

 instrument is switched to read alternating voltage, all contacts are altered to 

 position 2, and the circuit reduces to Figure 6.9. When any alternating 



Appropriate 

 multiplier 



o VVV^A 



©-^ 



Figure 6.9 



voltage of any waveform is connected, the deflection of the pointer is propor- 

 tional to the average of the rectified current. If the input voltage is sinusoidal, 

 the average value is 



TT Jo 



V . sin oit . dt 



2V 



TT 



0-637 V 



and the pointer deflection will be proportional to this. The reading which is 

 required on the scale is the R.M.S. value of V, which is 0-707K. Thus, in 

 order that the instrument shall read correctly on alternating voltage ranges 

 it has to have its sensitivity increased by a factor 0-707/0-637 = Ml, and this 

 is achieved by removing automatically from the circuit the diverter resistance 

 R^. The reading is then right for a pure sinusoidal alternating vohage input 

 but not for any other kind of waveform. Thus, a waveform which often 

 occurs in electronics is the so-called square wave {Figure 6. JO) which has the 



+1/ 



i 



-V 



t 



Figure 6.10 



property that its R.M.S. and average after rectification are the same. If 

 this is of amplitude V, the reading indicated by the pointer will be IM per 

 cent high. If the meter is of the kind which contains a transformer it may be 

 worse still. Much could be written about the interpretation of alternating 

 voltage voltmeter readings but in the author's opinion if the waveform is at 

 all peculiar it is best to look at it on a measuring oscilloscope; for special 

 waveforms it is in any case a moot point how useful or significant an average 

 reading is. 



SMOOTHED RECTIFICATION 



To smooth the output of a rectifying circuit we can either: (1) put an induc- 

 tance in series with the load; or (2) put a capacitance in parallel with the 

 load. The first method is called the choke input circuit, while the second is 

 the capacitor input circuit. 



99 



