VISUAL INDICATORS 



Electrical protection of moving-coil movements 



Meters may be protected from overload by the connection in series of 

 Wollaston wire fuses, and the construction of these has been described 

 by Strafford^. Alternatively, use may be made of the non-linear resistance 

 properties of semi-conductor diodes {Figure 32.5). When a potential differ- 

 ence across the movement is within the normal range, neither diode is 

 switched completely to full-conduction, and the diode resistances are 

 therefore high; most of the current supplied passes through the meter. 

 Under overload conditions, however, one or other of the diodes conducts 

 strongly, the resistance is low and most of the applied current flows through 

 that diode instead of through the meter. The scale is no longer linear, 

 so in cases where both are applicable the choice between fuse protection 



♦ o- 



- o- 

 Figure 32.5 Figure 32.6 



and rectifier protection is determined by whether a linear or a logarithmic 

 scale is required. For details of rectifier protection methods Scroggie^ and 

 de Gruchy^ may be consulted. Scroggie also quotes a method for the 

 electrical protection of meters reading voltages above 100 or so, due to 

 T. E. Burnup. A cold cathode diode is connected across the meter and an 

 appropriate fraction of the series resistor, such that the tube strikes just 

 before the applied voltage becomes excessive {Figure 32.6). 



Use of meter amplifier 



The cost and fragility of moving-coil movements rises steeply as the 

 full-scale deflection current falls. To obtain a meter with a full-scale 

 deflection of a few microamps it may be cheaper, and a robuster job be 

 obtained, to consider the use of transistor amplification. Johnson^ has 

 described a push-pull transistor amplifier, small enough to fit into the 

 meter case, which converted a 50 /^A F.S.D. movement into an instrument 

 giving a full-scale deflection for less than 2 /^A. Apparently no difficulty 

 was experienced from drift. 



A group of meters is shown in Plate 32.1. These are all of the 'flush- 

 mounting' type: (A) is 1 in., end-zero, round, moving coil; (B) is 2 in., 

 end-zero, square, moving coil; (C) is 2|in., centre-zero, round, moving 

 coil; (D) is 3 in., end-zero, round, moving iron; and (E) is 4| in., end-zero, 

 rectangular, moving coil. 



450 



