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Transcript
S S P P AMMETER, VOLTMETER, AND OHMMETER DESIGN The Ammeter The maximum current that the dâArsonval movement can read independently is equal to the current sensitivity of the movement. However, higher currents can be measured if additional circuitry is introduced. This additional circuitry, as shown in Fig. 7.44, results in the basic construction of an ammeter. 1 mA, 50 a Ammeter b Im Is c d Rshunt Imax = 1 A FIG. 7.44 Basic ammeter. The resistance Rshunt is chosen for the ammeter of Fig. 7.44 to allow 1 mA to flow through the movement when a maximum current of 1 A enters the ammeter. If less than 1 A should flow through the ammeter, the movement will have less than 1 mA flowing through it and will indicate less than full-scale deflection. Since the voltage across parallel elements must be the same, the potential drop across a-b in Fig. 7.44 must equal that across c-d; that is, (1 mA)(50 ) Rshunt Is Also, Is must equal 1 A 1 mA 999 mA if the current is to be limited to 1 mA through the movement (Kirchhoffâs current law). Therefore, (1 mA)(50 ) Rshunt(999 mA) (1 mA)(50 ) Rshunt 999 mA 0.05 In general, RmICS Rshunt Imax ICS (7.3) One method of constructing a multirange ammeter is shown in Fig. 7.45, where the rotary switch determines the Rshunt to be used for the 1 mA, 50 I + Imax = 1 A 0.005 External terminal Rotary switch 0.05 Imax Imax = 10 A = 100 A 0.0005 FIG. 7.45 Multirange ammeter. â I External terminal  231
