You are given a d\'Arsonval (panel) meter that gives a full-scale deflection whe

Question

You are given a d'Arsonval (panel) meter that gives a full-scale deflection when passing a current of 1mA with the voltage dropped across the meter bemg 128mV. Design a current meter (ammeter) ckt for a full-scale reading (FSR) of 20mA. Draw the ckt diagram and show all calculations. Repeat for a voltage meter (voltmeter) with an FSR of 20V. Figure out a way to realize (as accurately as possible) the shunt and series resistors in (a) and (b). respectively. using series/parallel combinations of standard resistor values.3 There may be some unavoidable inaccuracy. Let this be called the "realization error.". For both meters, calculate the error bounds on the FSR resulting from the combination of component tolerance4 and realization error. Express this error as a plusminus% of FSR.

Explanation / Answer

a) For 20 mA metering, we need to shunt 19 mA across anexternal R while 1 mA goes through the meter.
Since the V for this is .128 V, the external R must be.128/.019 = .632 . I won't draw this here, but theR is in parallel to the meter.
b) for full scale voltage = 20 V, we need a series resistor. For 20 Volts across the R and meter, there is 20 - .128across the R, or 19.19.872 Volts, at a current of 1 mA. ThisR must be 19.872/.001 = 19,872 , or 19.872 k. I won'tdraw this either, but the R is in series with the meter.
c, d) I don't have the book, so I can't look up the resistorvalues or tolerances.

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