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Ammeter design _ Dc Metering Circuits

Ammeter design _ Dc Metering Circuits - Ammeter desig n Dc...

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1/22/2014 Ammeter design : Dc Metering Circuits http://www.allaboutcircuits.com/vol_1/chpt_8/4.html 1/8 Ammeter design A meter designed to measure electrical current is popularly called an "ammeter" because the unit of measurement is "amps." In ammeter designs, external resistors added to extend the usable range of the movement are connected in parallel with the movement rather than in series as is the case for voltmeters. This is because we want to divide the measured current, not the measured voltage, going to the movement, and because current divider circuits are always formed by parallel resistances. Taking the same meter movement as the voltmeter example, we can see that it would make a very limited instrument by itself, full-scale deflection occurring at only 1 mA: As is the case with extending a meter movement's voltage-measuring ability, we would have to correspondingly re-label the movement's scale so that it read differently for an extended current range. For example, if we wanted to design an ammeter to have a full- scale range of 5 amps using the same meter movement as before (having an intrinsic full-scale range of only 1 mA), we would have to re-label the movement's scale to read 0 A on the far left and 5 A on the far right, rather than 0 mA to 1 mA as before. Whatever extended range provided by the parallel-connected resistors, we would have to represent graphically on the meter movement face. Using 5 amps as an extended range for our sample movement, let's determine the amount of parallel resistance necessary to "shunt," or bypass, the majority of current so that only 1 mA will go through the movement with a total current of 5 A:
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1/22/2014 Ammeter design : Dc Metering Circuits http://www.allaboutcircuits.com/vol_1/chpt_8/4.html 2/8 From our given values of movement current, movement resistance, and total circuit (measured) current, we can determine the voltage across the meter movement (Ohm's Law applied to the center column, E=IR): Knowing that the circuit formed by the movement and the shunt is of a parallel configuration, we know that the voltage across the movement, shunt, and test leads (total) must be the same: We also know that the current through the shunt must be the difference between the total current (5 amps) and the current through the movement (1 mA), because branch currents add in a parallel configuration:
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1/22/2014 Ammeter design : Dc Metering Circuits http://www.allaboutcircuits.com/vol_1/chpt_8/4.html 3/8
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