Electrical Engineering ⇒ Topic : DC Ammeter
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Maninder
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Ammeters and voltmeters are classed together because the principle of operation involved is the same for both the types of meters. An ammeter carries a definite fraction of the current to be measured, whereas a voltmeter carries a current proportional to the voltage to be measured. An ammeter is connected in series with the circuit whose current is to be measured. Insertion of any meter in the circuit should not change the circuit conditions and hence the resistance of an ammeter must be very small. On the other hand, a voltmeter is connected across a voltage source whose voltage is to be measured. The current drawn by the voltmeter must be as small as possible in order to keep the circuit conditions unaltered. Therefore, the resistance of the voltmeter must be very high compared to the resistance of the load connected across the circuit. Generally, the power loss in ammeters and voltmeters should be kept small. Suppose ra is the resistance of an ammeter carrying a current of /2 amperes, then the power loss in the instrument will be I. ra. If this power loss must be small, then ra must be small. On the other hand, if r, is the resistance of an voltmeter to which a voltage V is applied, then the power loss in the voltmeter will be | |
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David
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Ammeters and Voltmeters An ammeter is used to measure the flow of current in circuit. It is thus connected in series with the circuit under test See Fig.(a) so that current to be measured or a *fraction of it passes through the instrument itself. The ammeter must be capable of carrying this current without injury to itself and without abnormally increasing the resistance of the circuit intc which it is inserted. For this reason, an ammeter is designed to have low resistance figure (a) | |
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Sonali
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DC Ammeter A PMMC meter movement gives full scale deflection for very low current of the order of 10 μA to 1 mA. In order to measure high currents, the meter has to be shunted by low resistances to bypass the excess current. The shunt resistance is given by where n is the multiplication factor, I is the maximum value of current to be measured after shunting, and Im, is full scale current of the movement. The constant temperature shunt resistance may be inbuilt with the meter or may be connected externally to measure current up to 500 A. External shunts are made of magnanin or constantan. Large current can be measured without using a number of meters with different ranges, one single PMMC movement is used with a series of shunts selected by a ring or range switch. Such a shunt is called universal or Aryton shunt. Another advantage of this type is that the meter never remains unshunted so that the chances of getting burnt out is eliminated. Such instruments are called multimeters. The range may extend from 10 mA to 10 A. The following important points must be noted:
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