# The rectifier exhibits capacitance properties when

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temperature. The rectifier exhibits capacitance properties when reverse biased, and tends to bypass higher frequency. The meter reading may be in error by as much as 0.5% decrease for every 1kHz rise in frequency. Cont’d…

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12/07/18 Chapter 1 : AC Meters 17 A general rectifier type ac voltmeter arrangement that use in half-wave rectification have additional diode and shunt resistor as shown in Figure 3.4. The additional diode D2 is reverse-biased on the positive half-cycle and has virtually no effect on the behavior of the circuit. In the negative half-cycle, D2 is forward-biased and provides an alternate path for reverse biased leakage current that would normally flow through the meter movement and diode D1. The purpose of R sh is to increase the current flow through D1 during the positive half cycle so that the diode is operating in a more linear portion of its characteristic curve. Cont’d…
12/07/18 Chapter 1 : AC Meters 18 Cont’d… D 1 R sh I sh AC Input R s R m D 2 I f I m Figure 3.4 Half-wave rectification using an instrument rectifier and a shunt resistor for improved linearity. Example 3.1: Design a voltmeter for a 10V rms ac range on the circuit shown in Figure 3.1 and Figure 3.2. Determine the ac sensitivity of both design .

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19 Example 3.2 In the half wave rectifier shown in Figure 3.5, diodes D1 and D2 have an average forward resistance of 50Ω and are assumed to have an infinite resistance in the reverse direction. Calculate: a) R s b) ac sensitivity c) dc sensitivity Cont’d… D 1 R sh = 200 I sh AC Input 10Vrms R s R m = 200 Ifsd=100 A D 2 I f I m Figure 3.5 Half-wave rectification using an instrumentation rectifier and a shunt resistor
20 Example 3.3 Each diode in the full wave rectifier circuit in figure 3.6 has an average forward resistance of 50 and is assumed to have an infinite resistance in the reverse direction Calculates: a) R s b) ac sensitivity c) dc sensitivity Cont’d… Figure 3.6 AC voltmeter circuit using full-wave rectification and shunt

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12/07/18 Chapter 1 : AC Meters 21 3.4 Other types of AC meter movement Electrodynamometer movement Figure 3.7 Electrodynamometer Movement
12/07/18 Chapter 1 : AC Meters 22 Has the same basic operating principle as the D’Arsonval meter movement, except that the permanent magnet is replaced by fixed coils. The moving coil and pointer, which are attached to the coil, are suspended between and connected in series with the two field coils (fix coil). The two field coils and moving coil are connected in series such that the same current flows through each coil. Current flow through the three coils in either direction causes a magnetic field to be produced between the field coils. The same current flow through the moving coil causes it to act as a magnet exerting a force against the spring Cont’d…

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If the current is reversed, the field polarity and the polarity of the moving coil reverse, and the force continues in the same direction.
• Fall '17
• puan nor azilah binti ahmad
• Rectifier, AC Meters

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