Lessons In Electric Circuits -- Volume II Chapter 12 AC METERING CIRCUITS AC voltmeters and ammeters AC electromechanical meter movements come in two basic arrangements: those based on DC movement designs, and those engineered specifically for AC use. Permanent-magnet moving coil (PMMC) meter movements will not work correctly if directly connected to alternating current, because the direction of needle movement will change with each half-cycle of the AC. Permanent-magnet meter movements, like permanent-magnet motors, are devices whose motion depends on the polarity of the applied voltage (or, you can think of it in terms of the direction ofthe current). In order to use a DC-style meter movement such as the D'Arsonval design, the alternating current must be rectifiedinto DC. This is most easily accomplished through the use of devices called diodes. We saw diodes used in an example circuit demonstrating the creation of harmonic frequencies from a distorted (or rectified) sine wave. Without going into elaborate detail overhow and why diodes work as they do, just remember that they each act like a one-way valve forelectrons to flow: acting as a conductor for one polarity and an insulator for another. Oddly enough, the arrowhead in each diode symbol points againstthe permitted direction of electron
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