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FSD in an Ammeter or Electric Circuit FSD in an ammeter or electrical measuring instrument stands for Full Scale Deflection . What Full Scale Deflection (FSD) Means Full Scale Deflection is the maximum current that causes the pointer (needle) of an analog meter to move to the end of the scale . In simple words: When the meter needle goes all the way to the maximum reading , The current flowing through the meter is the FSD current . Example Suppose an ammeter has: FSD current = 5 mA This means: When 5 mA flows through the meter movement , The needle will reach the maximum mark on the scale . Why FSD is Important FSD helps in: Designing ammeters and voltmeters Calculating shunt resistors (to measure larger currents) Determining the range of the instrument Example in Circuit Design If a meter movement has: FSD current = 1 mA To make it measure 10 A , engineers add a shunt resistor so that: Only 1 mA goes thr...