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Metering of Rectified CurrentAuthor: E.E. Kimberly The average value of onehalf of a sine wave of current (beginning at a time when the current is zero) whose maximum value is Im is
For the whole cycle of current, only onehalf of which is used, (273) This is the value read on a directcurrent ammeter. The effective value of onehalf of a sine wave of current (beginning at a time when the current is zero) whose maximum value is Im is
For the whole cycle of current, only onehalf of which is used, the effective value is
Hence, an ac ammeter in series with a dc ammeter in a resistive circuit carrying halfwave rectified current would read higher than the dc meter in the ratio of A directcurrent ammeter is provided on a rectifier intended for battery charging because the charging value of any current is proportional to its average value, and not to its effective value. The effective value as read by an ac ammeter is always the heating value. The ratio of the reading of an ac ammeter to the reading of a dc ammeter in a resistive circuit carrying fullwave rectified current is 0.707/0.636 = 1.11 The ratio of the readings of ac and dc ammeters in a batterycharging circuit carrying halfwave or fullwave rectified current depends on the relative magnitudes of the battery voltage and the maximum value of the charging voltage; and so the foregoing ratios do not apply accurately in that case. Before a charging current can flow, it is necessary that the voltage of the charging device be greater than the emf of the battery. Hence, the chargingcurrent impulse is not a complete halfcycle of a sinusoid.


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