Metering of Rectified Current

Author: E.E. Kimberly

The average value of one-half 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 one-half of which is used,

The effective value of one-half 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 one-half of which is used, the effective value is

Hence, an a-c ammeter in series with a d-c ammeter in a resistive circuit carrying half-wave rectified current would read higher than the d-c meter in the ratio of

A direct-current 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 a-c ammeter is always the heating value.

The ratio of the reading of an a-c ammeter to the reading of a d-c ammeter in a resistive circuit carrying full-wave rectified current is 0.707/0.636 = 1.11

The ratio of the readings of a-c and d-c ammeters in a battery-charging circuit carrying half-wave or full-wave 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 charging-current impulse is not a complete half-cycle of a sinusoid.