Fig. 8 Effect of changes in the duty cycle on constant-average-current and constant-voltage pulsed-current plating. (a) 50% duty cycle, with average current 50% of the peak value. (b) Duty cycle reduced to 25% in constant-voltage mode; average current drops with duty cycle. (c) Duty cycle reduced to 25% in constant-current mode; the peak current changes inversely to the duty cycle.

Although a change in frequency also changes the pulse width, it does not effect either peak or average current, regardless of output mode (Fig. 9). Unlike conventional plating rectifiers, which are rated by average current capacity (ignoring the ripple), modulated periodic reverse pulse units are normally rated by their peak current capacity. Because both peak and average current values are intrinsic to modulated power pattern plating, both output capacities must be considered. Depending on the internal circuitry of the unit, the average current output capacity of some models can be as low as 25 or 30% of the peak capacity. With such a low value for average current, the rated peak current output would be attained even at average current capacity only if a duty cycle as low as 25 or 30% was used. Attempting to push average current up would drastically shorten the life of the unit. Experience has shown that effective duty cycles are usually not less than 50% (although they can be as low as 10% for pure precious metals), and most units are designed to deliver an average current capacity of 50 to 60% of the peak current capacity rating. However, any desired duty cycle can be used or specified, but the operator must keep in mind that the average current is the percentage (duty cycle) of the peak rating.

Fig. 9 Effect of change of frequency on current pattern in pulsed-current plating. Only pulse width is altered; peak current, average current, and duty cycle remain constant.
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