Equipment Modification

One factor that should always be checked when planning a change from conventional to pulsed-current power is the tank electrical contact system. Some anode and/or cathode contacts that may be perfectly suitable for conventional plating may present unwanted resistance to high-frequency peak currents. Overlooking this factor may prevent the realization of the full benefits of a modulated power supply.

The major consideration, of course, is the power system itself. Existing rectifiers may or may not be suitable for use with modulated periodic reverse or direct pulse units. For pulse plating, a high-voltage, quick-response rectifier is required, and the lower the ripple, the more precise and predictable the output. Although pulse units are available for use with existing power supplies, models with self-contained rectifiers give greater assurance that full benefit of the control system will be realized.

Pulse units with self-contained power can be operated in either a constant-average-current or constant-voltage mode. The significance of this option is illustrated in Fig. 8. Figure 8(a) depicts a pulse train with a 50% duty cycle. The average current delivered is 50% of the peak value. Figure 8(b) shows the effect of reducing the duty cycle to 25% when in a constant-voltage mode. The peak current remains the same, but the average current changes directly with the duty cycle, in this case dropping to half its former value. The current density of the pulsed current remains the same, but twice as much real time is required to deliver the same amp-minutes of current. Figure 8(c) shows the effect of reducing the duty cycle from 50 to 25% when operating in constant-average-current mode. In this case, the peak current changes inversely to the duty cycle, increasing in value to maintain the same average current delivered as before but in shorter pulses.

Average current

Average current

Average current

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