The direct current used in the plating process can be supplied by either a low-voltage generator or a rectifier. At one time the low-voltage generator was used exclusively as the plating power source. These generators have been largely replaced with rectifier units to cut down on the high initial cost and the maintenance required during operation of the generators.
The dc power supply has undergone as much development over the years as the electrochemical process. In the early days of electroplating, around 1920, generators were the only available source of dc power. By 1950 the rectifier had almost exclusively replaced the generator due to its many advantages.
A rectifier is a device that converts alternating current from the plant source into direct current that is used in the plating process. A rectifier consists mainly of a transformer to reduce the voltage, diodes to convert the current to dc, a voltage and current control for the output, overload protection, and a cooling device. Silicon is the most common material used in the diodes. Several diodes are used in order to handle the current load. Heat is generated during operation of a rectifier. This heat must be removed, and either fan or water cooling may be used.
When specifying a rectifier for electroplating it is important to specify a ripple of 5% or less. Several of the plating processes will not operate properly with a higher ripple factor.
Another factor to be considered when specifying a rectifier is the current wave form. A rectifier may be built to put out either a half-wave or full-wave current. It is important to use only a full-wave unit since many of the processes, especially chromium plating processes, will not operate properly with half-wave.
All rectifiers used in electroplating have a variable output control. Both tap switches and infinitely variable controls can be used. Fixed output units are available; they are somewhat less expensive, but are obviously not desirable due to the ever-changing load factors of a busy plating shop. Other controls that are available on modern rectifiers include automatic voltage, current, and current density control. These special options are becoming increasingly more popular as they provide a consistent output control for the plater.
The commercial use of a totally new power source, the pulse rectifier, was introduced in 1978. A pulse rectifier is essentially a switching device that turns the dc output on and off at millisecond intervals. It has a high-frequency square wave output that has been used to solve many deposition problems and has greatly improved the quality of electroplating. The important characteristic of a square wave output is that the rise time from zero power to full current is practically instantaneous. The power then remains on for a preset period of time, when it is then turned off for a preset period of time. Then the cycle repeats itself. This cycling can be set at intervals up to 1000 times per second.
A modern high-frequency square wave pulse rectifier has controls such that the output frequently can be set to the optimum conditions of the electroplating process. This optimum frequency is different for each process due to the specific electrochemical conditions associated with it. The pulse rectifier is effective due to its ability to control I polarization as well as the cathode film during deposition. This control has greatly improved throwing power, deposition rates, internal stress, and hardness of the deposit. Due to these advantages, pulse plating is finding wide acceptance in industry.