Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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Page 636 of 707

Fig. 2. Secondary thyristor. utilized. The function of the diodes, as stated earlier, is to allow conduction of cur- rent in only one direction. When the diodes are used, as shown in Fig. 1, they will rectify the transformer output and provide DC. Another method is to place the thyristors on the secondary side of the trans- former, as shown in Fig. 2. This is known as a secondary thyristor design. In this configuration, the thyristors perform both the regulation and rectification operations, and no diodes are required. Either design can provide the desired DC output, and although each method has its advantages and disadvantages, the cost is usually the determining factor. The advantages of the primary method are as follows: Soft start—Because the controlling element is in the primary side of the trans- former, it can control the inrush current to the transformer. Efficiency—It is slightly more efficient than some secondary designs. The advantages of the secondary method are as follows: Reliability—Fewer components mean greater reliability. It has greater voltage safety margin on SCRs. It is less susceptible to line voltage transients. Reversing—It is able to achieve solid-state reversing. PLATING Direct Current Plating Direct current electroplating covers a broad range of processes. These include, but are not limited to, chromium, nickel, copper, zinc, cadmium, silver, and gold. Whereas each of these processes vary somewhat in their particular voltage and cur- rent requirements, they all require some form of DC power to deposit the met- al out of solution onto the part being plated. A typical DC plating power supply will have a three-phase input of either 230 or 460 V AC. The output will be somewhere in the range of 6 to 18 V and 635

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