Metal Finishing Guide Book


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5. Spread a small amount of thermal compound on the new thyristor, taking care not to get any compound on the thyristor threads. 6. Insert the stud in the heat sink, reassemble the flat washer and the star washer, and then tighten the retaining nut. 7. Attach all leads to the proper locations, being sure that all connections are clean and tight. Flat-pack thyristors and diodes, sometimes referred to as "hockey pucks," are used in higher power rectifiers. They range from 2 through 4 in. in diameter. As with the stud-mount devices, the only difference between a flat-pack thyristor and diode is the presence of gate and cathode leads on the device. A flat-pack device is secured between two current-carrying bus bars by a clamping mechanism. Some clamps have indicators built in, whereas others do not. When replacing a device secured with a gauged clamp, note the reading before removing the device. The other type of clamps used are either 5,000- or 10,000-lb clamps. These systems consist of a pair of clamping bars, connected by two studs, between which is sandwiched the bus bars, a Belville washer system, and the semiconductor device. Replacement of thyristors or diodes utilizing these types of clamps requires the use of measuring devices. The following steps should be taken to replace a flat-pack thyristor or diode (refer to Fig. 8). 1. Note the clamping arrangement being used. If a gauge is present on the lamp, record the indication. Mark and remove the gauge and cathode leads if replacing a thyristor. 2. Uniformly and slowly loosen the nuts on the clamp studs. Remove the Belville washer assembly and the device. Note that the Belville washer is made up of four parts: a centering section, a flat washer, and two concave washers. 3. Clean the surfaces of both bus bars and the new thyristor or diode. Clean both clamping bars, and check that the insulated surfaces of the clamp have not been damaged. 4. Apply heat sink compound sparingly to both surfaces of the device and to the bus bars. 5. Place the new flat pack in the clamping mechanism, ensuring that the device is oriented properly. Check the other devices to verify this. There are typically roll pins in the bus bars that align with depressions in the device. Make sure the roll pins do not damage the flat-pack surfaces. 6. Reassemble the Belville washers as shown, making sure the two concave washers are back to back. Now place the washers in the clamp. 7. Finger tighten the clamp nuts, ensuring all parts are situated properly, and tighten the nuts with a wrench one-quarter additional turn. Check that approximately the same number of threads are visible beyond the nuts on each stud. 8. Using a depth gauge, measure through the center of the hole in the bus bar and Belville washer system. Note this reading. 9. Tighten each nut one-half turn, and recheck with the depth gauge. Continue this tightening procedure until the difference from the original reading is 0.048 ± 0.004 in. for a 10,000 lb clamp, and 0.026 ± 0.002 in. for a 5,000 lb clamp. 10. Reattach the gate and cathode thyristor leads. 751

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