Metal Finishing Guide Book


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ing the clamp-on ammeter around the flexible lead. A diode that is open will draw no current, whereas a diode that is shorted will draw excessive current. In either case, the diode should be replaced. As these diodes are removed, the DVM may be used on the diode range to verify that the diode being removed is, in fact, bad. A defective diode will read either open or shorted in both directions. The DVM may be also used to determine possible metering circuit defects. To check the power-supply voltmeter, measure the voltage across the output terminals of the rectifier and the terminals at the back of the panel voltmeter. Compare these readings with that of the panel voltmeter. They should all agree. Current is typically determined by measuring the voltage drop across a precision resistor placed at the output terminals known as a shunt. This voltage drop at full output will typically be 50 mV. This low-level voltage signal has to be multiplied by a factor before comparing it to the actual meter reading. The oscilloscope is useful in locating problems where complete diode circuit branches have burned open and left a missing section in the wave shape; however, this may also be a symptom of thyristor problems on the primary of the main transformer. If the AC ripple component of the output is important to the process, then an oscilloscope with a built-in true rms feature can be used to view the ripple waveform, as well as determine the AC to DC ratio of the ripple using the AC and DC coupling of the scope. The electronics are the most complex part of the power supply. Electronic circuits are usually indicated on schematics by boxes with terminal numbers and functions labeled along the edges. The DVM is commonly used in the testing of these electronic circuits to measure signal and control voltages. Although there are many different types of electronic circuits, two are found in every power supply and must function correctly for proper power supply operation. These are the drive circuit and the firing circuit. In some cases, these will be on one circuit board, whereas at other times, they will be on separate boards. The drive circuit is an analog amplifier circuit. It receives current and voltage reference signals from the operators ACC and AVC potentiometers. These control signals will typically range between 0 and 2.5 V DC, depending on the position of the operator controls. To check a typical drive circuit initially, verify that there is 120 V AC on the power terminals and that there are reference voltages on the ACC and AVC input terminals. You should then have a voltage at the output terminals. If no signal is available at these output terminals, the drive circuit may be defective or seriously out of adjustment. Remove and further test the drive circuit using the test procedures found in your operators manual. The firing circuit accepts the output signals of the drive circuit and produces synchronized gate pulses that fire the thyristors in the AC power circuit, which in turn regulates the voltage to the primary of the main power transformer. To test this circuit, ensure there is a signal of more than 2V DC at the input from the firing circuit. Then measure the signals at the gate outputs to the thyristors with the DVM. They should typically be about 1 V DC. Perform these measurements with great care against shorting any of the leads to ground or to another pair of terminals, as there may be line voltages of up to 600 V AC between these terminals and ground. As with the drive circuit, if any signals are missing or incorrect, remove the board and bench repair using the procedures outlined in the operators handbook. 781

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