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| SEC. 4-1 THE HALF-WAVE
RECTIFIER The half-wave rectifier has a diode in series with a load resistor. The load voltage is a half-wave output. The average or dc voltage out of a half-wave rectifier equals 31.8 percent of the peak voltage. SEC. 4-2 THE TRANSFORMER The input transformer is usually a step-down transformer in which the voltage steps down and the current steps up. The secondary voltage equals the primary voltage divided by the turns ratio. SEC. 4-3 THE FULL-WAVE RECTIFIER The full-wave rectifier has a centertapped transformer with two diodes and a load resistor. The load voltage is a fullwave signal whose peak value is half the secondary voltage. The average or dc voltage out of a full-wave rectifier equals 63.6 percent of the peak voltage, and the ripple frequency equals 120 Hz instead of 60 Hz. SEC. 4-4 THE BRIDGE RECTIFIER The bridge rectifier has four diodes. The load voltage is a full-wave signal with a peak value equal to the secondary voltage. The average or dc voltage out of a half-wave rectifier equals 63.6 percent of the peak voltage, and the ripple frequency equals 120 Hz. SEC. 4-5 THE CHOKE-INPUT FILTER The choke-input filter is an LC voltage divider in which the inductive reactance is much greater than the capacitive reactance. The type of filter allows the average value of the rectified signal to pass through to the load resistor. SEC. 4-6 THE CAPACITOR-INPUT FILTER This type of filter allows the peak value of the rectified signal to pass through to theload resistor. With a large capacitor, the ripple is small, typically less than 10 percent of the dc voltage. The capacitorinput filter is the most widely used filter in power supplies. SEC. 4-7 PEAK INVERSE VOLTAGE AND SURGE CURRENT The peak inverse voltage is the maximum voltage that appears across the nonconducting diode of a rectifier circuit. This voltage must be less than the breakdown voltage of the diode. The surge current is the brief and large current that exists when the power is first turned on. It is brief and large because the filter capacitor must charge to the peak voltage during the first cycle or, at most, during the first few cycles. SEC. 4-8 OTHER POWER-SUPPLY TOPICS Real transformers usually specify the secondary voltage at a rated load current. To calculate the primary current, you can assume that the input power equals the output power. Slow-blow fuses are typically used to protect against the surge current. The average diode current in a half-wave rectifier equals the dc load current. In a full-wave or bridge rectifier, the average current in any diode is half the dc load current. LC filters and LC filters may occasionally be used to filter the rectified output. SEC. 4-9 TROUBLESHOOTING Some of the measurements that can be made with a capacitor-input filter are the dc output voltage, the primary voltage, the secondary voltage, and the ripple. From these, you can usually deduce the trouble. Open diodes reduce the output voltage to zero. An open filter capacitor reduces the output to the average value of the rectified signal. SEC. 4-10 CLIPPERS AND LIMITERS A clipper shapes the signal. It clips off positive or negative parts of the signal. The limiter or diode clamp protects sensitive circuits from too much input. SEC. 4-11 CLAMPERS The clamper shifts a signal positively or negatively by adding a dc voltage to the signal. The peak-to-peak detector produces a load voltage equal to the peak-to-peak value. SEC. 4-12 VOLTAGE MULTIPLIERS The voltage doubler is a redesign of the peak-to-peak detector. It uses rectifier diodes instead of small-signal diodes. It produces an output equal to 2 times the peak value of the rectified signal. Voltage triplers and quadruplers multiply the input peak by factors of 3 and 4. Very high voltage power supplies are the main use of voltage multipliers. |