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1、电子信息类专业英语,http:/,机械工业出版社同名教材配套电子教案,温丹丽 高源 制作,Reading Material Power Supplies,Text 1. Information on power supplies There are many types of electrical and electronic power supplies, providing various alternating（交替的）current (AC) and direct current (DC) voltages for equipment operation. DC voltages ar
2、e needed for most electronic circuits, and generally an electronic power supply is considered as a device that converts AC into DC.,机械工业出版社 http:/,Reading Material Power Supplies,The AC voltage from the power lines can be rectified（电整流）directly or passed through an isolation transformer（变压器）(a turns
3、 ratio of 1:1). Depending on the value of DC voltage needed, the transformer may be of either a step-up or step-down type. After the transformer, the AC is rectified into pulsating DC by diodes in the form of a half-wave rectifier, a full-wave rectifier, or a bridge (full-wave) rectifier. He pulsati
4、ng DC is then filtered（滤波）or smoothed out by capacitors, inductors, and resistors so as to produce a constant DC output.,Reading Material Power Supplies,2. Bridge (full-wave) rectifier The bridge rectifier produces a full-wave DC output with the use of four diodes (Fig.2-5). When the positive half-c
5、ycle appears, the anode of diode 1 is positive and it conducts, and the cathode of diode 2 is negative and it conducts. Current flows from the AC source through diode 1, through RL, through diode 2, and back to the source. Diode 3 and 4 are reversed biased and no current flows through them. They app
6、ear as open circuits. On the negative half-cycle the anode of diode 4 is positive and it conducts, and the cathode of diode 3 is negative and it conducts. Current flows from the AC source through diode 4, through RL, through diode 3, and back to the source. Diode 1 and 2 are reverse. Voltage output
7、U0 is the peak voltage less two times the UF across the diodes (two diodes are conducting at a time):,Reading Material Power Supplies,Full-wave bridge rectifier modules containing four diodes in a single unit are commercially available.,Reading Material Power Supplies,a) Negative half-cycle b) Full-
8、wave rectifier circuit c) Total output voltage waveform,Fig.2-5 Rectifier,Reading Material Power Supplies,3. Filtering Capacitors are used in power supplies to smooth out pulsating DC and to produce steady DC voltage. A simple capacitor filter is connected across the output. When this peak voltage b
9、egins to decrease, the stored electrons in the negative side of the capacitor will discharge （电放电）and flow through the load in an attempt to keep the voltage constant across the capacitor.The slight charging and discharging of the capacitor produces a ripple（物波纹）voltage (AC component) that is superi
10、mposed（添加）on the top of the steady DC (Fig.2-6).,Reading Material Power Supplies,Fig.2-6 Output Wave,Reading Material Power Supplies,4. Zener Diode A zener diode is a specially doped diode that operates the same as a regular diode in the forward-biased condition (it allows large current to flow). Ho
11、wever, in the reverse-biased condition, it will not conduct until it reaches the zener voltage (UZ) at which it is designed. At this point the zener diode conducts current in the reverse direction, while maintaining the zener voltage across its terminals ( shown in Fig.2-7 and in Fig.2-8). The amoun
12、t of current that flows through it is determined by two factors, a series (current-limiting) resistor (RS) and the parallel load resistance (RL). Resistor RS is found by the formula where . With no load, a specific amount of current ( ) flows through the zener diode and RS. Voltage drop plus UZ will
13、 equal Usource (Usource should be at least 1V greater than UZ). When a load is connected across the zener diode, the diode current decreases by the amount drawn by the load so that the current through RS remains constant ( ). The zener diode maintains the voltage across its terminals by varying the current that flows through it.,Reading Material Power Supplies,Fig.2-7 Zener Circuit,Reading Material Power Supplies,Fig.2-8 The Output Characteristic or a Zener,Class is over!,Have a rest!,追 求,机械工业出版社 http:/,
