Lesson 8 Power Supply 电源供应器

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Power Supplies are electronic circuits designed to convert AC (Alternating Current) to DC (Direct Current) at any desired level. A power supply can be broken down into a series of blocks（Fig. 3-6）: transformer, rectifier, filter, and voltage regulator, each of which performs a particular function.

The transformer can convert AC electricity from one voltage to another with little loss of power. Most power supplies use a step-down transformer to reduce the high AC mains voltage (230V or 110V) to a safer low AC voltage (Fig. 3-7) .

The rectifier uses the ability of a diode to conduct during one half cycle of AC to convert AC to DC[1]. A single diode rectifier only uses the positive half part of the AC wave to produce a half-wave varying DC[2]. A bridge rectifier uses four diodes connected in a bridge network to produce a fullwave varying DC output (Fig. 3-8) .

Smoothing is performed by a large value electrolytic capacitor connected across the DC supply[3] to act as a reservoir, supplying current to the output when the varying DC voltage from the rectifier is falling. The diagram Fig. 3-9 shows the unsmoothed varying DC (dotted line) and the smoothed DC(solid line). The capacitor charges quickly near the peak of the varying DC, and then discharges as it supplies current to the output, smooths the DC from varying greatly to a small ripple[4].

Voltage regulators are circuits designed to maintain the DC output at constant amplitude. Voltage regulator ICs are available with fixed（typically 5, 12 and 15V）or variable output voltages[5]. The regulated DC output is very smooth with no ripple（Fig. 3-10）. It is suitable for all electronic circuits.

New Words and Technical Terms

Notes to the Text

[1]The rectifier uses the ability of a diode to conduct during one half cycle of AC to convert AC to DC：to conduct during one half cycle of AC 动词不定式作定语，修饰ability。to convert AC to DC动词不定式作整个句子的状语。句子可以翻译为：整流器利用二极管在交流的半个周期内导通的能力,将交流电转化成直流电。

[2]to produce a half-wave varying DC：动词不定式作整个句子的状语。句子可以翻译为：产生一个半波的变化直流电。

[3]by a large value electrolytic capacitor connected across the DC supply：by表示通过某种途径和手段，connected过去分词引导状语。句子可以翻译为：通过一个大容值的电解电容器跨接在直流电源上。

[4]to smooth the DC from varying greatly to a small ripple：动词不定式作整个句子的状语，表示行为的目的和结果。句子可以翻译为：从而将直流电从变化大的（波形）滤波平滑成一个小波纹（波形）。

[5]Voltage regulator ICs are available with fixed or variable output voltages：with介词短语修饰voltage regulator ICs。句子可以翻译为：现在已经有了具有固定或可变输出电压的电压调节器（稳压器）集成块。

Exercises

Ⅰ.Decide whether the following statement is true or false, and explain your answers.

1.There are many types of power supply. Most are designed to convert high voltage AC mains electricity to a suitable low voltage supply for electronics circuits and other devices.

2.Unregulated power supplies contain three basic components: a transformer, rectifier, and filter capacitor. This type of power supply, because of its simplicity, is the least costly. The disadvantage is that the output voltage is not constant.

3.The transformer receives on the primary winding an AC voltage and delivers on the secondary winding a different DC voltage (a lower one) .

4.The bridge rectifier is an arrangement of four diodes in a bridge circuit configuration to give a full-wave pulsating DC output. Full-wave rectifiers conduct on both halves of the input AC cycles.

5.The filter, formed by one or more capacitors, smoothes the previous wave eliminating the alternating current (AC) component delivered by the rectifier.

6.Voltage regulators are circuits designed to maintain the output of power supplies at constant amplitude despite variations of the AC source voltage or changes of the resistance of the load.

Ⅱ.Translate the following sentences into Chinese.

1.Power supplies for electronic devices can be broadly divided into linear regulated and switching power supplies. Linear regulated power supplies have very little ripple and very little output noise. A switched-mode supply will be smaller, is usually more efficient, but will be more complex.

2.The linear regulated supply is usually a relatively simple design, but it becomes increasingly bulky and heavy for high-current equipment due to the need for large mains-frequency transformers and heat-sinked electronic regulation circuitry.

3.Transformers waste very little power so the power out is almost equal to the power in. The ratio of the number of turns on each coil, called the turns ratio, determines the ratio of the voltages.

4.Voltage regulator ICs are available with fixed (typically 5, 12 and 15V) or variable output voltages. Negative voltage regulators are available, mainly for use in dual supplies.