题目内容
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.
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Unregulated power supplies contain three basic components: a transformer, retifier and filter capacitor. Thi type of power supply, because of its simplicity, is the least costly. The disadvantage is that the output voltage is not constant.
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The filter, formed by one or more capacitors, smoothes the previous wave eliminating the alternating current(AC) component delivered by the rectifier.
乒乓球正手快攻在球的（）击球。
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