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热敏电阻

热敏电阻器是敏感元件的一类,按照温度特性不同分为正温度系数热敏电阻器(PTC)和负温度系数热敏电阻器(NTC)。PTC热敏电阻器在温度越高时电阻值越大,NTC热敏电阻器在温度越高时电阻值越低。

目前普遍使用的热敏电阻器中有陶瓷热敏电阻器和有机材料热敏电阻器。如图甲、乙分别为这两种热敏电阻的阻温曲线图。

热敏电阻常用在过流保护上。室温下电阻较小,而当电路因短路或过载故障而出现过大电流时,热敏电阻由于发热功率增加导致温度上升,当温度超过一定值(转变点)时,电阻瞬间剧增,回路中的电流迅速减小到安全值而起保护作用。当故障排除后,热敏电阻还可正常使用,所以其又经常被人们称为自恢复保险丝。当温度超过转变点温度时,电阻随温度变化更大的更适宜制作自恢复保险丝。

热敏电阻还可用在温度控制及温度补偿上。生活中的电蚊香加热器(如图丙)就是采用图甲所示的陶瓷热敏电阻代替电热丝作为恒温加热器的。它的工作原理如图丁,当把电蚊香片放入后,接上电源即可对其加热驱蚊、灭蚊。航天器在太空工作,当进入地球阴影时温度会急剧降低,航天器上的一些金属电阻元件的阻值会减小,而当太阳直射时温度又会升得很高,它们的阻值会增大。这一问题对于需要精确工作的航天器而言,常常是致命的伤害。为了防止这一现象的发生,可以串联一个热敏电阻对其进行补偿。


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The World's Coolest Places

    There's nothing wrong with a relaxing week at the beach. But travel can also be an education, a step toward empathy(共鸣) and international understanding, so for those longing to escape the crowds this summer, encounter new worlds and return with a tale, here're a few little-known travel destinations I recommend.

⒈Bikini Atoll, Marshall Islands.

    This coral island in the Pacific Ocean, site of nuclear weapons tests decades ago, is now brilliantly beautiful and the deserted white-sand beaches are lined with coconut palms and scattered with seashells. The island is a reminder of the redemptive(救赎的) power of time and nature.

⒉Potosi, Bolivia.

    No city in Latin America is more beautiful than Potosi, Bolivia, where Europeans discovered a huge silver mountain centuries ago. Tourists can descend(下降) the silver mines to experience extremely hot and dusty tunnels, talk to the miners, get a glimpse of life image for Bolivians there and have new empathy for the miners.

⒊Victoria Falls, Zimbabwe.

    Maybe our family start at Victoria Falls, which wets you with spray, making Niagara Falls a backyard stream. Then we rent a car and make our way to Hwange National Park, empty of people but crowded with various animals. Zimbabwe has fewer tourists than South Africa, and there's less crime as well.

⒋Amritsar, India.

    The Golden Temple, Sikhism's holiest shrine(神殿), is a delightful place to contemplate the draw of faith. A four-century-old temple set in a lake attracts Sikhs(锡克教徒) worldwide. Walking the circuit of the lake, with your head covered, and putting up in temple buildings can be a full experience.