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原题现要组装一个由热敏电阻控制的报警系统,当要求热敏电阻的温度达到或超过60℃时,系统报警。提供的器材有:热敏电阻,报警器(内阻很小,流过的电流超过I_c时就会报警),电阻箱(最大阻值为999.9Ω),直流电源(输出电压为U,内阻不计),滑动变阻器R_1(最大阻值为1 000Ω),滑动变阻器R_2(最大阻值为2 000Ω),单刀双掷开关一个,导线若干。在室温下对系统进行调节。已知U约为18 V,I_c约为10 mA;流过报警器的电流超过20 mA时,报警器可能损坏;该热
The original question is to assemble a thermistor-controlled alarm system that warns the system when the temperature of the thermistor reaches or exceeds 60 ° C. Equipment provided are: thermistor, alarm (resistance is small, the current will flow more than I_c will alarm), resistance box (maximum resistance of 999.9Ω), DC power supply (output voltage U, internal resistance Excluding), sliding rheostat R_1 (maximum resistance of 1 000 Ω), sliding rheostat R_2 (maximum resistance of 2 000 Ω), a single-pole double-throw switch, a number of wires. The system is conditioned at room temperature. It is known that U is about 18 V and I_c is about 10 mA; the alarm may be damaged when the current flowing through the alarm exceeds 20 mA; the heat