Finding Balance. You decide to weather-proof your home by installing a temperatu

Question

Finding Balance.

You decide to weather-proof your home by installing a temperature alarm system. If the temperature dips below a certain threshold (which you can directly input), an alarm is activated. In the accompanying circuit, a sensor (termed a thermistor) has a variable resistance. The thermistor’s resistance varies as the exposed temperature changes. A decrease in temperature causes the resistance of the thermistor to increase, while an increase in temperature causes the resistance of the thermistor to decrease. At some temperature, T, and for the given resistances: R1, R2, R3, and the given potential difference, +V, the measured potential difference (between points A and B) is zero. You would like to know what value for RT yields a measured potential difference (between points A and B) of zero.

Explanation / Answer

If potential drop between A and B is zero, it means it will act like wheatstone bridge.

In a wheatstone circuit

If RT is unknown circuit and R1, R2 and R3 is known resistors. Then

RT/R3 = R1/R2

By this we can solve that

RT = R1*R3/R2

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