2–94 Thermocouple Alarm Sensor A type-K thermocouple produces a voltage that is proportional to temperature. The characteristic of a type-K thermocouple is shown in Figure P2−94(a). In an application, this transducer is used to detect when the temperature reaches 1250C and then to cause a safety shutoff to trip and stop an operation. The safety shutoff can be modeled by a 5-kΩ input resistance, while the transducer can be modeled by a variable voltage source, vðTÞ, in series with a resistance of 33 Ω to account for the transducer’s wires and internal resistance as shown in Figure P2−94(b). The safety shutoff will trip when exactly 10mVis applied. Select an appropriate resistance R that will cause the safety shutoff to trip at exactly 1250C.

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E/ Thermocouple output voltage (mV) K 0 S 500 1000 1500 2000 2500 Temperature (C)