Heat detector having an increased accuracy alarm temperature threshold and improved low temperature testing capabilities

摘要

A heat detector that includes a temperature sensing and testing circuit that provides an increased accuracy temperature sensing range and a push-to-test circuit that is electronically functional at low ambient temperatures. The heat detector includes a sensing circuit consisting of a voltage divider formed from a supply resistor and a thermistor. The thermistor is connected directly between a detector voltage input to a control unit and ground. The push-to-test circuit includes a parallel combination of a test resistor and a transistor each connected between the supply resistor and a voltage source. During normal operating conditions, base current is supplied to the transistor such that the transistor functions as a short circuit connection between the supply resistor and the voltage source. Upon depression of a push button, the base of the transistor is connected to the emitter such that the transistor acts as an open circuit. When the push button is depressed, a test resistor is connected in series with the supply resistor to simulate an increase in the ambient temperature to test the heat detector.