| 摘要 |
<p>1310904 Alarm systems TEXAS INSTRUMENTS Inc 1 April 1970 [1 April 1969] 15424/70 Heading G4N In a cooling system, the flow of coolant is monitored by a positive temperature coefficient thermistor having a greater rate of increase of resistance above an anomaly temperature than below it. The thermistor is heated by current through a series resistor so as to attain a temperature below the anomaly temperature in the presence of a first level of heat transfer to the coolant and to a temperature above the anomaly temperature in the presence of a second level of heat transfer. As shown in Fig. 6, a positive temperature coefficient thermistor 10 carries sufficient standing current to operate in its self-heating region and is situated in a fluid flow channel. Increase in thermistor temperature due to variation in the fluid medium causes breakdown of voltage regulator diode 54 and conduction of transistor 52 which operates a visual or audible indicator 56 or a relay which controls a load circuit. Transistor 52 also releases transistor 46 so as to extinguish the normal indicator lamp 50. Fig. 8 shows a flowmeter in which PTC thermistor 10 is heated by an auxiliary PTC thermistor having a lower anomaly temperature. SCR 70 becomes conductive when the heat exchange between thermistor 10 and the fluid medium becomes reduced. In Fig. 7 (not shown) three PTC thermistors are coupled via a diode OR gate and a Zener diode to the gate of an SCR.</p> |
