DRAUGHT_DETECTOR

Published:2009/7/13 4:01:00 Author:May | From:SeekIC

The complete circuit diagram for the draught detector is shown. The key components are R2 and R4; these form a pair of identical positive-temperature-coefficient (p.t.c.) thermistors. It is essential to use this particular type of thermistor. With switch S1 on, current flows from the nominal 18-V supply to the rest of the circuit. Thermistors R2 and R4 are included in a pair of potential dividers. The first comprises fixed resistor R1 in the upper arm and thermistor R2 in the lower one. The second consists of potentiometer VR1, connected in series with fixed resistor R3, in the upper arm and thermistor R4 in the lower one. With room temperature below 60℃, the current in the thermistors will be 150 mA approximately, sufficient for them to self-heat. They will then approach the threshold temperature within a few seconds. The thermistors will stabilize when the temperature reaches about 75℃, corresponding to a resistance of some 500 Ω. In the absence of a draught, each thermistor will be surrounded by a blanket of warm air. However, therrnistor R4 is arranged so that it does not receive any draught impinging on detector R2. When a draught is detected, the warm air around R2 is disturbed, and this thermistor is cooled slightly. This results in a lower resistance and hence a falling voltage being developed across it. This sounds the alarm. R5 supplies a little hysteresis to the alarm circuit.



Reprinted Url Of This Article:
http://www.seekic.com/circuit_diagram/Measuring_and_Test_Circuit/DRAUGHT_DETECTOR.html

Print this Page | Comments | Reading(3)