GB1454794A - Electrical warning devices - Google Patents

Abstract

1454794 Detecting fluid level electrically AUTOMOTIVE PRODUCTS Ltd 21 Feb 1974 [25 April 1973] 19602/73 Heading G1N A fluid level warning device, wherein a selfheated temperature-sensitive resistance senses a difference in thermal conductivity between the fluid and air, comprises an element of semiconductor material having a negative and a positive thermal coefficient of resistivity respectively above and below a critical temperature, one face of the element being in thermal and electrical contact with a container provided therefor to which a first electrical connection is made, another face of the element having a second connection attached thereto. A doped Ge or Si chip 1 is attached by one face to the inner end 2 of a metallic can 3 held by a plastics holder 8 screwed into the wall 10 of a hydraulic reservoir of a motor vehicle. Electrical connections to the chip are by leads 5 and 6 passing through a seal 4 and attached respectively to the interior of the can 3 and to another face of the chip 1. An electric supply is applied by an ignition switch 14 to a lamp 13 in series with the chip 1, and the lead 5 is earthed to the vehicle chassis. The heat generated by electric current in the chip 1 is quickly dissipated if fluid 11 is present, so that the temperature of the chip 1 does not rise high enough to cause a fall in resistance sufficient to light the lamp 13. If the fluid 11 gives way to air of lower thermal conductivity, the chip 1 now heats to above the critical temperature, whereupon its resistance falls and thermal runaway ensues, until an equilibrium is reached wherein most of the supply voltage is dropped across the lamp 13. In very cold weather the change from n.t.c. to p.t.c. below the critical temperature avoids a highresistance condition wherein the chip 1 might fail to draw sufficient current to generate the heat necessary to begin the thermal runaway in the presence of air.