GB922887A - A transistor monitoring circuit - Google Patents

Abstract

922,887. Transistor monitoring circuits. COURTAULDS Ltd. Dec. 6, 1961 [Dec. 7, 1960], No. 42070/60. Class 40 (6). [Also in Group XXXVI] A circuit for monitoring the magnitude of a variable parameter comprises two complementary transistors T 1 , T 2 , the base of one transistor being connected to a common base line, to which is applied a voltage derived from the variable parameter, through a variable resistor and the emitter of the other transistor being connected to a variable potentiometer. As shown in the Figure, the two transistors are biased so as to be non-conductive when the parameter to be measured applied to terminals A is within preselected upper and lower limits. If, however, the input current through resistor R 1 increases the voltage of the common base line L 1 becomes more negative with respect to the earth line L 2 , so that when the voltage on the base of transistor T 1 , determined by the variable resistor R 2 , reaches the preselected upper limit of the parameter, transistor T 1 conducts, causing the high limit alarm 1 to operate. The second transistor T 2 has its emitter bias regulated by potentiometer R 3 so as to be cut-off when the parameter to be monitored is above the low-level limit. However, if the input current through resistor R 1 decreases the voltage of line L 1 becomes less negative and the base of transistor T 2 becomes positive with respect to its emitter resulting in this transistor conducting and the lowlimit alarm 2 operating.