GB863144A - Improvements in or relating to time control circuits employing transistors - Google Patents

Abstract

863,144. Transistor circuits. SQUARE D CO. Oct. 1, 1958 [Oct. 25, 1957], No. 31331/58. Class 40(6) [Also in Group XXXV] In a time control circuit, upon operation of an initiating switch a capacitor which had previously been charged through two electrodes of a transistor, and a source of voltage discharges and then commences to charge from the source, the charging continuing until the transistor conducts to its third electrode and so terminates the timing period. In the quiescent state of the circuit shown, a capacitor 36 is charged due to a circuit from a battery 10 through the base-emitter paths of transistors 20 and 30. Upon operation of a control switch 15 a relay 18 operates and its contact 47 causes a relay 44 to operate. One contact 43 of this relay connects a power supply 40 to a load 42 while a further contact 16 provides a holding circuit for relay 44. Capacitor 36 now discharges through a circuit comprising resistors 37 and 38, a relay contact 25 which was made when relay 18 operated, the battery 10 and contact 16. The voltage across capacitor 36 accordingly decreases to zero and then increases to a value at which transistor 30 conducts. This renders a further transistor 20 conducting causing a relay 21 to operate. Contact 48 of this relay interrupts the circuit of relay 44 causing the circuit to restore to its initial condition. A diode 23 prevents reverse current flow through the transistors and resistor 38 permits the time during which relay 8 is operated to be varied. Transistor 20 may be omitted, in which case relay 21 replaces resistor 32. If relay 18 is less sensitive than relay 21 then providing battery 10 has sufficient power to operate relay 18, relay 21 will also operate so that the circuit "fails safe" in this repect.