Patent GB-920007-A (GB)
Title: Improvements in or relating to electric circuit arrangements
Inventor: DAVEY PETER GORDON
IPC: H03M1/00
Filed: 1960-10-31
Abstract: 920,007. Electric selective signalling. GENERAL ELECTRIC CO. Ltd. Sept. 21, 1961 [Oct. 31, 1960], No. 37337/60. Class 40 (1). [Also in Gioup XL (c)] The digital voltmeter described in Specification 920,006 has the analogue " circuit " 6 of Fig. 1 and the switching transistor 21 of Fig. 2 replaced respectively by the circuits 9 and 10 of Fig. 1, of the present Specification. In the present Fig. 1 a selectable one of a number of thermocouples 1-6 is connected by a uniselector SR1, SR2 to a series gating circuit comprising a filter 17, 20, 22 for smoothing voltage changes occurring during switching, a resistance network comprising resistor 18, 53, 23, 24, the collectoremitter path of a temperature-stabilized gating transistor 27 and the primary winding of output transformer 28. The whole of this series circuit is isolated from earth to reduce induced interference voltages. Resistors 18, 21 are also chosen to make them a minimum. The resistance network provides a voltage to back-off the cold-junction voltage of the thermo-couples, a further voltage for neutralizing the emittercollector voltage drop of the transistor when conducting and a voltage supplied from a series of bi-stable circuits (of which 35,37 are typical) which by successive steps of diminishing amplitude back off the thermocouple voltage until near equality is reached. The first bistable switch is triggered by a pulse at 64 so that it connects the negative terminal of a supply through a resistor 59 to the thermo-couple circuit and a pulse at 50 operates a further bistable circuit 34 which connects the negative supply to the base of transistor 27 to render it conducting. The resulting output from the transformer 29 represents the magnitude and polarity of the difference between the thermocouple voltage and the voltage supplied through resistor 59. A pulse of opposite polarity to the initial pulse is applied at 50 to restore the bistable circuit 34 so that the gate transistor is blocked, and the circuit output at 29 causes the bi-stable circuit to be restored to reconnect resistor 59 to the positive supply line only if a comparison indicates that the voltage at 59 is greater than the thermo-couple voltage. A register circuit (not shown) thereupon provides a pulse which causes the next bi-stable switch to connect resistor 60 to the negative supply line and so provide an additional voltage of half that provided by resistor 59 and a further pair of pulses at 50 operate or restore the switch to cause the gate transistor 27 to effect a further comparison. In this manner the comparison is effected with successively reducing voltage additions, each bi-stable circuit being left in the " ON " condition only if the thermo-couple voltage is greater than the comparison voltage. The bi-stable circuits each comprise a pair of complementary transistors (Fig. 2, not shown) connected in series between positive and negative supply lines and cross-connected with a further transistor so that one or other of the complementary transistors is conducting and connects the positive or negative supply, respectively to their common connection. Triggering of the bi-stable circuits is effected by windings 48, 49 connected respectively in the collector circuits of the complementary transistors.