Patent GB-1312333-A (GB)
Title: Bistable control amplifier device
IPC: G05F1/45
Filed: 1970-05-04
Abstract: 1312333 Photo-electric control AUCO N V 4 May 1970 [2 May 1969] 21439/70 Heading G1A [Also in Divisions H2 and H3] A switching circuit comprises a breakdown diode 9 or equivalent circuit connected in parallel with a transistor 11 and with a voltage divider 10, 13 having its tap coupled to the base of the transistor. If the potential at the base of transistor is lowered as by varying resistor 10 (which may be light-sensitive), the current through the transistor will rise robbing the diode of its holding current so that it will begin to switch off. The voltage across the diode will thus rise, increasing the conductivity of the transistor and by this action the diode rapidly switches off. Similarly if the voltage at the base of the transistor is raised the voltage across the diode can be increased until it begins to break down. The resulting decrease in voltage thereacross reduces the base-emitter voltage and thus the current in the transistor and the diode rapidly switches on. By suitably dimensioning the components, switching can be effected by a small change of base voltage. If a capacitor and resistor are connected across resistor 10 the circuit will oscillate between its states with a mark-space ratio dependent upon the value of the resistor, a change of resistor outside the range bringing the circuit to rest in one or other of its states dependent upon the resistor value. As shown the circuit is used to operate a triac 1 connected to an A.C. supply the control circuit being supplied by a rectifier system 7, 8. A capacitor 12 is provided to prevent operation by interference pulses. The capacitor may also be connected in the base circuit of transistor 12 for this purpose Figs.8 and 9, (not shown). The control resistor 10 may be a transistor controlled by a potentiometer (16,17) connected to its base whereby increased sensitivity is obtained Figs.7  and 9, (not shown). A contact (19) may be connected in these circuits to break the collector supply to the additional transistor so that the potentiometer is connected to the control transistor 11 through the base emitter diode of the additional transistor whereby reduced sensitivity is obtained. A further transistor of field effect type preceding the additional transistor will provide increased sensitivity. The control transistor 11 may be of the opposite conductivity type and may be connected to the line 6 through a Zener diode so as to provide a higher value threshold Figs.10 and 11, (not shown). In Fig.13, (not shown) a triac is fed from a separate alternating current supply which may be of different frequency from that of the control circuit supply and it is stated that the control circuit may be fed directly from D.C. A neon indicator may be connected across the load and a resistor across triac Fig.14, (not shown) to give a light depending upon whether the triac is off, on or the line is disconnected. A further triac may be controlled by the transistor current so that it is on when triac 1 is off and vise versa Fig.12, (not shown). Instead of the diac breakdown shown there may be provided an equivalent device formed from a combination of thyristor Zener diode and ordinary diode Fig.15a, b, (not shown) a four layer diode and an ordinary diode, Fig.15c, (not shown), or a combination of complementary transistors and diodes, Figs.15d and e, (not shown). The upper end of the potentiometer may alternatively be connected a tap on resistor 4.