Patent GB-1078987-A (GB)
Title: Three-state trigger circuits
IPC: G05B11/18
Filed: 1965-04-28
Abstract: 1,078,987. Three-state circuits. SIEMENSSCHUCKERTWERKE A.G. April 28, 1965 [April 30, 1964], No. 17973/65. Heading H3T. [Also in Division H2] A circuit capable of assuming one of three states depending on the value of an input voltage comprises two stages p5 and p6 having supply voltages N and P respectively, and driven by an input signal X w  through stages p3 and p4; the latter drives p5 directly and p6 through a Zener diode (or equivalent) n1, and positive feedback is taken from a voltage divider point C to the input. Transistors p5 and p6 are initially conducting so that the potentials at U and U<SP>1</SP> are close to zero (i.e. point M). If the input potential X w  exceeds a certain value in either the positive or negative direction, conduction of either p5 or p6 will fall, and the positive feedback will make this cumulative so that either p5 or p6 will be cut off, and one of the output terminals U will assume the potential of the line N. The input potential is applied to an emitter-coupled pair p1 and p2 to compensate for threshold values of the transistors and their temperature dependence. The positive feedback needed for the relaxation operation is applied from point C to the base of p2. Stabilization of the response boundaries is achieved by negative feedback from the output of p4 via resistor r8 to the base of p2. When used in a servo system, a differential component can be included by additional negative feedback from point D to the base of p1 with a time-constant determined by r30 and k2. The circuit is initially balanced by r0 so that with the input shorted the potentials of A and B are equidistant from M in opposite directions. Diodes n2 and n3 provide a threshold voltage which must be overcome before p5 and p6 can be cut off; when these diodes conduct they effectively short the base/emitter paths of the transistors and reduce their conduction, thus initiating the relaxation operation which proceeds as a result of the positive feedback along line RK. The phase-reversing stage p7 can be omitted if the outputs U need not be of the same sign, U<SP>1</SP> then becoming equal to P in one state. To avoid undesired operation of the circuit by short-duration changes in X w  the voltage at the junction of r10 and r11 is tapped off, integrated (Fig. 3, not shown) and applied to another relaxation circuit of the same type as Fig. 2 whose outputs U are fed to a NOR circuit. The NOR output is applied to terminal F of Fig. 2 to inhibit relaxation of the circuit until the integrator has reached its limiting value when the signal at F falls to zero. For operating an A.C. servo motor (Fig. 4, not shown) the U outputs are both applied in parallel through diodes to transistors which also receive two square wave signals in antiphase. When the input X w  is less than the threshold value both U outputs are zero and the transistor inputs are short-circuited. When X w  goes outside the predetermined range in either direction one of the U outputs goes negative, the corresponding diodes are cut off and the transistors conduct alternately on the two half-cycles of one of the square-wave signals. The phase of the signals supplied to the motor by the transistors depends on which phase of the square-wave signals is passed by which transistor, and this depends on which of the U outputs has gone negative.