Patent GB-1261663-A (GB)
Title: Controller for a temperature control system
IPC: G05D23/24
Filed: 1970-05-07
Abstract: 1,261,663. Automatic temperature control systems. VAPOR CORP. May 7, 1970 [Oct. 3, 1969], No.22044/70. Heading G3R. A temperature control systems comprises means providing a difference signal dependent on whether sensed temperature is above or below a set point, and a pulse source coupled to a means for energizing one of a plurality of leads in a directional sequence which vary power in steps, supplied to a heating means, dependent on the sensed temperature. A resistance sensor 5 is situated in a bridge network 43 fed from a stabilized supply, a diagonal of which is connected to a high gain amplifier 9 having a logical 0 output when the temperature sensed is high and logical 1 when low, and a positive feedback resistor 57 giving a snap action. An input protective circuit comprising resistors 61, 63 and Zener diode 65 is provided. The amplifier 55 output is applied to a bidirectional counter 11 which also receives pulses from a source 67. The counter output is applied on one of a plurality of leads odd numbers 13-21 which energize switch means e.g. 23, 25 in turn to connect a heating load 7 to successive taps on a transformer winding 217. The pulses are blocked by an AND gate 97 when there is no output from a supply voltage zero crossing circuit 99 in order to prevent a register shift during this time which may result in two of load switching means e.g. 23, 25 being turned on simultaneously. To ensure that counter 11 is set for zero power on start up, a zero reset circuit 131 resets register 69 to zero each time power is initially connected. A flip-flop 71 forming the fifth stage of the register is also caused to be in its zero state. During start up, the frequency of the source 67 is reduced by circuit 151 including a bi-stable flip-flop 153. The output of amplifier 55 is connected to flip-flop 153 which changes state on the first high to low transition of the amplifier indicative of the sensor exceeding the set point, which opens a switch 155 breaking a C.R. circuit 157, 159 thus reverting the multi-vibrator to normal frequency of operation. Switch means e.g. 23 in the load circuit include reverse parallel connected thyristors 205, 207 which are switched on and off at each supply voltage zero detected by a circuit 163 and a signal is passed through a differentiating circuit to gate 161 so long as a logical l input is present on one of leads e-g. Output of gate 161 passes through a gate 173 to a complementary bi-stable flip-flop 175 having an output resistor 183 connected through a capacitor 195 to the primary winding of a transformer 199 delivering firing pulses to thyristors 205, 207. Protective circuits 209, 211 prevents spurious firing on stray signals. A sensor short circuiting detector 165 is also connected to the bridge network and to gate 161. An over temperature circuit 233 resets counter 11 to zero through a reset circuit 131 which also disconnects the supply through an amplifier 241, and gate 243 causing a thyristor 259 to fire and a switch 265 which may be a transistor to conduct thus energizing a relay coil 267 which opens a contact in the supply circuit of transformer 215. Counter 11 may be alternatively a bi-directional ring counter or a forward and backward counter with an output decoder. The pulse source 67 may be a free running multi-vibrator whose output is connected to a differentiating circuit and preferably the period of the pulses is arranged to match the thermal response of the load 7. Lamps (not shown) may indicate when the sensor is below and above the set point.