GB1432024A - Control circuits - Google Patents

Abstract

1432024 Automatic control of temperature WAYNCO Inc 9 July 1973 32582/73 Heading G3R In the temperature control of an electric heater, the heater is connected in a bridge circuit and in effect functions also as its temperature sensor. A sampling pulse is applied periodically to the bridge and any deviation from a desired temperature produces an output from the bridge which controls a power transistor in series with a main power supply to the bridge and heater. In Fig. 1, the desired temperature for a heater 11 is pre-set on R45. A pulse 31 Fig. 3 is produced by a pulse generator 13 substantially at zero voltage crossings of A.C. supply, being initiated at 5 volts before zero and terminating at 5 volts after zero. Each pulse 3 1 switches ON transistors 38, 39, 40 momentarily to conduct a sampling pulse 41 Fig. 4 to the bridge. If the heater temperature, is below the set temperature then the sampling pulse will appear more positive at Y than at X so that an amplifier 46 with positive feedback will switch to its most positive output which is applied to switch ON transistors 38, 39, 40. Full wave rectified unsmoothed D.C. voltage Fig. 6 will then be applied to the bridge including heater 11 until the temperature of the heater increases to a level whereat the potential at Y is slightly less than that at X causing the transistors 38-40 to switch OFF. The power control transistor will then stay OFF until the next sampling pulse causes them to switch ON momentarily. Since the power transistors will be switched ON by the amplifier 46 in response to a temperature sensed during a sampling pulse, the supply of full power to the heater will be initiated substantially at zero voltage crossings (e.g. when the voltage drops to 5 volts before zero). Excess load current is sensed by an amplifier 53 connected across R42, which amplifier switches ON a transistor 59, thereby blocking control current to 38, 39, 40 so that the heater is deenergized. The amplifier 53 will be latched in its more positive output mode by positive feedback and will not unlatch until the supply is removed there from. Where the supply is D.C. it is caused to fluctuate by a relaxation oscillator Fig 7 (not shown) which determines the sampling pulse rate.