GB1302760A - - Google Patents

Abstract

1302760 Triggering thyristors MATSUSHITA ELECTRIC INDUSTRIAL CO Ltd 20 April 1970 [22 April 1969 27 May 1969] 18852/70 Heading H2F [Also in Division G3] In an electric blanket temperature control arrangement the blanket heater wire 17 (Fig. 3) is supplied during positive half cycles of an A.C. source 11 via a thyristor 15 and a diode 16. The thyristor is triggered only during negative half cycles by a pulse from a transformer 22 when a neon tube 21 fires and at this time a capacitor 23, which was charged during the negative half cycle via a diode 25, discharges through Th 15 and R24. The time constant of C23, R24 is such that the discharge of C23 continues to flow through Th 15 maintaining a holding current therein until current begins to flow therethrough from the source 11 during the next positive half-cycle. Thereby zero voltage switching of the heater load is obtained and radio frequency interference is substantially reduced. The desired temp. is preset on a resistor 18 and the actual temperature is sensed by a thermistor 19 between the heater wire 17 and a defector wire 20. The lower the temp. the higher the impedance of the thermistor and the greater the proportion of the supply voltage thereacross. When this voltage proportion is such that the firing potential of neon tube 21 is exceeded during a negative half cycle, the thyristor is fired and the heater is subsequently energized during the next position half cycle. If Th 15 is not triggered during a negative half cycle it will be blocked by the charge on C23. In an alternative embodiment Fig. 6 (not shown) the thermistor material is between a pair of signal wires one of which is connected to the neon tube pulse generator, and there is a pair of heater wires with insulation therebetween which insulation becomes molten above the desired temp. whereby the heaters will short circuit and blow a fuse cutting off the blanket heater current.