| 摘要 |
A circuit is described in which a triac is to be fired to allow voltage to be supplied to start a motor which powers the compressor of a refrigerator. In order that the triac is fired at zero-crossings of the voltage, the voltage is supplied to two comparators which are connected antiparallel with one another, and which are supplied with a reference voltage such that the voltage difference at the input terminals of each comparator represents the voltage at the triac terminals with no added d.c. component. The outputs of comparators supply a square-wave voltage with zero-crossings synchronized perfectly with those of the main voltage at the terminals of the triac. The square-wave voltages are passed via compacitors and diodes to a comparator which reserves a string of negative pulses synchronized with zero-crossings of the triac. If the comparator is gated by an IN signal indicating that the temperature is above a predetermined value, the pulses are fed via the transistor of the gate of the triac to fire it. The invention may be applied to other household appliances such as washing machines, cookers and the like.
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