| 摘要 |
<p>A circuit for controlling the supply of a.c. to a load RL comprises a triac TR1, the conduction angle of which is controlled by pulses from a pulse generator 4. The circuit includes a low voltage power supply 1 and a timer 2 which is adjustable to set the overall time period between 0-4 minutes during which the load is energised. A delay circuit 3 produces pulses at each zero crossing of the supply, the width of the pulses being determined by the resistance of a P.T.C. thermistor TH1 associated with the load. The trailing edges of the pulses trigger the pulse generator which in turn supplies trigger pulses to the triac TR1 so that the triac is switched on early in each half cycle when the load is cold and late in each half cycle when the load is hot. The load may be a heating element which melts adhesive in a book binding machine or the circuit may be used to dim a lamp. <IMAGE></p> |
