Solid state switching system for coupling an ac power supply to a load

Claim 1

A solid state switching system for controlling the appliCation to a load of an alternating voltage provided by a two-terminal AC power supply, comprising: a triac having first and second main terminals and a gate terminal; means for connecting one of the power supply terminals through the load to said first main terminal and for connecting the other power supply terminal directly to said second main terminal; a triggering circuit coupled between said gate terminal and said second main terminal and including a series-connected resistor and a series-connected electronic switch having a complementary pair of parallel-connected transistors, the emitters of which are joined together and connected directly to said second main terminal, and in which the collectors of said transistors are connected through respective ones of a pair of oppositely-poled diodes to one terminal of said resistor, the other terminal of said resistor being connected to said gate terminal; and control means for establishing and maintaining said transistors forward biased to permit conduction thereof in alternation in response to polarity changes of the alternating voltage in order to close said electronic switch and effect bidirectional current flow between said gate terminal and said second main terminal to trigger said triac into conduction at the beginning of each half cycle of the alternating voltage, thereby effectively applying that voltage continuously to the load via the conducting triac while at the same time deriving the bidirectional triggering current entirely from the AC power supply so that no loading is placed on said control means.