Solid state AC switch with self-synchronizing means for stealing operating power

In apparatus for controlling the supply of alternating electric current to a load, the alternating current being of a known frequency and the apparatus including a switch controllable to low and high impedance states for respectively permitting and effectively blocking the flow of current to the load, a method for stealing electric power from the controlled supply of current during periods when current to the load is called for, comprising the steps of:

• diverting a portion of the electric current which would otherwise flow through the switch, diversion being possible only when voltage of at least a predetermined magnitude exists across the switch;
• controlling the switch to a high impedance state for a power stealing interval during each of equally spaced half cycles of alternating polarity of the alternating electric current;
• determining whether the magnitude of the diverted current reaches a predetermined threshold;
• shifting timing of succeeding power stealing intervals to a portion of the half cycles of the alternating current characterized by higher instantaneous current if the magnitude of the diverted current is less than the predetermined threshold;
• and
• shifting timing of succeeding power stealing intervals to a portion of the half cycles of the alternating current characterized by a lower instantaneous current if the magnitude of the diverted current exceeds the predetermined threshold.

Self-powered electronic switching apparatus for controlling the supply of alternating current to a load comprising:

• first and second power terminals;
• first and second substantially identical power field effect transistors, each having source and drain power electrodes, a gate electrode and a body diode between the source and drain power electrodes;
• first connecting means connecting said first and second power field effect transistors in a back-to-back arrangement between said first and second power terminals, a pair of corresponding power electrodes of said first and second power field effect transistors being connected at a first junction;
• a reference potential conductor;
• second connecting means connecting said first junction to said reference potential conductor;
• first and second diodes connected in series in an oppositely poled arrangement between said first and second power terminals, said first and second diodes being connected at a second junction, and, together with the body diodes in said first and second power field effect transistors, forming a full-wave rectifier for producing a unipolar voltage between said first and second junctions;
• a current supply conductor for supplying direct current at a first voltage;
• third connecting means connecting the second junction to said current supply conductor;
• a current detector operable to produce a current threshold signal indicative of whether the electric current flowing through said reference potential conductor and said current supply conductor exceeds a predetermined threshold;
• pulse generator means responsive to the current threshold signal to produce a pulse train of a first repetition rate corresponding to a pulse spacing shorter than the duration of a half cycle of the supplied alternating current when the magnitude of the current detected by said current detector is less than the predetermined threshold, and of a second repetition rate corresponding to a pulse spacing between the durations of a half cycle and a full cycle of the supplied alternating current when the magnitude of the current detected by said current detector is at least as great as the predetermined threshold;
• and
• a gate control circuit requiring direct current at substantially the first voltage for operation, said gate control circuit being responsive to the pulse train produced by said pulse generator means and a signal indicative of a command for supply of current to the load, said gate control circuit further being operable to supply a gate signal to the gate electrodes of said first and second power field effect transistors effective to cause said first and second power field effect transistors to assume a low impedance state when supply of current to the load is commanded except during individual pulses received from said pulse generator means;
• and
• fourth connecting means connecting said current supply conductor to said gate current for supplying current at the first voltage said gate control circuit.