Timed induction motor start switch utilizing positive temperature coefficient thermistor and semi-conductor switching device

Claim 1

A motor start switch circuit comprising: a. a symmetrical switching means including a gate electrode and first and second power electrodes, said switching means normally exhibiting a high impedance between said first and second power electrodes but being switched to a quasi-stable low impedance state when a control signal is applied to its gate electrode and remaining in the low impedance state until the current flowing through the power electrodes falls below a latching level; b. means for connecting said symmetrical switching means by said first and second power electrodes in series with a motor start impedance means and a source of alternating supply voltage; c. a charging capacitor for developing a potential thereacross; d. reference voltage means for applying to said charging capacitor an alternating potential derived from said voltage source; e. a phase shift capacitor coupled between said charging capacitor and said reference voltage means for advancing the potential developed across said charging capacitor; f. means for supplying a control signal to the gate electrode of said symmetrical switching means responsive to the charge on the charging capacitor attaining a predetermined level, g. said reference voltage means including at least one positivE temperature coefficient impedance means, which is heated as a result of current flow through the reference voltage means, h. the capacitance of the phase shift capacitor being sufficiently large with respect to the charging capacitor to cause the charging capacitor to be charged to the predetermined level at or shortly after the zero crossing point of the applied voltage when the resistance of the positive temperature coefficient impedance means is that corresponding to the beginning of a start cycle.