Motor speed control circuit with overload protection

A switching circuit responsive to two complementary constant frequency pulses of variable duration for the controlled application of direct current power through a load, comprising:

• a first controlled rectifier having an anode, a cathode and a gate electrode, said cathode being adapted to be connected to a common circuit ground, said gate electrode and said common ground being adapted to be connected to a source of one of two complementary constant frequency pulses of variable duration for rendering said first controlled rectifier conductive;
• a second controlled rectifier having an anode, a cathode and a gate electrode, said cathode being connected to said common circuit ground, said gate electrode and said common ground being adapted to be connected to a source of the other of said two complementary constant frequency pulses of variable duration for rendering said second controlled rectifier conductive;
• a turn-off capacitor connected between the anodes of said first and second controlled rectifiers for rendering said first controlled rectifier non-conductive when said second controlled rectifier is rendered conductive;
• a transformer having primary and secondary windings, each of said primary and secondary windings having one end thereof connected to said common circuit ground, the polarity of the secondary winding being selected such that a current is conducted from the other end of said secondary winding in response to a current entering said one end of the primary winding;
• a charging diode having an anode and a cathode, said anode being connected to said other end of said secondary winding of said transformer, said cathode being connected to said anode of said second controlled rectifier and to one end of said turn-off capacitor for conducting a current from said secondary winding to charge said turn-off capacitor when said first controlled rectifier is rendered conductive;
• a second diode having an anode and a cathode, said anode being connected to said other end of said primary winding of said transformer, said cathode being connected to said one end of said primary winding;
• and
• a charging capacitor being connected between said anode of said first controlled rectifier and the other end of said primary winding of said transformer, said charging capacitor being charged through said second diode when said second controlled rectifier is rendered conductive, said charging capacitor being discharged through said primary winding of said transformer and thereby inducing a current in said secondary winding to charge said turn-off capacitor when said first controlled rectifier is rendered conductive.