DC To AC power inverter

In a DC to AC power inverter of the class B,C,D or E type which includes a battery, at least one power SCR and associated capacitor circuitry, and at least one input winding on a main transformer core, which also has thereon at least one secondary output winding, the improvement comprising self-detecting load demand circuit means coupled to a line from said output winding for cyclically energizing the inverter, for sensing a minimum AC load across said secondary winding, and upon sensing a minimum AC load, for holding said inverter in an energized state until less than a minimum AC load is sensed during an energizing cycle;

• said load demand circuit means including a current transformer circuit having a primary winding comprised of at least one turn formed by a line from one end of the output secondary winding;
• and a multi-turn secondary winding, such that a load current flowing through said line generates a voltage across said secondary winding;
• means for cyclically energizing said inverter;
• means for sensing a load current flowing in the output secondary winding;
• and, means for holding said inverter in an energized state when and while a minimum load current is flowing through the output secondary winding, said energizing means including a timer which supplies an enable signal every few seconds to said holding means.

In a DC to AC power inverter of the class B,C,D or E type which includes a battery, at least one power SCR and associated capacitor circuitry, a main transformer core, at least one input winding on the main transformer core, and at least one output secondary winding on the main transformer core, the improvement comprising automatic power factor correction circuitry for supplying full time leading power factor correction to a load, said automatic power factor correction circuitry being sensitive to light reactive loads and including a power factor correction capacitor, switching means in series with said capacitor and operable to place said power factor correction capacitor in parallel with the load on said output secondary winding, a signal source in said inverter producing a signal having a frequency greater than the inverter output frequency, means for differentiating said signal, first circuit means coupled to said differentiated signal and coupled to the voltage across the load, said first circuit means having a dv/dt sensing circuit and an output coupled to said switching means and being operable to operate said switching means to place said power factor correction capacitor in parallel with the load on a trailing edge of a cycle of the inverter output waveform when a dv/dt above a certain threshold dv/dt is sensed by said sensing circuit and second circuit means connected to have an input voltage proportional to an inductive reactive load and an input coupled to said differentiated signal, the output of said second circuit means also being coupled to said switching means, and said second circuit means being operable to operate said switching means to hold said power factor correction capacitor in parallel with the load after the dv/dt sensed by said sensing circuit has fallen below said threshold dv/dt as long as the load is connected to the inverter.