Patent US 4,047,235

In a power conversion system wherein AC input current is supplied through contacts of a circuit breaker and phase controlled rectifiers, a control derives a DC signal, I.sub.o, proportional to the square of the RMS value of actual current. The summation of I.sub.o and I.sub.ref, a signal representative of desired AC current limit value, is applied to an integrator whose output is coupled to an amplifier. A tripping circuit for opening the contacts is coupled through a first diode from the integrator output. A current limit circuit for regulating the phase controlled rectifiers is coupled through a second diode from the amplifier output. A clamping circuit establishes proper integrator and amplifier outputs. When the AC current is below the critical limit value, and I.sub.o is less than I.sub.ref, both diodes are back biased and neither the tripping nor current limit regulator circuits are enabled. When the AC current attains the critical limit value, I.sub.o is equal to I.sub.ref, the second diode conducts permitting current limit regulation. If AC current is not limited, due to a malfunction, I.sub.o is greater than I.sub.ref, the integrator output varies sufficiently for conduction of the first diode and tripping of the contacts.