functions as an inductor for transistor 221 and the secondary of transformer t3 functions as an inductor for transistor 222 differential amplifier 219 provides a signal on its output line 232 which is proportional to the ac voltage across capacitors 225 227 corresponding to load power drain and capacity charging pwm circuit 224 receives input ea the output of differential amplifier 219 on line 232 and a reference voltage ref from pwm 224 on line 234 input ea divided down by resistors 235 236 pwm circuit 234 compares the signals on its ea and ea inputs and provides driving pulses on line 238 e1 e2 to switching transistors 221 222 to cause recharging of capacitors 225 227 transistors 221 and 222 are connected in parallel to increase the current handling capacity of the transistors as a switch current transformer t4 couples the current to a bridge rectifier 246 which provides a voltage across resistor 248 proportional to the current drawn from the ac supply resistor 248 is selected so that a 1 100 turns ratio of transformer t4 provides 200 mv across resistor 248 when the ac input current is about 20a peak the voltage across resistor r248 is supplied to pwm circuit 224 input is and cause shutdown of pwm circuit 224 the voltage reaches 200 mv operational amplifier 250 provides a 2 v dc output for test purposes corresponding to 200 mv across resistor 248 integrated circuit 252 is an opto coupler which functions as an isolator for the control signal enpwm1 from the microprocessor which when high enables operation of the pwm circuit 224 amplifier 250 also provides.