those skilled in the art it should be noted that the current sensing resistor 50 will typically include appropriate filtering to provide an output voltage thereacross which is representative of the mean or average current a diode 52 is connected across the output terminals for protecting the switch 48 from high inverse voltages as is pointed out in the 39 405 patent this diode may be replaced with a back emf limiter to increase the current throw at the expense of reverse voltage spikes if desired an additional breakdown of the components for use in a pulsed d c source are shown in fig4 wherein an a c source supplies current to d c to d c converter 58 via full wave bridge rectifier 56 the output of the d c to d c converter is applied to a group of silicon controlled rectifiers u201c scrs u201d 60 62 64 and 66 which are controlled from a frequency control circuit 67 via a conventional trigger circuit 68 to form in conjunction with capacitor 70 a capacity charge discharge circuit the capacitor 70 is connected between the anode cathode junctions of the scrs as shown also functions to double the voltage from the converter 56 scrs 60 66 and 62 64 are triggered to conduct alternately in a conventional manner.