The design of a constant-current 30 A 4 kw d.c. power supply is described. A bridge rectifier incorporating silicon controlled rectifiers is followed by a low-pass filter and a series control element consisting of several power transistors in parallel. The current is controlled, and stabilization is provided, by the series transistors. By altering the firing angle of the rectifiers the bridge output voltage is controlled in such a way as to maintain a low collector voltage and a low transistor dissipation. The output current can be maintained constant to better than 0.1% at any value between 5 and 30 A. At maximum output the efficiency is 83% and the input power factor is 0.85 lagging.