10 through series diode cr21 during normal operation the gate electrode of q14 is at about the supply voltage while the anode is about 2 3 of the supply voltage if a line failure occurs capacitor c16 discharges towards ground potential at such time that the gate potential of q14 is within 1 2 volt of its anode voltage q14 will become conductive thereby transferring the potential existing across c15 to the gate of the output scr 19 rendering it conductive and thereby completely discharging the energy storage capacitors an alternate embodiment of the circuitry of the series thyristor is shown in fig3 as shown therein the thyristor is a pair of silicon controlled rectifiers 22 23 connected in a back to back relationship the anode 24 of scr 23 is connected to the cathode 25 of scr 22 and to one side of secondary 30 of transformer t2 the cathode 26 of scr 23 is likewise connected to the anode 27 of scr 22 and to one side of another secondary winding 21 of transformer t2 the gate electrodes 28 of scr 23 is connected to the opposite side of winding 31 while gate electrode 29 of scr 22 is connected to the opposite side of winding 30 the combination of back to back scr 39.