one embodiment of the invention will now be described with reference to the drawing . the input terminal 1 of the mains supply unit illustrated in the figure receives a non - stabilized direct - current voltage of about 6 . 9 - 7 . 6 v . there is a stabilized direct - current voltage of + 5 v at output terminal 2 . the unit contains a known stabilization circuit that includes transistors t3 and t4 , resistors r5 , r6 , r7 , and r8 , and zener diodes d3 and d4 . zener diode d3 supplies a reference voltage to the emitter of transistor t4 . a control voltage that is proportional to the output voltage is tapped from resistors r7 and r8 and controls the base of transistor t3 through transistor t4 for purposes of stabilization . a smoothing capacitor c1 and a protective resistor r1 are also connected to input terminal 1 . the aditional circuitry that turns the unit on and protects it against short circuits consists of transistors t1 and t2 , resistors r2 and r3 , capacitor c2 , and diodes d1 and d2 . the voltage at control terminal 3 is zero when the mains supply unit is switched off . transistor t2 is accordingly blocked and , since it can convey no collector current , transistor t1 receives no base current and is also blocked . this keeps input terminal 1 disconnected from transistor t3 , and the overall unit is out of operation . to switch the mains supply unit on , to initiate operation of a passage of television - screen text stored in the unit for instance , a voltage of + 5v is applied to control terminal 3 witha remote - control unit . capacitor c2 charges through diode d2 and through the previously uncharged capacitor c3 . when capacitor c3 is charged to a fraction , approximately 0 . 6 v , of its final operating voltage of 5 v , diode d2 blocks , disconnecting the base of transistor t2 from charging capacitor c3 . charging capacitor will now already have a voltage of + 0 . 6 v . the mains supply unit , however , will still be kept off by transistors t1 and t3 . the current that charges capacitor c2 will now flow on through the base - emitter section of transistor t2 and through diode d1 . this makes transistor t2 conductive and generates a base current at transistor t1 . transistor t1 also becomes conductive and switches completely through , so that all of the illustrated circuitry , including transistors t3 and t4 begins to operate . the stabilizing voltage at output terminal 2 is now + 5 v . when a short circuit occurs at terminal 2 , the cathode of diode d2 will be grounded , so that diode d2 is conductive and a its current voltage of about 0 . 6 v will be at the base of transistor t2 . for transistor t2 to be conductive , however , the base - emitter voltage needed for switchng through and the current voltage from diode d1 will have to be overcome . since the sum of these two voltages , however , is higher than 0 . 6 v , transistor t2 will be blocked , and the base of transistor t1 will no longer be controlled , so that the transistor will block and the control circuit that involves transistors t3 and t4 will no longer obtain voltage . the voltage at output terminal 2 will collapse as desired , and the short circuit will not endanger any of the components . once the short circuit has been eliminated , the unit will turn on as previously described . one advantage of this circuitry is that , when transistor t1 switches through and the control circuit goes into operation , charging capacitor c3 will already be charged to + 0 . 6 v . if the capacitor were discharged at that instant , the circuitry would immediately register a short circuit at the output when turned on and would turn off again . the aforesaid charging procedure results in a desirably weak voltage switch - in at terminal 2 subsequent to a certain delay . the individual components in one practically proven embodiment of the type illustrated in the figure have the following values : since the switchng off attained with transistor t1 can basically also be attained with transistor t3 , the former can be eliminated .