The mains voltage is transformed to the appropriate level. On the secondary side of the transformer is a thyristor controlled rectifier stage (phase cutting circuit), the output of which is used to charge a capacitor bank. This capacitor bank is also connected to the final series regulating transistor output stage. By controlling the conduction angle of the thyristors after each zero-crossing of the sinusoidal voltage, the flow of energy is regulated in such a way as to have a defined voltage drop across the final series transistor stage (pre-regulation). The performance of the final series transistor stage defines the final stability of the output voltage (main regulation).