This clever design uses 4 diodes in a bridge to produce a fixed voltage power supply capable of supplying 35mA. All diodes (every type of diode) are zener diodes. They all break down at a particular voltage. The fact is, a power diode breaks down at 100v or 400v and its zener characteristic is not useful. But if we put 2 zener diodes in a bridge with two ordinary power diodes, the bridge will break-down at the voltage of the zener. This is what we have done. If we use 18v zeners, the output will be 17v4. When the incoming voltage is positive at the top, the left zener provides 18v limit (and the left power-diode produces a drop of 0.6v). This allows the right zener to pass current just like a normal diode but the voltage available to it is just 18v. The output of the right zener is 17v4. The same with the other half-cycle. The current is limited by the value of the X2 capacitor and this is 7mA for each 100n when in full-wave (as per thiscircuit). We have 10 x 100...