Here we used the 12-0-12 step-down 500mA power transformer. The output of the transformer is supply to the bridge rectifier made of D2 , D3, D4, D5 which is use to convert the Ac supply to the DC supply. Capacitor C1 is used as a filter the DC output. We used 470 μF capacitor but you can used any. More the value of capacitor more pure DC can be obtained. Resistor R2 of 2.2K is used as bleeder. Here you can see the transistor T1 [BC147B] and transistor T2 [SL100] are use for regulator compressor. The DC output is fed to these transistors. T1 acts as a series pass driver or a current regulator. Base bias for transistor T1 is achieved from the supply through resistor R3 of 680 ohms as resistor R2 of 10k is a base bleeder and capacitor C2 1 μF filters base potential. When the test probe is fully open with no zener connected, the base potential of transistor T1 is around 32V that is across resistor R4 or capacitor C2. New Zener Diode Circuit Diagram ...