A variable voltage "Zener" can be done using this electronic scheme. Compared to Zener diodes normal input impedance is higher (20 ... 50 ohms), lower peak load and temperature coefficient of only about -2 mV / ° C. However, the circuit is quite simple and use can be modified in the field of 3 ... 25 V. Once the voltage of T1 is greater than 0.6 V, the transistor leads, therefore, lead and voltage T2 can not grow, like a Zener diode. R2 sets the ratio between P1/R2 and Zener voltage of the circuit. To fix Zener voltage, the circuit must be connected to power through a resistance of 10 k and P1 potentiometer must be adjusted to the desired Zener voltage is reached. If the circuit is used to replace a Zener diode in an existing installation is required additional resistance of 10 k.