Variable zener diode circuit

I used to design and calculate the value of the equipment. By false starts. Frequently have all the equipment. Except that the Zener diode can not find what you need. I tried using this way.


This circuit features as zener diode voltage can be adjusted. And Zener diode features more than normal, are higher input impedance (20 to 50 ohm) Rate under the maximum load. and there is the temperature coefficient just only about -2 mV per degree celsius. Zener voltage Uz can be adjusted in the 3 to 25 volts.

This circuit works. When the voltage at pin T2 base of more than 0.6 volts. T1(?BC547) conduct makes T2(BC557) conduct current according, And the voltage drop across this circuit. It can not be increased, same as a normal zener diode. This voltage zener determined by the ratio of P1/R1 and R2. When adjust the voltage Zener This circuit is connected in series with a 10 kilo-ohm resistor to the power supply. And then adjust P1 until the zener voltage as needed. But if you want to put this circuit is to change the Zener resistor P1 as constant. This circuit limits the current to 100 mA. (Transistor T2, can endure up to 100 mW power loss.)

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