This is the Variable Zener diode circuit that we can use it instead of a general Zener diode. We can adjust its voltage. And it has a few components that you can easy to buy at a local store near you.
Often we use a fixed reference Zener diode. Although, it will have the breakdown voltages (Vz) from about 2V to 200V. But we can not adjust its voltage level because it is a specific voltage. For example, we use a 12V Zener diode. We cannot adjust voltage fo 10.5V.
We design and calculate the value of the equipment. When false starts, we Frequently have all the equipment to change. Except for the Zener diode, we cannot be looking for them in us store.
Thus, this Variable Zener diode circuit is a better way for us. circuit features as zener diode voltage can be adjusted. And Zener diode features more than normal, are higher input impedance (20 to 50 ohms) rate under the maximum load.
And there is the temperature coefficient just only about -2 mV per degree celsius. Zener voltage Vz can be adjusted in the 3 to 25 volts.
Variable zener diode circuit working
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 VR1 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.)
Parts you will need
Q1 = BC547_NPN transistor 0.4A 40V
Q2 = BC557_NPN transistor 0.4A 40V
VR1 = 250K Potentiometer
R1 = 100K_0.25W Resistor
R2 = 4.7K_0.25W Resistor
R3,R4 = 10K_0.25W Resistor
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