How does the voltage stabilizer circuit work

Voltage stabilization with Zener diode and transistor (collector circuit)

One also speaks of "stabilization with Zener diode and series transistor" or "series stabilization with series transistor".

Z-diodes are usually used to stabilize voltages easily. But Z-diodes can only be loaded with a small current. Especially when you have to choose a Zener diode with a high Zener voltage.
To avoid this problem, a transistor in a collector circuit (emitter follower) is used as a current amplifier. Because the voltage itself is not amplified, this method is called an impedance converter (resistance converter): The output resistance between emitter and ground (GND) is lower by the factor of the current gain of the transistor than the input resistance at the base in relation to ground (GND ).

What is the purpose of the resistor RC.?

A short circuit at the output will destroy the transistor. Therefore a protective resistor RC. of approx. 10 ohms must be built into the load circuit. The stabilization then decreases somewhat, but the load resistance can then be smaller.
This shows that there is only a small margin for the choice of RC. there and must remain in the lower resistance range. However, this circuit is not permanently short-circuit proof. This would require more complex control measures. If you want to know more about it, you can read about it under voltage control circuit with electronic hum filter by Thomas Schaerer.

What is the purpose of the resistor RE.?

The resistance RE. is used to set the operating point of the transistor when there is no load connected to it. The reason: If RE. does not exist and no R.L. is connected, no base current flows and so no base-emitter voltage of 0.7 V can develop.
The resistance RE. must be so low that the voltage UBE is around 0.7 V. When a current through RL. flows and the collector or emitter current increases, then U also increasesBE a little on.

Why a Zener diode?

The Zener diode is therefore necessary to keep the output voltage reasonably stable even when the input voltage fluctuates. It can therefore not simply be exchanged for a resistor.
If you need a stabilized voltage at the output, you can use a Zener diode. Otherwise this circuit doesn't make much sense.
As a reminder: In order to achieve a comparatively good stability, the cross current Iz due to the Zener diode at least 5 times greater than the base current at maximum I.L..

Calculation of the output voltage

The output voltage Ua is determined by the Zener voltage UZ minus the base-emitter voltage UBE certainly.

Load resistance of the Zener diode

With this circuit, the load on the Zener diode is reduced by the factor of the current gain of the transistor.

Dimensioned example circuit

Ue+15 V
T1Small signal transistor
R.V.390 Ω
R.C.10 Ω
R.E.10 kΩ

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