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Example circuits where Zener diode is used as voltage stabilizer. The goal is to have 10V voltage and 1A current at output for unregulated input voltage. Examples use 15V input voltage as reference. To simplify things, example load is 10V/10W bulb.
FYI: Top left circuit is added only as visible desired voltage and load current reference for this simulation.
1st row, 2nd circuit use only current limiting resistor and Zener diode with 10V Zener voltage as output voltage limiter. Because excess current is dissipated on resistor, larger current require resistor rated for higher power and turn particular circuit into very inefficient space heater. This circuit is useful to power loads with current demand only up to 200 mA.
1st row, 3rd circuit use NPN power transistor as voltage regulator. Desired reference voltage for transistor base is set by 12V Zener diode. Resistor between collector and base bias the transistor. Both resistors works as current limiter for Zener diode. This circuit is simplest voltage regulator without excessive heat generation. Unfortunately it still lose 10% of available power and is susceptible to input voltage fluctuations and temperature changes.
2nd row, 1st circuit is improved 1st row, 3rd circuit. Instead of single NPN power transistor it use NPN Darlington pair where upper transistor is power transistor for voltage regulation. This circuit is more efficient and less susceptible to input voltage fluctuations. Although it still require additional resistor for base voltage correction thus circuit is still affected by temperature changes.
2nd row, 2nd circuit introduce a transistor for Darlington pair base reference voltage regulation. This transistor compensate unwanted Darlington pair base voltage changes due to input voltage and temperature fluctuations. 4.7V Zener diode set the reference voltage for both Darlington pair and compensation transistors.
Note: Two resistors and a diode in block around Darlington pair are present to emulate NPN Darlington pair transistor 2SD1825 with extra resistors for faster switching and flyback diode for inductive load support.
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