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Kevin Weddle

zener diodes

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A voltage regulator needs to have a voltage reference. Low voltage and adjustable voltage regulator ICs use a "band-gap" circuit. Higher voltage regulator ICs use a zener diode.

Voltage regulators have a power error amplifier that supplies power to the load, not the little low current zener diode.

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A zener diode is used as the voltage reference in a voltage regulator, not for biasing. Constant current sources, current-mirrors and resistors (accurate ratios) are used for biasing.
The zener diode is temperature stable when its voltage is 5.6V to 6.2V. For a higher stable voltage then it is simply amplified.
Why don't you see this on the datasheets for 78xx regulator ICs?

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A voltage regulator circuit has a zener diode voltage reference plus an error amplifier that drives the load. The zener diode voltage reference always has a constant load current.
The error amplifier has the variable load current.

A zener diode is a shunt regulator so its current is fairly low so it doesn't waste a lot of power when the load current is low. Its output current is always low.
But a voltage regulator circuit is a series regulator that has a low current when the load current is low.
It can provide a high current when the load current is high.

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A zener diode has low output impedance, like a battery, so you have said before. But even being a shunt regulator, the voltage isn't good enough to act as a voltage regulator. Otherwise a simpler circuit using zener diodes would substitute for a voltage regulator, with a little waste of power.

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The datasheets for zener diodes list their output impedance that is no where near as low as a voltage regulator circuit.

A zener diode is a shunt regulator that wastes its entire max output when it has no load. Its voltage drops when loaded because it has no high gain error amplifier like used in series voltage regulators.

A voltage regulator circuit is a series regulator that has low wasted current with no load.
When loaded if its output voltage drops then its high gain error amplifier boosts it to what it is supposed to be.

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