The basic Zener diode circuit does give us a regulated voltage but has a limitation. In order to keep working the current we can pull out is limited to a few mA of current. The current through the Zener should be 5 to 10 times the load current and that is limited by the wattage of the Zener. We will also notice that the voltage will vary somewhat as load current varies. So how can we adapt this circuit to improve it.
By adding an op amp in a buffer configuration we can get more current out, 10 mA or so depending on the op amp. This also isolates the load current changes from the Zener itself.
10 mA isn't enough? Let's add a transistor on the output to provide more current.

 

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The basic Zener diode circuit does give us a regulated voltage but has a limitation. In order to keep working the current we can pull out is limited to a few mA of current. The current through the Zener should be 5 to 10 times the load current

Nope. Picture this:
10 V source > resistor > 5.0 V zener (easier math than 5.1 V) > TTL circuit that draws exactly 25 mA

Without the zener, a 200 ohm resistor will drop 5 V at 25 mA, and the circuit will be happy. Happy and susceptible to any voltage fluctuations caused by its own output current requirements. Not good.

Dropping the resistor to 150 ohms with the zener in place, the current through the zener is 33.3 mA unloaded, and 8.3 mA with the circuit attached. As long as the circuit's current requirement does not increase by 8 mA, the zener keeps regulating. As long as the circuit's current does not drop to zero, the zener stays cool.

In short, what sets the zener current is the *range* of the output current, not it's absolute value.

ak

 

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Super!