I'm wondering what the 1.2k resistor does in the following circuit series pass voltage regulator. I should also mention, \$Q1\$ is a Darlington transistor.
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Interesting circuit - the zener could be biased from the incoming power but instead it's biased from the output of the regulator and the reason is this: -
For a typical NPN transistor such as the BC547 the max reverse voltage that can be applied to the B-E junction is 6V and if the 1k2 fed the zener from the incoming power supply the emitter would always be at about 6.2V - if the output were at zero volts you've exceeded the max limits of the device.
However, with the 1k2 being where it is, exceeding the max limit is impossible because the output level sources both base voltage and emitter voltage.
It supplies current to the zener diode which acts as a reference voltage to the emitter of Q2. The 470R and 56OR form a voltage divider that produces a signal at the base of Q2 (6.9V). This is compared with the zener voltage (6.2V) which is lower by the base -emitter voltage of Q2 (0.7V). When this condition is met the output will be 12.7 volts. If the output is pulled lower (large load current) more current is allowed to go into the base of Q1 (less through Q2) and raise the output voltage to the desired level (12.7V)