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Re: Can anybody tell me how to understand the following circ
The circuit can be thought of as a cascaded regulator. The first stage regulates the voltage to 26V-2*Vbe, while the second regulates it to 12V.
The advantage in general of a cascaded regulator is the better line regulation (smaller changes in output voltages due to changes in input voltages). It is easy to see why: the second regulator sees already a somewhat regulated voltage at its input, so its output changes less than it would otherwise.
However, in some cases this circuit can be used to limit the power dissipation in the second regulator, if the input voltage is too high, because the dissipation in the second regulator is the product of the load current and the input-output voltage diffeerential. Since the input voltage is limited to about 25V, there is only 13V across the regulator. If the first stage was not there, the second regulator would have seen the entire input voltage, thus increasing its dissipation.
The rest of the power is then dissipated by the transistor.
Finally, such a circuit can be used as a clamp, to provide the second regulator with an acceptable input voltage. Suppose it can only accept 30V at the input, but your input voltage is 40V. The circuit shown will "clamp" the input voltage to about 25V, that is, if the input voltage is less than 25V, it will appear at the YYY point, minus 2Vbe. But if it is higher, then the voltage at YYY will stay at 25V, being regulated by the transistors/ Zener circuit.
So this is how the circuit works. One of the reasons above mandated its use: either there was a need for better regulation than the second regulator alone could provide, or the power dissipation was too great for the second regulator to handle, or the input voltage was too high for it.