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Series pass transistor on a linear voltage regulator

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boylesg

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As per the datasheet for ST L7812CT voltage regulator, I have wired up a high current voltage regulator. Except that I have used a BD140 rather than the tranny they specified because I don't believe that I need that much extra current. I have used their formulas to calculate the required resistance value and it comes out at about 8R however I have used 10R since I only had 0.25W resistors at 8.2R. The 10R resistor seems to work well enough.

But I want to understand how this works. If I have 24V going into the regulator then I have a sort of direct parallel connection between the 24V in and the 12V out through the transistor.

So what happens to the extra 12V as it passes through the transistor?
 

With respect for high current regulator with short circuit protection.................

How do you calculate Vbeq2 and Isc?
 

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