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100R means that the transistor will turn on whenever the 7812 is using any more than 6.5 mA of input current. Since some 7812s could have 10mA input with no load, this might cause the output to be too high. When operating, the transistor would be carrying almost all of the load current which means the 7812 has no idea when there is an overload condition.
Google "Web Bench" and sign up. They have a neat program where you just enter your input voltage, output voltage and current and it gives you a design and all the parts needed.
Flinty, The 7812 cannot supply 5 or 10 amps.
ah that's why we use transistor,mm used as an amplifire??
Yes, kind of like a current amplifier.