Current limit and current gain bjt question

bjtcurrentsemiconductorsvoltage-regulator

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Contact potential of Vbe in Q2=Vbe-on
I worked out that the maximum current that can go through R2 is Vbe-on/R2. So my question is why does Q2 turn on when the emitter current of Q1 reaches this maximum current?
Thanks!

Best Answer

What is the voltage across R2?

When the voltage across R2 reaches Vbe-on, what happens with Q2?

Answer those questions and come back with more questions of your own if necessary.

As a side note, it is customary to place a low-value resistor (1k) in series with the base of Q2. This prevents damaging the base of Q2 when a sudden short occurs at the output.

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