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Can anyone explain this circuit

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Arya Kumar

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Can anyone explain this circuit.

circuit - http://www.reuk.co.uk//OtherImages/lm317t-pnp-pass-transistor-voltage-regulator.jpg
link - http://www.reuk.co.uk/wordpress/electric-circuit/lm317-high-current-voltage-regulator/

I made this circuit and it works fine..
I know about Lm317 and how to get variable voltage. I dont know how PNP darlington transistor is bypassing current with particular value of resistance.
to achieve high current.

I want to get more current from it so i am interested with this.

If anyone has any other circuit..or any other idea please tell.

Thanks Arya.

- - - Updated - - -

Analytically what i see is with more difference in input and output voltage there will more current flowing through R3
and as Ic proportinal to Ib more current will flow thru collector. similarly vice versa..
that is what i see .Not sure.
 

In your circuit R3 is connected between B-E of P-N-P Darlington .
To turn it ON , sufficient voltage must develop across Base to emitter (Vb-e of pnp Darlington)

This voltage comes from I*R3
given R3 as 22 ohms , and knowing Vbe , you can find at what current PNP turns on.
Once ON , it shares the load current with LM317.
 

You find similar circuits in LM317 data sheets. Using a darlington doesn't make much sense, at least for output currents up to 10 A. Better use a regular PNP power transistor for lower voltage drop.
 

Ok thanks,
I should have made that out. That was a simple circuit.
 

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