Current boosting 78 series regulators with PNP transistor

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eXnol

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Hi everyone I am trying to boost the current output of LM7805. A circuit is available here : https://www.onsemi.com/pub_link/Collateral/MC7800-D.PDF

On page 21 the circuit for Current boost with Short Circuit Protection is given.

My question is, in the given circuit, the transistor used is of low gain type. I would like to use TIP127 instead. I do not think there would be a problem. Can you tell me if there would be a problem with using TIP127?
 

Re: Current boosting 78 series regulators with PNP transisto

I don't see major problem in using TIP127 as an external current boost transistor ..
Just select the resistor "R" (Fig.12) wisely bearing in mind that roughly the first 100mA of the 5V output current suppose to go through 7805, and above that the resistors "R+R[sc]" should “produce” enough voltage to turn the bypass transistor on ..
Also, as the TIP127 is a double-transistor (Darlington) the U[BE] voltage is ≈1.3V ..

IanP
 

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