regulator question

[fezz]

i have checked in the data sheets where it explains roughly how to do this, but i dont understand it all and really need it explaining to me (sorry)

how can i get a 1A voltage regulator to work with higher currents... i know youve got to use a pnp transistor but i do not understand the rest of the circuit... could someone show me what circuit i will need to do this (please)

thanks alot

 

[pinkmouse]

This is for a 12V reg, sorry about the quality, but it's from my collection of web snippets from a while ago

 

[Circlotron]

For those that care about such things, the drawing is really a complementary feedback pair (CFP) aka Sziklai pair as is also sometimes used in one leg of an audio power amp. A power supply reg is just a DC power amp.

Feed audio into Vref of a power supply? - who said that?

 

[fezz]

thanks for that, i assume it is the same for a higher voltage circuit - but i still do not understand how R is defined - and why it is there

 

[moamps]

thanks for that, i assume it is the same for a higher voltage circuit - but i still do not understand how R is defined - and why it is there

Hi
here is my calculation, maybe help

First choose max current thru 78xx (must be smaller then 1A to avoid excessive heating, and bigger then 0,1A for stable operation) I choose in my PS I(reg)=0,25A),

Then R =Vbe/Ireg=ca 0,7/ 0,25=2,8 E, ( power transistor will start conduct when Vbe reach ca 0,7V)

Power dissipation on R is Pr=0,7*0,25=0,13W, I use 1W resistors.

If calculated R have not standard value, take bigger one (smaller current thru 78xx), or smaller one (bigger current thru 78xx). In this case god values are 2E2, 2E7, 3E3.

regards

 

[fezz]

ah, its clear now, thanks very much - i promise i wont ask anymore stupid questions for at least a week

 

[JOE DIRT®]

ah, its clear now, thanks very much - i promise i wont ask anymore stupid questions for at least a week

LMAO@Fezz.......ask away!!!!!....thats why we all come here!~!!

 

[fezz]

JOE DIRT®: thanks, looks like i lied anyway, ive got one more question - do i change the PNP transistor to an NPN transistor when using a negative regulator?

 

[JOE DIRT®]

yes you do change it....same circuit as a positive

Cheers!!The DIRT®

 

[Tube_Dude]

Put a NPN...

do i change the PNP transistor to an NPN transistor when using a negative regulator?

Yes!!!

 

[moamps]

Hi, Fezz

You can use this schematic too. For +V here is used small PNP for sensing current thru regulator and driving high power NPN transistor. For –V schematic is similar (small NPN and high power PNP).

regards

 

[Circlotron]

One last thing - with an extra transistor added the cct is no longer short circuit proof like the reg is when used by itself.

 

[jan.didden]

reg question

... except that in this circuit the LM195 itself is short-circuit, over-dissipation etc proof...

Jan Didden