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Old 14th April 2003, 09:51 PM   #1
fezz is offline fezz  United Kingdom
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Default regulator question

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
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Old 14th April 2003, 11:24 PM   #2
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This is for a 12V reg, sorry about the quality, but it's from my collection of web snippets from a while ago
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File Type: png reg1.png (2.0 KB, 274 views)
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Old 15th April 2003, 02:48 AM   #3
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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?
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Old 15th April 2003, 08:17 AM   #4
fezz is offline fezz  United Kingdom
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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
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Old 15th April 2003, 09:10 AM   #5
moamps is offline moamps  Croatia
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Quote:
Originally posted by 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
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
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Old 15th April 2003, 09:56 AM   #6
fezz is offline fezz  United Kingdom
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ah, its clear now, thanks very much - i promise i wont ask anymore stupid questions for at least a week
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Old 15th April 2003, 02:30 PM   #7
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Quote:
Originally posted by fezz
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!~!!

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Old 15th April 2003, 06:55 PM   #8
fezz is offline fezz  United Kingdom
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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?
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Old 15th April 2003, 06:57 PM   #9
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yes you do change it....same circuit as a positive

Cheers!!The DIRT®
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Old 15th April 2003, 10:06 PM   #10
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Default Put a NPN...

Quote:
do i change the PNP transistor to an NPN transistor when using a negative regulator?
Yes!!!
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