Op-amp + a voltage regular pair...

[Zach]

http://www.national.com/ds/LM/LM338.pdf
On that page there's a nice 10A circuit, using an UA741 op-amp (I think it's an op amp).

But, I'm still looking at this schematic here: http://www.aaroncake.net/circuits/supply.htm

And I see that the latter acheives up to 25v with a little different resistor combination, as opposed to 20v (since the output voltage of each LM338 is about 1.25(R2/R1 + 1)).

So, I need to know what the hell that UA741 does. I notice that a 15A circuit is 4.5v min, and I think each chip supplies 1.5v min due to an internal thing where they need that voltage. So, does the op-amp, since the resistors are there on the chip inputs, with the UA741 pulling it's + from the v+ of the secondary and - from the v+ of the primary, "turn off" the extra LM338?

Anyone? Please?

 

[SCSIRAID]

Are you talking about the circuit on page 14? The op amp in that circuit seems to be enforcing an equal split in current being supplied by each regulator. If one reg passes more current, its series .1ohm R will drip a little more voltage. The op amp will compare the drops across the .1 ohm sharing resistors and adjust the vref of the upper reg to compensate.

 

[Zach]

Aaahhh, that makes sense! Odd that this circuit can go down to 1.2v then.

So, I winder what will happen when I much with R1 and R3 to get 25V out of it, hopefully I don't need top play with R3. I could put in a 5K pot I guess, to be sure.

Sorry about not having the page too, yeah it was 14. I lost three times what I wrote playing with the back/forward buttons in my browser. Odd too, I've been doing that before.