Regulator

[knuckles_mctug]

I realise this kind of question is asked a lot, but i've gone through all the posts and i don't seem to be able to figure it out. I'm using the circuit as attached, with the sense lines directly connected to the output for now. This circuit is working perfectly with +12V, but i also want +5V so i swapped out the 6.9V LM329 for the LM336 2.5V and used 1/1 for the divider network on the -ve side of the opamp. This according to theory should give me 5V. However the reference seems to breakdown much lower and therefore give me around 3V output, which i can't explain.

I've tried using an opa277 because i read somewhere that the common mode voltage was important but it did exactly the same. I've even tried putting the lm329 6.9V in and fitting a resistor across C16 to drop the reference, but with similar results. Its like there is a minimum voltage of the circuit, but i don't understand why - pass transistor maybe?

Is there something fundamental i'm missing?

Sorry if this has been answered before but i'm banging my head trying to work it out.

Thanks

 

[knuckles_mctug]

I took out R24 and used a second supply instead of the zener. It works fine, which i assume means that the pass transistor and opamp used are fine. Can anyone see what i'm doing wrong with the zener?

Thanks

 

[knuckles_mctug]

Obvious

OK that's obvious really, not enough reverse current on zener, dropped r26 to 2k7, did the job. Sorry to bother people!

Andrew