Thank you Adjuster. Really what i was looking for was to verify if I was using the correct reference voltage. This is not a circuit I am using, it's for a lab question.Apparently, Fairchild did sanction this kind of arrangement for 78XX: http://www.fairchildsemi.com/ds/LM/LM7805.pdf (see P.23).
You will see that they give a formula for Vout: VO = VXX(1 + R2 / R1) + IQR2 , provided that I(R1) ≥ 5 IQ
Vref is indeed 5V for 7805 but you also have to allow for the quiescent current IQ. This current is about 5 to 6mA, so you end up wasting an extra 25mA or more to get an adjustable output.
As beenthere suggests, if you want an adjustable regulator, why not use something designed for the job?
by Don Wilcher
by Jeff Child