# Help with circuit

Discussion in 'Homework Help' started by ///m3dave, May 3, 2011.

1. ### ///m3dave Thread Starter New Member

Dec 8, 2009
23
0
In the attached circuit I can't figure out how to calculate reference voltage. Can someone help with this?
Thanks.

File size:
58.1 KB
Views:
45
2. ### beenthere Retired Moderator

Apr 20, 2004
15,815
283
That's interesting. Usually, 7805's are used as intended. The output will try to be some fixed voltage above apparent ground. The currents through the fixed resistors may tell you something.

Next time, use an LM317.

3. ### wmodavis Well-Known Member

Oct 23, 2010
737
150
I see no reference to reference voltage on the referred schematic. What reference voltage are you referring to? Just for reference.

4. ### ///m3dave Thread Starter New Member

Dec 8, 2009
23
0
I'm using Vout =Vref (1+r2/r1) to figure out Vout. I have r2 and r1. So would Vref be 5.0 V?

Dec 26, 2010
2,147
300
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?

6. ### ///m3dave Thread Starter New Member

Dec 8, 2009
23
0
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.