allright. I pulled up the LM317 spec sheet and found the following
so....here is the work I have based on this equation
I am wayyyyy off base here. what am I doing wrong?
Agreed - I misread the data sheet. Looked at the adjustment pin current change row rather than the actual value. I can only put it down to deteriorating vision.Iadj is 50uA typical, and 100uA=0.0001A max.
Iadj is 50uA typical, and 100uA=0.0001A max.
But this circuit can be treat as a voltage divider.
Since R2 = 100Ω and voltage across R2 is equal 1.25V we have
IR1 = 1.25V/100Ω = 12.5mA
And the current that is flow through R1 is equal
IR2 = Iadj + IR1 = 12.6mA
And we want Vo = 15V----> VR1 = 15V - 1.25V = 13.75V
R1 = 13.75V/12.6mA = 1.09126984KΩ
\(R1=R2*\frac{Uwy-1.25V }{1.25V+I_{ADJ}*R2}\)
Or
\(R1=R2*(\frac{Uwy }{1.25V}-1)\)
Well in my country "Uwy" means exactly the same as Vout in english.
And sorry for confusion
In R2 of your schematic (your diagram and the application notes have reversed designations) you have 100Ω. Since the method the LM317 uses is to maintain 1.25V between the output pin and adjust you should be able to figure current. The current through R2 is the same as through R1. From there it is gravy.
Maybe answer is wronghowever, this answer was listed as wrong
what are we doing wrong here?
but my schematic looks different from the one that is in that PDFHello,
The PDF in the following link will give you some more insight in the voltage regulator:
http://www.techlearner.com/Apps/basregulators.pdf
The LM317 you will find at page 22.
Bertus
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