# Is this impossible to solve due to lack of information

1. Aug 7, 2016

### Enochfoul

1. The problem statement, all variables and given/known data
I came across this question in another forum and the members of the group said it can't be solved due to lack of information. Can anyone solve it?

2. Relevant equations

3. The attempt at a solution

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2. Aug 7, 2016

### Enochfoul

I should have said 3b can't be solved

3. Aug 7, 2016

### CWatters

That question has been posted on this forum recently. I don't think it can be solved. Too many unknowns. I think the best you can do is calculate R2 as a function of Vin for 3b(i) but then you are stuck for (ii).

4. Aug 8, 2016

### Tom.G

And for (ii) solve for VIN as a function of R2.

5. Aug 8, 2016

### donpacino

Where did this problem come from?
any chance a Vout value is given on a previous page?

If you had Vout you could get both as a function of Vout

6. Aug 8, 2016

### Enochfoul

Its an assignment question which I believe they have messed up on by omitting information.

7. Aug 13, 2016

### Enochfoul

Hi

The missing piece has been found Vout= 7.5V

Using the formula: Vout=Vref*(R1+R2/R2)

Vout=1.25(10+2/2) = 7.5

R2 must be 2 does that look right?

Last edited: Aug 13, 2016
8. Aug 15, 2016

### donpacino

looks good to me!

9. Aug 29, 2016

### rude man

What happened to part (ii)? We await with bated breaths ...

10. Sep 4, 2016

### Enochfoul

The problem stated had missing info. The Vout should have been 7.5V. Sorted it now thanks the lecturer missed it off by accident

11. Sep 4, 2016

### rude man

Yes I know, and part (a) was I presume solved. But what about part (b)? It's still an interesting problem.

12. Sep 4, 2016

### Enochfoul

D=Vout/Vin. There are 2 knowns in the question Duty Cycle which is 30% and Vout which is 7.5V. So Vout is 30% of Vin.

13. Sep 4, 2016

### Enochfoul

Im happy to show the solution for 3bi if you would like

14. Sep 4, 2016

### Tom.G

In post #7:
There seems to be some parens missing: Vout=Vref*((R1+R2)/R2)
Which reduces to the usually seen: Vout=Vref*(1+R1/R2)

15. Sep 5, 2016

### rude man

The answer is correct. Good going.