Check your math.

15 = 1.25 * ( 100 + R2)/100
1500 = 1.25*(100 + R2)
1500 = 125 + 1.25R2
R2 = 1375/1.25 = 1100

yea your right. i made a mistake but that answer is still incorrect

 

But this is the correct answer or if we take into account Iadj current then for
R1=100 R2 = 1.09126984KΩ = 1.1K

 

I also calculated 1.1KΩ, but the question asks to select the best answer of the choices below. It would be the one with 1075Ω as that is the closest to the actual calulations.

 

of the answers that are listed

D) 1075Ω

and

E) 1200Ω

have both been deemed incorrect.

this problem is due tonight at 12p.m. and I cant afford to take another "guess" at the answer. I NEED to figure this out

 

heres another one

If I use that formula Vout = 1.25(1+R2/R1)
I get the same thing

R1=1100

ughhhhhhhhhhhhhhhhhhh

 

Hello,

Keep in mind that the reference voltage can be a little diffrent from the 1.25 Volts.

This can be of influence of the the value of the resistor.

Bertus

 

Hello,

Keep in mind that the reference voltage can be a little diffrent from the 1.25 Volts.

This can be of influence of the the value of the resistor.

Bertus

but it specifically states in the problem that the OUT-to-COM voltages is regulated at 1.25V

is that not the same?

 

Even with the worst case variation in the reference voltage (1.2V min or 1.3V max) the range of R2 would only be from 1150Ω to 1054Ω, making one of the nominal answers (D) or (E) appear the most likely.

Sometimes one needs to stand their ground and believe in their viewpoint. Often the disagreement arises because the 'protagonists' are reading from a different page or perhaps even a different script!

 

i have been staring at this so long I have decided to try a different method

I figured that I could put the answer into the equation to see what kind of Vout I would receive

here are my results

R1=1200Ω.....Vout=16.25V
R1=1075Ω.....Vout=14.69V
R1=1060Ω.....Vout=14.5V
R1=960Ω.......Vout=13.25V
R1=860Ω.......Vout=12V

the question states "determine the size of the resistor R1 required to provide for a 15V output"

based on my data, the minimum resistor needed to get 15V would obviously be the 1200Ω resistor.

that answer HAS to be correct.

 

Yes that seems a reasonable assumption if the output has to be at least 15V.

There is also another anomaly in the original question statement that leaves a strong sense of ambiguity.

The question at one point states that "the minimum load current through resistor R2 is 10mA". I'm not sure if this is meant to say the minimum load current is 10mA - which would make more sense in the context of the voltage regulator goal. In any case it would have no relevance to the immediate design goal in the problem statement.

The uncertainty with the statement that the minimum current in R2 is 10mA would indicate the regulator is either running in partial drop-out condition which is highly undesirable or that the reference voltage is only 1V which is an untenable assumption. The minimum current in R2 would be nominally 1.25/100 or 12.5mA with the regulator correctly set up.

If Vref were only 1V then to obtain 15V would require R1 to be 1400Ω.

Trying to analyze the circuit when operating in partial drop out mode would presumably be entirely beyond the intended scope of the question.

 

I answered what I believe the answer to be even though it is wrong. I emailed the Prof. and pleaded my case. well see what he says. thanks for all your help