Can someone explain how I would solve this diode problem?

Ian0

Joined Aug 7, 2020
9,668
It turns a simple current-limiting resistor calculation into something a lot more challenging. That's all it does!
 

WBahn

Joined Mar 31, 2012
29,978
Thank you for all the help, I

The question is to solve for R2 though, and the only way I can do that is by assuming a value of R3, and there was nothing mentioned about it, and it seemed like the teacher wanted an exact value
Did the problem actually say that the answer needed to be a actual value?

Whether it did or not, you can't give a specific actual value for R2 because it is a function of R3. So the best you can do is give R2 as a function of R3.

But aside from that, solving the problem first symbolically will usually get you nearly all the points even if the problem does give you all the information you need to arrive at a numerical example and even if you don't follow through to the end and come up with that actual value.

But DON'T just ASSUME a specific value of R3. You have NO justification for doing that. Just parameterize your solution in the unknown constraint.
 

WBahn

Joined Mar 31, 2012
29,978
It turns a simple current-limiting resistor calculation into something a lot more challenging. That's all it does!
Far from it. R3 and R1 may not be accessible and R2 is used to shunt current away from the diode to set it to the desired current.

This is almost exactly how bias adjust circuits frequently work on an IC. You use a shunt resistor to either the positive or the negative supply in order to adjust the on-chip generated bias current.
 

djsfantasi

Joined Apr 11, 2010
9,156
Thank you for all the help, I

The question is to solve for R2 though, and the only way I can do that is by assuming a value of R3, and there was nothing mentioned about it, and it seemed like the teacher wanted an exact value
“Seemed like”? If he wanted an exact value, he would have given you a problem that could be solved for an exact value.

I see that you have three choices.

1) you don’t answer the question. Expected # of points = 0

2) you give some gobbledygook answer. Again, the expected # of points = 0

3) you solve it symbolically. At least with this choice, your answer would technically be correct and the expected # of points > 0

Up to you...
 

Thread Starter

FuneralHomeJanitor

Joined Oct 12, 2019
48
But is it actually possible to arrive at a numerical value for R2 without any value for R3? Or do absolutely have to make up that value on my own and write an algebraic expression to actually have a number for my answer? I am still a little confused as to if this is something that is possible
 

djsfantasi

Joined Apr 11, 2010
9,156
But is it actually possible to arrive at a numerical value for R2 without any value for R3? Or do absolutely have to make up that value on my own and write an algebraic expression to actually have a number for my answer? I am still a little confused as to if this is something that is possible
No! And no! Making up a value is the wrong approach and lead to the wrong answer. As stated the problem has no way to get an actual number. This is NOT something that is possible.
 

Thread Starter

FuneralHomeJanitor

Joined Oct 12, 2019
48
the exact question was “what should the value of R2 be to have a diode current of .25 mA” and I tried to find an actual value for an hour an a half and thought there was some circuit technique I didn’t know since I have taught myself most of what I know
 

djsfantasi

Joined Apr 11, 2010
9,156
the exact question was “what should the value of R2 be to have a diode current of .25 mA” and I tried to find an actual value for an hour an a half and thought there was some circuit technique I didn’t know since I have taught myself most of what I know
Note that the problem statement does NOT require an actual value.

Does it?

Or are you overthinking and reading something that was not stated.

A symbolic answer acknowledging the dependence on R3 is the ONLY possible answer. You should have recognized this long ago and be finished with this problem.

Asking over and over is NOT going to change the answer. I won’t comment on what it DOES say.
 

MrAl

Joined Jun 17, 2014
11,389
But is it actually possible to arrive at a numerical value for R2 without any value for R3? Or do absolutely have to make up that value on my own and write an algebraic expression to actually have a number for my answer? I am still a little confused as to if this is something that is possible
No it is not possible period.

The simplest solution given your comments on the diode voltage being 0.7 volts is:
R2=(30000*(R3+2800))/(15200-R3)

and there is no way to factor out the R3 or eliminate it in any other way without some other constraint. The constraint could come in many forms not just the value of R3. R3max, R3min (if there is one), R3 average, R3 from some other problem, pick your own R3 value, etc.

What else you could do is present the solution in the form of a graph of R2 vs R3. The graph would have to be limited because R2 could go to an open circuit if R3 is max.

Here is an example...

DiodeCircuitProblem-1.gif
 
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WBahn

Joined Mar 31, 2012
29,978
But is it actually possible to arrive at a numerical value for R2 without any value for R3? Or do absolutely have to make up that value on my own and write an algebraic expression to actually have a number for my answer? I am still a little confused as to if this is something that is possible
No, it is not possible to arrive at a numerical value for R2 without knowing the value for R3. Imagine that R3 happened to be the value such that, with R2 removed completely, there is 0.25 mA of current in the diode. Then any value of R2 will result in the current going down, so you have to set R2 to infinity. Now pick another value of such that the diode current is 0.30 mA. If you leave R2 as infinity, you have too much current in the diode, so you have to use a finite value of R2 to shunt 0.05 mA of current away from the diode.

But, no, you do NOT make up your own values. You solve for it symbolically. You give an equation for R2 that is a function of R3 so that, when given a numerical value for R3 you can plug it into your function and get the corresponding value of R2 that is needed to meet the requirements. You can even plot it out showing the value of R2 over the allowed values of R3 for which a solution is possible.
 

WBahn

Joined Mar 31, 2012
29,978
If R1 and R3 are inaccessible, how do we know their values?
Just because they are inaccessible, at least in the sense that we can't change them, does not mean we don't know their values. In many cases you look at the schematics and see what they are. If you don't have the schematics then there are a host of ways you can make measurements to determine what they are, either directly or indirectly. But when running the device, you have to control the current in an internal branch by shunting current either into it or out of it and the simplest way to do that is with a shunt resistor to one of the supplies.
 

WBahn

Joined Mar 31, 2012
29,978
the exact question was “what should the value of R2 be to have a diode current of .25 mA” and I tried to find an actual value for an hour an a half and thought there was some circuit technique I didn’t know since I have taught myself most of what I know
If you are given a circuit in which there are two resistors in series, R1 and R2, with 12 V across them, then the answer to the question of what the value of R2 should be to have a current of 10 mA would be:

R2 = 1.2 kΩ - R1

That is the value that R2 has to have in that circuit to result in a current of 10 mA.
 

WBahn

Joined Mar 31, 2012
29,978
Your answer is:
R2 = R1.(250uA.R3+Vd)/ Vs-Vd-250uA.(R3+R1)
I think you need to patch that up to reflect order of operations. Otherwise you have a resistance minus a voltage, minus a voltage.

Why use the specific numerical value for the current but leave supply voltage and the top resistor symbolic even though they also have specified values?
 

Ian0

Joined Aug 7, 2020
9,668
No good reason why I put 250uA rather than I3, except that I3 looks rather like 13.
You're correct - there's brackets missing because when I wrote it on paper the division was a long horizontal line so wouldn't need them. I don't think that this forum's software includes an easy way of typing equations!
R2 = (R1.(250uA.R3+Vd))/ (Vs-Vd-250uA.(R3+R1))
Otherwise it's consistent (Resistance *(Voltage+Voltage))/(Voltage-Voltage-Voltage), so the units are consistent, because 250uA.R3 is a voltage as is 250uA.(R3+R1)
 

Thread Starter

FuneralHomeJanitor

Joined Oct 12, 2019
48
Thank you for all the responses, I was going crazy trying to figure out what I was doing wrong. I had arrived at a value of R2 in terms of R3 in just a few minutes but the problem seemed like he wanted an exact value, and after working hard to improve my circuit analysis I felt like this was a major setback until you all confirmed there is no way to find an actual numerical value without assuming R3
 

BobaMosfet

Joined Jul 1, 2009
2,110
I was asked to find a value of R2 to make the diode current = .25 mA on a recent quiz. I breezed through most of the quiz and thought it seemed extremely simple but completely hit a wall with this question and tried everything I knew including node and mesh analysis which usually works for me but everything I did resulted in me having too many unknowns since the value of R3 isn't given. I ran out of time after trying for an hour and a half on this and it is still bugging me a day later. Can someone explain this to me, because it seems very difficult in comparison to the rest of the quiz so I must be doing something very wrong. Thanks in advance.
View attachment 217490
Think parallel circuit.... they have worded the original question to confuse.
 

WBahn

Joined Mar 31, 2012
29,978
No good reason why I put 250uA rather than I3, except that I3 looks rather like 13.
You're correct - there's brackets missing because when I wrote it on paper the division was a long horizontal line so wouldn't need them. I don't think that this forum's software includes an easy way of typing equations!
R2 = (R1.(250uA.R3+Vd))/ (Vs-Vd-250uA.(R3+R1))
Otherwise it's consistent (Resistance *(Voltage+Voltage))/(Voltage-Voltage-Voltage), so the units are consistent, because 250uA.R3 is a voltage as is 250uA.(R3+R1)
You can use MimeTex to do equations -- whether it qualifies as "easy" is in the eye of the beholder.

\(R_2 \; = \; R_1\frac{\left( V_d \; + \; I_3 R_3\right) }{\left(V_s \; - \; V_d \right) \; - \; I_3\left(R_1 \; + \; R_3\right)}\)
 
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