Can someone explain how I would solve this diode problem?

WBahn

Joined Mar 31, 2012
29,979
Think parallel circuit.... they have worded the original question to confuse.
What do you think is confusing about is?

Yes, it's a parallel circuit, but that doesn't help a lot since R2 and R3 are not in parallel -- and even if they were, that wouldn't allow for finding a numerical value for R2 independent of knowing what R3 was.
 

Ian0

Joined Aug 7, 2020
9,680
You can use MimeTex to do equations -- whether it qualifies as "easy" is in the eye of the beholder.
I must have a go at that.
Did you type in "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)}", or is there software to do it for you?
 

WBahn

Joined Mar 31, 2012
29,979
I must have a go at that.
Did you type in "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)}", or is there software to do it for you?
I typed it. With just a little bit of practice it's not too bad - it's merely another scripting language to learn and the basics are pretty simple. There are editors out there that have a GUI interface that will then generate the code *tex code, but unless you find one that knows the specific flavor of *tex you need, you end up spending as much time patching things up as it would have taken to just type it in the first place.
 

sghioto

Joined Dec 31, 2017
5,383
No, it is not possible to arrive at a numerical value for R2 without knowing the value for R3.
Not necessarily. We know R3 has to be less than 18K if R2 is infinity.
So assigning 4 volts at the parallel junction then 4/.25ma = 16K.
To get 4 volts at the junction the equivalent resistance has to be 1/2 of R1 or 15K
Solving for R2... 15=16xR2/16+R2 ...R2 =240K

EEE resistor challenge.pngSG
 
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WBahn

Joined Mar 31, 2012
29,979
Not necessarily. We know R3 has to be less than 18K if R2 is infinity.
So assuming 4 volts at the parallel junction then 4/.25ma = 16K.
To get 4 volts at the junction the equivalent resistance has to 1/2 of R1 or 15K
Solving for R2... 15=16xR2/16+R2 ...R2 =240K

View attachment 217606SG
Are you REALLY claiming that R2 = 240 kΩ independent of the value of R3?

What is R3 = 1.2 kΩ? Is R2 still 240 kΩ?

Upon what basis do you assume that there is 4 V at the junction?

This is EXACTLY the same as assuming a value for R3 even though you have NO basis upon which do do so.

The voltage at the junction, which I'll call Va, is given by:

Va = 0.7 V + (0.25 mA)R3

So if you "assume" a value for one, you have assumed the corresponding value for the other.
 

sghioto

Joined Dec 31, 2017
5,383
How do you justify assuming 4V at the junction?
Why can't I? The total resistance of the diode and R3 has to be less than 18K if R2 is not infinity. I know if the equivalent resistance at the junction is 15K then the voltage is 4 volts. I need the resistance of the diode and R3 greater than 15K and less than 18K. So 4v/.25ma = 16K. Now I can solve for R2.
SG
 
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sghioto

Joined Dec 31, 2017
5,383
Yes you would get different answers and both would be right, but that wasn't the point. WBahn said: "No, it is not possible to arrive at a numerical value for R2 without knowing the value for R3." I didn't know the value of R3 I knew what the total resistance needed to be at certain voltage level to achieve the .25ma.
SG
 

Ian0

Joined Aug 7, 2020
9,680
As soon as you fix the voltage across R2, then R3 is "known". Just because you haven't yet pressed the buttons on your calculator to subtract 0.7V and divide the result by 250uA, doesn't make it "unknown".
 

djsfantasi

Joined Apr 11, 2010
9,156
As soon as you fix the voltage across R2, then R3 is "known". Just because you haven't yet pressed the buttons on your calculator to subtract 0.7V and divide the result by 250uA, doesn't make it "unknown".
For any arbitrary V at the node, you have to know the value of R3. Since you agree your solution changes with each different value of V, you are demonstrating that R2 is a function of V... which is a function of R3. You are playing on semantics and ironically proving that you can’t arrive at a value for R2 without knowing R3.
 

sghioto

Joined Dec 31, 2017
5,383
You are playing on semantics and ironically proving that you can’t arrive at a value for R2 without knowing R3.
[/QUOTE
Not true. I only needed to know what the total resistance was of R3 and D1. The fact that R3 can be determined from that calculation (which is not an assumption) is irrelevant.
SG
 

WBahn

Joined Mar 31, 2012
29,979
Yes you would get different answers and both would be right, but that wasn't the point. WBahn said: "No, it is not possible to arrive at a numerical value for R2 without knowing the value for R3." I didn't know the value of R3 I knew what the total resistance needed to be at certain voltage level to achieve the .25ma.
SG
But you DID know the value for R3. By assuming a specific voltage at the junction, you are assuming a specific value of R3, namely the specific value that results in a voltage of 4 V at the junction when 0.25 mA is flowing in the diode.
 

panic mode

Joined Oct 10, 2011
2,715
OMG such as long topic yet still so far from being over... And it is so hard to watch and not just blurt out the solution... but it is not the solution that matter, it is the PATH to solution. what is see over and over with homework (not just this but in general) is lack of systematic approach. way too often one tries to jump over something (obvious or not) and in doing so fails to complete previous step. yes simplifications are often necessary and justified but they only have place after one has mastered the topic, not while building the case trying to solve one of the first problems.

one glaring problem here is the "step 1"....

so without giving out the solution, i propose to work on a process - which is literally stated in the topic name of the thread. here is one "recipe" one may follow (and not just in this problem but in general):

STEP 1. ALWAYS mark the circuit... mark EVERYTHING... all voltages across every component, all currents through each components, make sure that all components have reference label and if needed add one yourself.

NOTE: after 50 posts in a thread with numerous attempts to help you i still see no sign of this. and.... this is REQUIRED in order to write any equations and ultimately solve the circuit. it is also REQUIRED to demonstrate to your professor what your thought process is. unfortunately your circuit is STILL marked pretty much exactly as it was when the problem is presented. this is colossal waste... this demonstrates basic technical literacy so by failing to do this step, it is evident that you have not learned how to write and apply what was taught so far in your courses. this will likely be a major blow to your grade when doing similar problems in an exam. so practice to do things properly from the very first step...

STEP 2. if something need to be simplified, do so, use equivalent circuit or symbol. Considering diode "resistance" would be last on my mind, that would look like a poor model, specially when considering general solution. it is much more appropriate to consider diode as a fixed voltage drop since diode is non-linear and voltage drop changes much less than current.

STEP 3. pick method to solve it

STEP 4. write ALL equations you can deduce from MARKED schematics.

STEP 5. solve set of equations using method of your choice (algebraic manipulation, matrices, whatever)



for example...

STEP 1... draw circuit and mark all components, all voltages, all currents. here i used I1,V1 to show R1 current and voltage, for R2 it is is simply I2/V2, for R3 it is I3/V3, for diode, just Vd/Id.

1600617276869.png

STEP 2. i would leave the diode just as a voltage drop Vd. actual value is irrelevant and could be inserted later. Vd is different for different diode types (Ge, Si, GaAs etc). Anyone can do that once the solution is complete... for Ge value is 0.1-0.3V, For Si it is usually 0.6-0.7V etc. irrelevant...

STEP 3. pick method (for example nodal analysis)

STEP 4. apply KVL/KCL to everything and write those equations. reason why things are the way they are. for example diode and R3 are in series so the current though them is the same (Vd=I3) but their voltage drops add up. this branch is in parallel with R2 so the voltages across both branches are same. supply voltage is fixed so V1 is actually a function of V2 (V1 depends on V2). then you can explain how I1,I2,I3 are related, or V,V1,V2, or V2,Vd,V3. It should be dead simple if you are familiar with Kirchhoff and Ohm now that step1 is done PROPERLY.

STEP 5. this is just algebraic manipulation of data gathered un previous steps


for bonus points evaluate various special cases.... like obvious ones are when R3=0, or when R2=infinity... but how about defining minimum value for R2 to have a possible solution? or how about maximum value for R3, or maximum Vd etc.
 

WBahn

Joined Mar 31, 2012
29,979
That wasn't an assumption that was an educated calculation based on the .25ma requirement ,R1 and the 12 volt supply.
SG.
You're just digging yourself a deeper hole.

YOU are the one that explicitly stated that it WAS an assumption.

Not necessarily. We know R3 has to be less than 18K if R2 is infinity.
So assuming 4 volts at the parallel junction then 4/.25ma = 16K.
To get 4 volts at the junction the equivalent resistance has to 1/2 of R1 or 15K
What "educated calculation"?

Your educated calculation results in a value of R3 that is non-standard.

What is "uneducated" about assuming 3 V? Or 2 V?

What is "uneducated" about assuming a junction voltage of 0.95 V (which corresponds to R3 = 1 kΩ) or 3.2 V (which corresponds to R3 = 10 kΩ)
 

Ian0

Joined Aug 7, 2020
9,680
STEP 1. ALWAYS mark the circuit... mark EVERYTHING... all voltages across every component, all currents through each components, make sure that all components have reference label and if needed add one yourself.

NOTE: after 50 posts in a thread with numerous attempts to help you i still see no sign of this. and.... this is REQUIRED in order to write any equations and ultimately solve the circuit. it is also REQUIRED to demonstrate to your professor what your thought process is. unfortunately your circuit is STILL marked pretty much exactly as it was when the problem is presented. this is colossal waste... this demonstrates basic technical literacy so by failing to do this step, it is evident that you have not learned how to write and apply what was taught so far in your courses. this will likely be a major blow to your grade when doing similar problems in an exam. so practice to do things properly from the very first step...
See posts 15 and 40.
"Let V2 be the voltage across R2, and Vs be the supply voltage." Isn't that good enough for you?
I think we can all comprehend what that means without having to draw a diagram.
 

WBahn

Joined Mar 31, 2012
29,979
OMG such as long topic yet still so far from being over... And it is so hard to watch and not just blurt out the solution... but it is not the solution that matter, it is the PATH to solution. what is see over and over with homework (not just this but in general) is lack of systematic approach. way too often one tries to jump over something (obvious or not) and in doing so fails to complete previous step. yes simplifications are often necessary and justified but they only have place after one has mastered the topic, not while building the case trying to solve one of the first problems.
Look at Post #3.

The problem is that the TS was steadfastly resistant to any approach that does not yield a numerical value for R2 without knowledge of the value of R3. Your suggestion does not somehow make that happen. The TS eventually stated that they got a symbolic (at least in some form) solution early on but was still in search of some technique that would somehow yield a numerical value of R2 without any knowledge of the value of R3. I think they are now convinced that this is not possible.
 

sghioto

Joined Dec 31, 2017
5,383
YOU are the one that explicitly stated that it WAS an assumption.
Yes I did say that but that was worded incorrectly my bad.
What "educated calculation"?
In order for R2 to have a definitive value less than infinity the voltage at the junction has to be less than 4.5 volts.
I think the objective of the question was to put the circuit in a real world scenario that would give definitive values for R2.
Why else would they give a value for R1 and a supply voltage.
SG
 

WBahn

Joined Mar 31, 2012
29,979
Yes I did say that but that was worded incorrectly my bad.

In order for R2 to have a definitive value less than infinity the voltage at the junction has to be less than 4.5 volts.
Agreed. That does not mean that the junction voltage has to be 4 V. It can be ANY voltage between 0.7 V and 4.5 V and no specific voltage in that range is more justifiable than any other, with the possible exception of values that correspond to standard resistor values for R3. Since R1 is a standard value from the E24 series, then it is reasonable (but by no means guaranteed) to stick to E24 values for R3 (and to choose an E24 value for R2 as well and give the resulting diode current that will be slightly different than the target).

I think the objective of the question was to put the circuit in a real world scenario that would give definitive values for R2.
Why else would they give a value for R1 and a supply voltage.
SG
It may well have been that the objective was for the student to come up with a definitive value. If so, they failed to accomplish that objective. That can't be fixed by making unjustified assumptions.

They may have meant (or perhaps even did) provide the value of R2 in the question or in a table or some other means so that they could vary the answer from student to student. It could be that they just did a poor job of proofreading the problem when they prepared it.

It is FAR more valuable to have an engineer that is able to recognize that a problem, as given, has no solution and point that out than to have one that just goes and makes things up in order to get an "answer".

In the real world, it is not uncommon at all for customers to ask for solutions to problems in which they don't provide all of the necessary information -- part of the reason that they need to hire an engineer to do it is because they don't know what information they do and don't need, even if they happen to think that they do. When we ran into that, we went back to the customer and hammered things out, we did NOT just make assumptions based on something that we were willing to claim was an "educated calculation". Sometimes those conversations were simple and short. Sometimes they were long and painful. It was not uncommon for major changes to the design specifications to result once the customer was aware of the missing information and pondered what it would take to deal with it. In once case they resulted in the customer realizing that their project was fundamentally flawed and had to be scrapped altogether. In no case were we ever justified in assuming what the missing information was.
 
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