Just realized that the first sentenced of the of the first post by the TS stated:
"I was asked to find a value of R2 to make the diode current = .25 mA on a recent quiz."
That sounds definitive to me and can only be calculated as described above.
SG

 

I think that the salient point was in post #16
"I have a feeling the value of R3 was accidentally cropped out, the rest of the quiz was incredibly simple"

 

Just realized that the first sentenced of the of the first post by the TS stated:
"I was asked to find a value of R2 to make the diode current = .25 mA on a recent quiz."
That sounds definitive to me can can only be calculated as described above.
SG

Again, it may be that the author of the problem was wanting to give a problem that could be solved for a numerical value of R2. They didn't accomplish that, however.

It's also possible that they were seeking a function for the value of R2. Maybe not, but far from impossible. Referring to symbolic expressions as "values" is not at all uncommon.

If I had to guess, I would guess that the author of the problem and/or the editor of the quiz messed up. That does NOT make it reasonable to make unreasonable assumptions just to get an answer. If that were the case, then why not just assume that the value of R2 is 10 kΩ and be done with it?

 

I think that the salient point was in post #16
"I have a feeling the value of R3 was accidentally cropped out, the rest of the quiz was incredibly simple"

Agreed.

 

Agree also.
Maybe the TS can confirm by giving us the answer that was expected on the quiz.

 

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.

It's not confusing to me- but apparently is to the TP. I'm just waiting to see if TP gets an answer on if R3 has a value or not. One way or another 250uA must appear on the R3 leg.

 

Just realized that the first sentenced of the of the first post by the TS stated:
"I was asked to find a value of R2 to make the diode current = .25 mA on a recent quiz."
That sounds definitive to me and can only be calculated as described above.
SG

Hello,

If you assume that the diode voltage is 0 volts you end up with R1 in series with two resistors in parallel where both parallel resistors are UNKNOWN. There is no way to know what the current is through an unknown resistor when the other parallel resistor is also unknown.
If you assume 0.7v for the diode the same thing happens it's just a little more complicated where since the current is known and the assumed voltage is 0.7v that makes it possible to replace the diode with a resistor of value:
Rd=0.7/0.00025=2800 Ohms.
So now we have a resistor of 2.8k in series with one of the two unknown parallel resistors and that does not help any in the calculation of any of the two unknown resistors.

A worthwhile secondary idea comes out of this too. That is, when placing a known resistor in series with an unknown resistor the total combined resistance is also unknown. So in this particular problem if we substitute R3+2800 for the diode and R3, we get what we can call R3b and then we have R2 directly in parallel with R3b and we can forget about the diode (or just set its voltage to zero). Since R2 is still unknown and R3b is also unknown we have the same situation where we can not calculate a single value for R2.

Another way of looking at it is that R2 is a regulating resistor maybe a pot. You are told to change the value of the pot in order to make the current though the diode and R3 equal to 0.25ma. You will note that without knowing the value of R3 you wont know what resistance to set the pot at.

So no matter how you look at it there is no single value for R2.

What there is though is interesting. Find the value of R2 when R3 is max, and find the value of R2 when R3 is min. Now we are asked to find the minimum value of R3 and the value of R2 that satisfies the 0.25ma criterion. We know the max for R2 is infinity and that sets the max for R3 depending on what the diode voltage is but let's assume 0.7v as the original poster mentioned. Now we have to find the min of both resistors.
Try that and see what you get, it leads to some interesting results.

Here is a formula for R2 with vd=0.7 volts:
R2=30000*(R3+2800)/(15200-R3)

Note that again there is no way to eliminate R3 from the calculation.
If we solve for R3 we get:
R3=(15200*R2-84000000)/(R2+30000)

and again there is no way to calculate R3 without knowing R2.
So we need to know one or the other.

 

One thing I might consider would be the Vf of the diode. Another would be what would it take to limit the current through the diode to a safe value. There are a couple of very basic assumptions that could be made.

 

So no matter how you look at it there is no single value for R2.

MrAL we already verified that back in post 49,50

Note that again there is no way to eliminate R3 from the calculation.

Yes you can but you cannot dismiss as you called it R3b

 

But shouldn't there be a diode in series with R2 that is thermally coupled to the other diode? Otherwise, when we have worked out the correct value for R2, it will only be correct at one temperature?

 

One thing I might consider would be the Vf of the diode. Another would be what would it take to limit the current through the diode to a safe value. There are a couple of very basic assumptions that could be made.

The VF of the diode was fixed at .7 volt by the TS in post #5. The current through the diode was fixed at .25ma in post #1. You don't need to make any assumptions if you have the facts.
SG

 

Where do I get one of those diode? What's the RS part number?

 

Actually the 0.7Vf for the diode was not given in #1. And the 0.25mA? Hmmm... Kinda low but if that's the value... But given that it is 0.7Vf, the answer is easily found.

 

What's the RS part number?

Available at your local Radio Shack!

 

Actually the 0.7Vf for the diode was not given in #1. And the 0.25mA? Hmmm... Kinda low but if that's the value...

You are correct the .7 volt was not given in #1 but it was in #5 which is what I said.

But given that it is 0.7Vf, the answer is easily found.

Give us your answer.

 

One thing I might consider would be the Vf of the diode. Another would be what would it take to limit the current through the diode to a safe value. There are a couple of very basic assumptions that could be made.

The 30 kΩ resistor almost certainly does that. I doubt there are many diodes (outside of an IC) that can't handle at least half a milliamp.

Leaving the Vf parameterized is quite reasonable because then you can always replace it with the Vf for the diode at the desired current and for the particular diode. Unless R3 is very small, however, the specific Vf will have limited impact unless it is a very different diode type such as a germanium or a Schottky diode.

 

But shouldn't there be a diode in series with R2 that is thermally coupled to the other diode? Otherwise, when we have worked out the correct value for R2, it will only be correct at one temperature?

In theory. But the two diodes would really have to be very well thermally coupled such as on the same die. Even then though the current will not be the same in the two diodes unless R3=R2.
Good idea though.

 

Actually the 0.7Vf for the diode was not given in #1. And the 0.25mA? Hmmm... Kinda low but if that's the value... But given that it is 0.7Vf, the answer is easily found.

The value of 0.7v for the diode was given some time after the first post by the original poster.

 

MrAL we already verified that back in post 49,50

Yes you can but you cannot dismiss as you called it R3b

What do you mean by "dismiss"?
Why would you think i did that?

 

Actually the 0.7Vf for the diode was not given in #1. And the 0.25mA? Hmmm... Kinda low but if that's the value... But given that it is 0.7Vf, the answer is easily found.

Forget the diode! Remove it. You have three resistors. R1 in series with the parallel combination of R2 and R3. The voltage across the whole thing is 12 V and R1 is 30 kΩ. Given ONLY that information, find the value of R2 that results in the current in R3 being 0.25 mA.

You CAN'T!

The value that is needed for R2 depends on what R3 is.

The best you can do is give R2 as a function of R3.