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

You misread my reply. I was saying the value of R3 was not relevant but the value of what you labeled R3b is.
Another thing you said there is no way to calculate R3 without knowing R2. Quite the opposite.

 

You misread my reply. I was saying the value of R3 was not relevant but the value of what you labeled R3b is.
Another thing you said there is no way to calculate R3 without knowing R2. Quite the opposite.

Given any two of the three resistances, you can determine the third (or whether no positive finite value exists).

 

You misread my reply. I was saying the value of R3 was not relevant but the value of what you labeled R3b is.
Another thing you said there is no way to calculate R3 without knowing R2. Quite the opposite.

Well ok, then show some proof.
If you do not feel comfortable posting in this thread then send me a PM with your value of R3 or R3b without knowing what R2 value is.

 

Well ok, then show some proof.

I did back in post #44.
SG

 

I did back in post #44.
SG

Again, you arbitrarily declared R3 to be 13.2 kΩ. If R3 is anything other than 13.2 kΩ, your value for R2 is wrong.

Claiming that you solved for R2 without knowing R3 is like saying that you solved a problem that requires the weight of the water in a tank without knowing the weight by assuming what the volume of the water in the tank was.

Let me put it another way. You assumed, declared, or "educationally calculated" a junction voltage of 4 V. That is just another way of saying that you assumed, declared, or educationally calculated that R3 has 3.3 V across it when 0.25 mA is flowing through it. In other words, you assumed, declared, or "educationally calculated that R3 has a value of 3.3 V / 0.25 mA which is 13.2 kΩ.

 

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.

He's not wrong.

 

Again, you arbitrarily declared R3 to be 13.2 kΩ. If R3 is anything other than 13.2 kΩ, your value for R2 is wrong.

Claiming that you solved for R2 without knowing R3 is like saying that you solved a problem that requires the weight of the water in a tank without knowing the weight by assuming what the volume of the water in the tank was.

I did not declare R3 to be 13.2k
We already agreed the voltage at the junction had to be less than 4.5 volts. I assigned a voltage of 4 volts. Therefore the total resistance of R3 and D has to be 16k to get .25ma. Now in order to get 4 volts at the junction the equivalent resistance has to be 15k. So I now solve for R2 and without actually having to know R3. The 13.2k was calculated after.

 

I did not declare R3 to be 13.2k
We already agreed the voltage at the junction had to be less than 4.5 volts. I assigned a voltage of 4 volts.

And, in doing so, you assigned R3 to be 13.2 kΩ.

There is a one-to-one correspondence between the voltage at the junction and the resistance of R3. Specify either and you have specified both.

Just like it doesn't matter whether I say that the weight of water in a tank is 833 lb or say that the volume of water in that tank is 100 gal; both are equivalent statements (in a simplified world in which a gallon of water weight 8.33 pounds, just like the simplified world in which the diode voltage drop is 0.7 V). Saying one dictates the other.

If you don't agree, then what is R2 if you assign the junction voltage to be 4 V but R3 turns out to be 10 kΩ?

 

And, in doing so, you assigned R3 to be 13.2 kΩ.

There is a one-to-one correspondence between the voltage at the junction and the resistance of R3. Specify either and you have specified both.

Just like it doesn't matter whether I say that the weight of water in a tank is 833 lb or say that the volume of water in that tank is 100 gal; both are equivalent statements (in a simplified world in which a gallon of water weight 8.33 pounds, just like the simplified world in which the diode voltage drop is 0.7 V). Saying one dictates the other.

If you don't agree, then what is R2 if you assign the junction voltage to be 4 V but R3 turns out to be 10 kΩ?

How could it be 10k? Are you assuming?

 

How could it be 10k? Are you assuming?

In the following discussion i assume that the diode voltage is 0.7v as given earlier in this thread.

If you set the junction voltage to be 4v then you get one value for R3, but if you set the junction to some other voltage then you get a different value for R3 and that would be 10k if the junction voltage was 3.2 volts instead of your arbitrary 4.0 volts.

Here are the two equations that calculate R2 and R3 if the junction voltage (vu) is set at some particular level:

R2=(6000*vu)/(9-2*vu)
R3=4000*vu-2800

and since we know that vu can only have a range of 0.7v to 4.5v that means we are limited to what we can set the value of vu to, but we dont have a single value of vu either it can be whatever it has to be and we can still satisfy the 0.25ma criterion.

It should be noted that the two equations above are not independent of each other they are linked by the variable 'vu'. If you change vu for one you have to make vu the same in the other.

 

How could it be 10k? Are you assuming?

That was a "what if" scenario.

R3 has a value, we just don't know what it is. You are claiming that, without knowing what it is, you can find a value for R2 that results in 0.25 mA flowing in the diode by . But your method assumes and requires that R3 have a specific value that is determined by your assumption. If it has any other value, your approach fails. But you seem to think that by declaring the junction voltage to be 4 V that some magical genie appears out of nowhere and turns R3 from whatever it really is into the one value that it has to be to make your declaration correct.

Consider it this way.

I build the circuit using a 12 V battery, a 30 kΩ resistor for R1, a 10 kΩ resistor for R3, and a diode. All of these parts are nice, ideal, and exact. I put a blob of epoxy over R3 so that you can't see what it is, but you can see all of the other components.

You are claiming that you can find a value for R2 that will make the diode current equal to 0.25 mA without knowing the value of R3.

So do it.

 

Let's look at this another way. Let's say the values of R3 was given at 10K and we know the resistance of the Diode is 2.8k then the minimum voltage at the junction has to be above 3.2 volts if R2 is anything less than infinity. Now you have to assign a voltage to the junction inorder to figure out what the equivalent resistance

Without R2 in the circuit I measure the voltage across R1. Now I know the voltage across the junction and the current and the resistance of R3+D. Now I assign a voltage higher then the

This makes almost no sense.

You don't get to measure the voltage across R1 to find the voltage at the junction. That's not what your method does. In your method you ASSIGNED a voltage to the junction, independent of what R3 was. You DECLARED the junction voltage to be 4 V. If you were willing to do that before, why aren't you willing to do it now? Nothing has changed.

As soon as you MEASURE the junction voltage (or any other measurement that would allow you to find it), this is equivalent to measuring R3. Whether you ever actually do the math to be able to write down the value of R3 or not is completely irrelevant -- you now have and are using information that equivalent to knowing the value of R3.

One last time.

I now have sitting before me a circuit that has a 12 V bench supply connected to that circuit (without R2). R1 is a 32.7 kΩ resistor. I'm not going to tell you what R3 is, since you claim you don't need to know it. At a current of 0.25 mA the diode has a voltage of 0.59 V across it.

You claim that, without knowing what R3 is, that you can determine what value of R2 is needed to get 0.25 mA of current through the diode.

So, do it!

Tell me what value resistor I can use for R2 and I will put that value resistor into the circuit and determine what the diode current is.

If you can't do that, then your claim that you can is wrong.

 

I wasn't able to finish was on my phone. Will continue when I get home
SG

 

I wasn't able to finish was on my phone. Will continue when I get home
SG

Sounds good.

 

I wasn't able to finish was on my phone. Will continue when I get home
SG

sghioto, assigning (same as "assuming" in this context) the junction voltage is exactly equivalent to assuming the current in R1. It's easy to see that if the current in R1 is known, the problem is solved.

To assume the current in R1 is to assume an answer to the problem.

 

I build the circuit using a 12 V battery, a 30 kΩ resistor for R1, a 10 kΩ resistor for R3, and a diode. All of these parts are nice, ideal, and exact. I put a blob of epoxy over R3 so that you can't see what it is, but you can see all of the other components.
You are claiming that you can find a value for R2 that will make the diode current equal to 0.25 mA without knowing the value of R3.
So do it.

Well considering it that way the answer is no. I can't do that because R3 is a fixed value and I'm not allowed to use a meter. However in the circuit posted R3 is not shown has having a fixed value. The quiz said, unless the TS did not word it correctly, find a value of R2 that will allow the .25ma through the diode D with the known values given including a fixed .7 volt across D.
It did not say find "the" value of R2 or find an expression/equation that defines R2. To me that means an actual value like 240K that will satisfy the requirements.
I never disagreed that R2 was not defined by R3 or vice versa. Once I chose to use 4 volts at the junction (which was mathematically derived not assumed) I did define the value of R3 at that moment only because I knew the resistance of the diode. I did not have to do the final calculation to know what that value was to figure out what R2 needed to be is all I was implying in my original post #44.
SG

 

Still hoping for a reply from the TS as to what was the expected answer to that problem on the quiz.
If it's a single definitive value for R2 than the value of R3 must have been stated but somehow on his version of the quiz in his words "I have a feeling the value of R3 was accidentally cropped out "
If it's multiple values for R2 then my analysis is correct.
If it's an expression/equation I stand corrected.
SG

 

Well considering it that way the answer is no. I can't do that because R3 is a fixed value and I'm not allowed to use a meter. However in the circuit posted R3 is not shown has having a fixed value. The quiz said, unless the TS did not word it correctly, find a value of R2 that will allow the .25ma through the diode D with the known values given including a fixed .7 volt across D.
It did not say find "the" value of R2 or find an expression/equation that defines R2. To me that means an actual value like 240K that will satisfy the requirements.
I never disagreed that R2 was not defined by R3 or vice versa. Once I chose to use 4 volts at the junction (which was mathematically derived not assumed) I did define the value of R3 at that moment only because I knew the resistance of the diode. I did not have to do the final calculation to know what that value was to figure out what R2 needed to be is all I was implying in my original post #44.
SG

You did NOT mathematically derive a junction voltage of 4 V. You determined that the maximum the junction voltage could be was 4.5 V. You then arbitrarily chose to use some random value that happened to be less than that and tried to then later pass it off as being "educationally calculated". Now you are trying to claim that it was mathematically derived?!

By your reasoning, I could just say that R2 = 1 MΩ. Or 100 kΩ. Or 5.6 kΩ, or 100 GΩ, or 47 kΩ and claim that it is "a" value for R2.

Finally, and I've resisted (no pun intended) pointing this out, but I just can't ignore your repeated use of the "resistance" of the diode any longer. Saying that the diode has a large signal resistance is meaningless and trying to claim that taking a some voltage and dividing by some current yields a resistance that has any meaning will only get you into trouble. By that reasoning, a 12 V car battery from which 1 mA is being drawn has a resistance of 10 kΩ, which of course is patently absurd. Resistance is the proportionality constant in a particular equation that governs the relationship between voltage and current in a device, namely Ohm's Law. If the device does not obey that law, at least to some reasonable approximation, then that equation has no meaning for that device and, likewise, neither does that proportionality constant.

 

Still hoping for a reply from the TS as to what was the expected answer to that problem on the quiz.
If it's a single definitive value for R2 than the value of R3 must have been stated but somehow on his version of the quiz in his words "I have a feeling the value of R3 was accidentally cropped out "
If it's multiple values for R2 then my analysis is correct.
If it's an expression/equation I stand corrected.
SG

Don't hold your breath for the TS. Their issue has been resolved and they've moved on to other things. Arguably a much saner thing to do that what we are doing.

 

You did NOT mathematically derive a junction voltage of 4 V. You determined that the maximum the junction voltage could be was 4.5 V. You then arbitrarily chose to use some random value that happened to be less than that and tried to then later pass it off as being "educationally calculated". Now you are trying to claim that it was mathematically derived?!

Yes I stand by my analysis.
Yes I determined the max voltage was 4.5 volts which was derived mathematically
Yes I had to select a voltage less than 4.5 volts to satisfy the requirements of the circuit based on the above statement.
Yes the above requires knowledge of ohm's law and math which equals education.
Educationally calculated means I didn't just pick some number out my you know what.

Finally, and I've resisted (no pun intended) pointing this out, but I just can't ignore your repeated use of the "resistance" of the diode any longer.

Give me a break. We are talking about a hypothetical problem here involving voltage,current and resistance. What's next you want to know the temperature of the diode, is it on a heatsink?
Finally I can't resist any longer. I believe you and others who disagree with my analysis can't think outside the box because you have over analyzed the problem. Kind of like can't see the forest for the trees.
SG