Did the problem actually say that the answer needed to be a actual value?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
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.It turns a simple current-limiting resistor calculation into something a lot more challenging. That's all it does!
“Seemed like”? If he wanted an exact value, he would have given you a problem that could be solved for an exact value.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
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.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
Note that the problem statement does NOT require an actual value.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
No it is not possible period.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 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
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.If R1 and R3 are inaccessible, how do we know their values?
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: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
I think you need to patch that up to reflect order of operations. Otherwise you have a resistance minus a voltage, minus a voltage.Your answer is:
R2 = R1.(250uA.R3+Vd)/ Vs-Vd-250uA.(R3+R1)
Think parallel circuit.... they have worded the original question to confuse.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.
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You can use MimeTex to do equations -- whether it qualifies as "easy" is in the eye of the beholder.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)
by Jake Hertz
by Aaron Carman
by Jake Hertz