allaboutcircuits worksheets correct?

Thread Starter

pmichaud

Joined Jan 25, 2017
5
It there a posted erratum for the worksheets, or can I assume all the answers are correct?
When I get different answers than what is posted, it pisses me off more than it should because I don't know who is correct and usually can't determine which. I've skipped a handful, assuming the online answer is wrong after spending 15 minutes trying to figure out what I did wrong. Sometimes it's just a matter of decimal point position, but which is correct? And 1.0 is certainly not the same as 10.0.

It's an awesome online resource, but not showing the answers worked out has turned into a huge waste of my time.
 

Thread Starter

pmichaud

Joined Jan 25, 2017
5
This is a general question about all of the worksheets. But currently, I'm on this one
https://www.allaboutcircuits.com/worksheets/time-constant-calculations/

#8 - I assume I'm using the wrong values. What are the actually values (8k1 and 2u7 mean what? 8000 and .000002 don't give Vc = -13.08v)
#15 - A precision issue? I get 10.886mA not 109.11mA
#16 - I guess, I'm doing it completely wrong? I have a time constant T=0.0594, so for 30ms Vc=31*e^(-.03/.0594) = 18.7 not 12.29
 

Jony130

Joined Feb 17, 2009
5,181
Q8

8k1 = 8.1kΩ = 8100Ω

2u7 = 2.7μF

The time constant is t = 8.1kΩ*2.7μF = 21.87ms so after T = 45ms we have:

Vc = -15V (1 - 1/e^(45/21.9)) = -13.0782126 ≈ -13.08V

And the current is

I = (15V - 13.08V)/8.1kΩ = 237μA

Next question

Q15

L = 250 mH and R = 100Ω the time constant is L/R = 2.5ms

I = 12V/100Ω *(1 - 1/e^(6/2.5)) = 0.109113846A = 109.11mA


#16 - I guess, I'm doing it completely wrong? I have a time constant T=0.0594, so for 30ms Vc=31*e^(-.03/.0594) = 18.7 not 12.29
The time constant is R*C = 27kΩ*2.2μF = 59.4ms

Vc = 31V (1 - 1/e^(30/59.4)) = 12.29V


As you can see the answers are good.
 

WBahn

Joined Mar 31, 2012
26,143
It there a posted erratum for the worksheets, or can I assume all the answers are correct?
When I get different answers than what is posted, it pisses me off more than it should because I don't know who is correct and usually can't determine which. I've skipped a handful, assuming the online answer is wrong after spending 15 minutes trying to figure out what I did wrong. Sometimes it's just a matter of decimal point position, but which is correct? And 1.0 is certainly not the same as 10.0.

It's an awesome online resource, but not showing the answers worked out has turned into a huge waste of my time.
While frustrating, having to deal with wrong answers can be a very good learning experience.

When you get an answer that doesn't agree with the book, first review your work to see if you find a mistake. This will quickly teach you the value of keeping your work neat and organized. If you don't find an error in your work, then set out to prove that your answer is correct. This is something you should always do, anyway, whether or not there is an answer available and before you look at one if there is. Even in the real world, the correctness of the answer to most engineering problems can, and should, be verified based on the answer itself.

Once you've verified that your answer is correct, the next step is to ask whether or not it's possible you misinterpreted the question. Look at it carefully and identify where you might have made assumptions about what was asked. Rework the problem based on any alternate interpretations you can come up with and see if any of those match the answer. If one does, ask if the associated interpretation is at least as reasonable as the one you made.

As some point down this path, you will come to the conclusion that the author's answer is wrong, but you will be in a position to defend that assertion by being able to prove that your answer is correct.
 
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