Problem understanding an answer given to a question in 'Complete Electronics Self-Teaching Guide'.

[Kain6622]

Evening everyone,

I'm working my way through 'Complete Electronics Self-Teaching Guide' and so far so good with understanding the material in it, The book has a review (Questions with answers) and pre-test approach to learning and I am working through the review of the second chapter on diodes by trying to answer the review questions myself first before comparing them to the answers given.

One of the questions (Q22) is not making sense as it makes an assumption about the current flowing through the circuit (1 Amp) and I don't understand where they got that value from, question shown below:



From the information given I can't see how they can say "Because 1 Ampere of current is flowing" and I'm worried i'm missing something here?

From my calculation of R = Vr / I and the given information from the circuit, it should work out as R = 100 / 0.02 = 5kΩ.

Please can someone help me understand, Have I missed something or is this maybe a typo in the book?

Kind Regards
Dave

 

[kpatz]

I don't get where they get 1A from, when the question specifically says a max of 20 mA.

I think it's a typo. Maybe the question was originally worded 1A and changed to 20 mA and the answer wasn't modified accordingly. I would say the correct answer is 5K ohms as well.

 

[Martaine2005]

I would have to agree. 5K.
If the diode can safely pass 20mA, then that is a red herring. It might as well not be there.
So the calculation is 100/0.02.
Unless we are missing something?

Martin

 

[WHONOES]

5K looks reasonable.

 

[Harald Kapp]

Nothing missing, sloppy work.
100 Ω @ 1 A
or
5 kΩ @ 20 mA

 

[Nanren888]

Here's the original which asks about the same at 1Amp and answers about 1 amp

https://www.oreilly.com/library/vie...elf-teaching/9781118282328/c02_level1_2.xhtml
Same figure number.
Seems to support the theory of a change in the question, but not in the answer text.

 

[Kain6622]

Thank you all for your response, that clears that up. Got slightly worried from all the talk in the chapter of assumptions that can be made under certain conditions of the diode so wanted to double check to be sure that I wasn't going mad. Thank you again.

Regards
Dave