According to the book:

http://img687.imageshack.us/img687/169/unt222copy.jpg

But when I do the loop I get a different equation and I don't know why. I'm using the passive sign convention and just writing my equation by the convention I use which is to write the term in the equation with whatever sign first appears around the loop for that term.

http://img291.imageshack.us/img291/9040/img0005bv.jpg

Thanks.

 

I might be missing the question, but it looks like if you rearrange your terms to match the book equation it would come out. The book solves for Vd and you solved for Vdo.

 

Sorry, that's precisely my question... why doesn't it come out?

If you rearrange my terms you get Vd = Vdo - IR (not what the book gets)

 

I think it's just the direction of your current.

 

They're basing the direction off of the source. Which would make it a positive Vdo in the model.

 

Thanks jlcstrat...

Anyone else have some input?

 

This blue rectangle represents the model of the real diode.
So voltage on the "real" diode is equal:
Vd=Vdo+I*rd
And KVL
Vdd = I*R +Vdo + I*rd

 

If you will make the first equation (Id=4.26) from your point of view, is it same with the book or your will be "Id= -4.26"? if it is negative then your equation will the same with the book.

from my point of view:

assigning polarity to R (rd actually) made your equation different from the book.

notice the direction of the assigned current flow (Id), this serve as my reference:

when the direction of the assigned current flow is along with your loop direction use (+IR).

when the direction of the assigned current flow is against with your loop direction use (-IR).

I use the term assigned current flow because in some cases your have to really assigned the direction of the current flow.

If you get a value of current (I) equal to negative value, this means that the direction of the current is in reverse.

I hope this can help....