# Diode Problem

Discussion in 'Homework Help' started by MatthewPerry, Feb 20, 2011.

1. ### MatthewPerry Thread Starter New Member

Nov 29, 2010
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Hi. I've been trying to understand the solution to a practice problem in my book (it only provides the answer), but I cant seem get it. The problem gives the circuit in the attachment and asks the following: Find a value of R such that V0 is equal to 2.4 V. Assume that the diodes available have 0.7 V drop at 1 mA.

The solution is R = 139 ohms. I keep getting about 160. If someone could tell me how I should go about solving this it would be great! Thanks.

2. ### Jony130 AAC Fanatic!

Feb 17, 2009
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You forgot to attach the diagram.

Nov 29, 2010
3
0
Here it is.

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4. ### Jony130 AAC Fanatic!

Feb 17, 2009
3,993
1,116
Te get the right answer you need to use Shockley diode equation
$Id = Is * e^{\frac{Vd}{26mV}}$
First you need to find Is current, because you know Id = 1mA; Vd= 0.7V so
Is = 2.03fA
Now you want Vo = 2.4V but you have three diodes so Vd = 2.4V/3 = 0.8V
Id = 2.03fA * e^(0.8V/26mV) = 46.816mA
So we know Id current which causes 0.8V voltage drop across the diode.
And now we can find resistor R.
R = (10V - 2.4V) / 46.816mA = 162Ω

So I get the same answer as you.

MatthewPerry likes this.
5. ### Georacer Moderator

Nov 25, 2009
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I support Jony130's method of solution.

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