Problem

Annu007

Joined Jun 12, 2006
11
Hello,
Can anybody show me how to solve this problem (Value of R2)?

kubeek

Joined Sep 20, 2005
5,587
Best solution for me is using Kirchhoff´s law:
1) Sum of currents in one joint equals 0
2) Sum of voltages in one loop (don´t know the right term) is equal to zero.

Look at www.allaboutcircuits.com for explanation

Dave

Joined Nov 17, 2003
6,960

hgmjr

Joined Jan 28, 2005
9,029
Once you have reviewed the material contained in the link that Dave has posted, then give the calculation a whirl and then post your calculations and your results. We will be happy to provide you a bit of guidance if needed.

hgmjr

Annu007

Joined Jun 12, 2006
11
I came up with 13K for R2.
Vo = 0.25ma * 5K +0.7V
Vo = 1.95V
I1= I2 + ID
I1 = 12V/30K =0.4mA
ID = 0.25mA (Given)
I2 = 0.4-0.25 =0.15mA
V0 = 1.95V
R2 = 1.95V/0.15mA = 13K

richbrune

Joined Oct 28, 2005
106
The part about 12v/30k doesn't look right; the voltage drop across the 30k can't be all of the 12v, it's got to be something less than that, seems like about 9 or 10 volts, because you've got to subtract the voltage drops caused by all of the 3 remaining components. I got 40678, but I don't have time to check it before I leave---Sorry, Rich

Annu007

Joined Jun 12, 2006
11
Vo = 12-1.95 = 10.05V

I1 = 10.05/30K = .335mA

I2 = 0.335-.25 = .85mA

R2 = 1.95/0.85 = 22.94K

Syed

Joined Dec 5, 2005
6
Vo = 12-1.95 = 10.05V

I1 = 10.05/30K = .335mA

I2 = 0.335-.25 = .85mA

R2 = 1.95/0.85 = 22.94K
Right..... except 0.335-0.25=0.085 and 1.95/0.85=2.2941 so 1.95/0.085=22.941

hgmjr

Joined Jan 28, 2005
9,029
Not a bad effort, annu007.

I used Millman's Theorem and arrived as 22.941K....

hgmjr