# Calculate an unknown resistance

Discussion in 'Homework Help' started by Kango, Oct 20, 2007.

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1. ### Kango Thread Starter Active Member

Sep 22, 2007
30
0
I need help calculating R3 in the circuit. I have calculated R2 and R1 to the following:

R1 = 2800Ohm
R2 = 5000Ohm

Specifications:
Vref = 5,6V
I14 = 2mA
Vo = 0-10V
The electric current throw the diode = 6mA

The circuit.

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2. ### Ron H AAC Fanatic!

Apr 14, 2005
7,050
657
Hint: have you studied Kirchhoff's Current Law (KCL)?

3. ### Kango Thread Starter Active Member

Sep 22, 2007
30
0
Yes...

However I was thinking this

V = 15V
I = 6mA from the zenerdiode

R = V/I = 2500Ohm, can that be correct?

4. ### Ron H AAC Fanatic!

Apr 14, 2005
7,050
657
No. One end of R3 is at 15 volts, but what is the voltage on the other end?

5. ### Kango Thread Starter Active Member

Sep 22, 2007
30
0
On the other end it is 5,6V.

6. ### Ron H AAC Fanatic!

Apr 14, 2005
7,050
657
OK, so what is the voltage across the resistor?

7. ### Kango Thread Starter Active Member

Sep 22, 2007
30
0
Now im getting tired of you, look at the picture, if you don´t wont to help me, then stop ask all this question!

I want a fast answer. I have no time playing this game of yours, the reason im asking here is because I dont now the answer. If you dont like to help me then you can stop thrasing this thread. I dont want this to take a week, this is just a small question that I had.

8. ### Ron H AAC Fanatic!

Apr 14, 2005
7,050
657
I thought this was homework. Giving you the answer is not going to help you. I was trying to lead you through the problem-solving process. Why don't you take a long walk off a short pier.

9. ### beenthere Retired Moderator

Apr 20, 2004
15,815
283
This thread has become unproductive and a waste. I am goin gto close it from further postings.

A notice to the OP - it is your responsibility to do the work. We - as the AllAboutCircuits community - are willing to help, but only if you have done some work. This is not an answer on demand site. You may either take this to heart or choose not to make postings in the future.