# Need help - resistance

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#### Ventilo

##### Newbie level 4 I need help with the resistor R3, the two others have I calculated to:

R1 =Vref/I14 = 2800OΩ
R2 = Vo*10V / R1 = 5000Ω

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

http://images.elektroda.net/46_1192958728.jpg

#### RFCMOS

##### Full Member level 2 Hi

You know the current through the zener diode as well as the I4 current, so you can deduce (thanks to kirchoff's law) the current through R3.
As you know Vref, you can calculate R3 easily thanks to Ohm's law.
Correct me if I am wrong....
Hope this helps

#### Ventilo

##### Newbie level 4 I was thinking like this

(Vref / I4) = (5,6V / 6mA) = 933Ω

Perhaps that is wrong...

#### RFCMOS

##### Full Member level 2 Hi,

I would have think:
R3=(15-Vref)/(6mA+I14)....
Correct me if I am wrong

### Ventilo

points: 2

#### Ventilo

##### Newbie level 4 Now Im getting confused, you have a point but im not sure it´s the correct way to do it.

So what do you other say? Can someone confirme it?

#### the_edge

##### Full Member level 3 Well this is true...
"R3=(15-Vref)/(6mA+I14)...."

Current trough R3 is: I=Id+I14=6mA+2mA=8mA (1st Kirchoff's law)
Voltage on R3 is: V3=Vcc-Vref=15V-5.6V=9.4V (2nd Kirchoff's law)
Now that you know voltage and current using Ohms law you can calculate resistance: R3=V3/I=1175Ω

"R1 =Vref/I14 = 2800OΩ":OK
"R2 = Vo*10V / R1 = 5000Ω ":Can you explain how did you get this? I dont think this is OK.

R2=Vo/I4... ### Ventilo

points: 2

#### Ventilo

##### Newbie level 4 Thanks!

I have wright it wrong.

R2 = R14*V0 / Vref = 5k

R2 = (2800*10) / 5,6V = 5k

#### the_edge

##### Full Member level 3 Ventilo said:
Thanks!

I have wright it wrong.

R2 = R14*V0 / Vref = 5k

R2 = (2800*10) / 5,6V = 5k

Still I dont understand, where is R14??? Does some part of schematic missing?

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