# Voltage Divider Rule question

#### Xufyan

Joined Aug 3, 2010
114
Please look at the attached circuit ,

According to Voltage divider rule ,

V1 should be equals to,

10volts x (33k + 68k) / ( 33k + 68k + 100k ) = 5.02volts .

but the circuit says its 2.812 volts

and V2 equals to,

10volts x (68k) / ( 33k + 68k + 100k ) = 3.38volts .

but according to the circuit its 1.641 volts,
please explain where i am wrong

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#### t_n_k

Joined Mar 6, 2009
5,455
Current will flow in the two 47k resistors which will upset the divider relationship.

#### Tealc

Joined Jun 30, 2011
140
I'm completely new to this but simulating the initial divider circuit in Multisim with just the 100k, 33k and the 68k give the results you have calculated but as soon as I add in the 47k and additional 100k the voltage on my meters changed and when adding the op-amp in they changed again to the figures you found in your simulation.

As a guess I would say that for the bottom meter reading the 47k and 100k are working in parallel with the 68k to change the voltage divider values.

#### Xufyan

Joined Aug 3, 2010
114
Current will flow in the two 47k resistors which will upset the divider relationship.
can you please drive the equation, how 47k resistors effect the voltage ?

#### t_n_k

Joined Mar 6, 2009
5,455
The attached analysis may help with understanding why the values aren't what you expect.

The difference amplifier effectively loads the voltage divider.

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#### t_n_k

Joined Mar 6, 2009
5,455
The image didn't show that well - I'll try again

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#### Xufyan

Joined Aug 3, 2010
114
thanks alot !!!! Great Explanation !!! Awesome !!!!

#### iONic

Joined Nov 16, 2007
1,654
Current will flow in the two 47k resistors which will upset the divider relationship.
Why wouldn't the 100K in the lower leg play a roll as well, given it connects to ground??

#### t_n_k

Joined Mar 6, 2009
5,455
Why wouldn't the 100K in the lower leg play a roll as well, given it connects to ground??
It does - included in my analysis.

#### iONic

Joined Nov 16, 2007
1,654
It does - included in my analysis.
Originally you didn't mention it, so I skipped the analysis, I'll read it now!

Thanks