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Ok, so I have the following circuit: enter image description here

click here for simulation

The source voltage is 5V.

The voltage measured between A and B is 2.44V, which differs from what I get by calculating that same voltage - 2.55. Here is how I am calculating it.

R1=1000

R2=1/(1/1.2*1000+1/4.7*1000) //adding voltages of parallel resistors using the 1/Rt = 1/R1+1/R2 formula

Rtotal=R1+R2

Vab=R1/Rtotal*5V

Vab=2.55 //calculated result using MS Mathematics because I am a lazy piece of shit

As you can see there is a difference of 0.11V. Is that normal or is there another reason as to why it happens? Am I doing something wrong? Is the simulator bugged, maybe?

UPDATE: Here is how I get the exact same value using excel: enter image description here

Gonna pick up a pen and paper and see what I get myself.

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  • \$\begingroup\$ my calcs are 2.44367V what exactly did you do \$\endgroup\$ – JonRB Sep 16 '15 at 21:44
  • \$\begingroup\$ @JonRB, see updated question \$\endgroup\$ – Phoenix Sep 16 '15 at 22:03
  • \$\begingroup\$ What did you get for R2? I get 955.96 Ohms, resulting in 2.4437 volts. You should show your intermediate results to aid in troubleshhoting. \$\endgroup\$ – Peter Bennett Sep 16 '15 at 22:05
  • \$\begingroup\$ @PeterBennett, I just updated the question with more details :) \$\endgroup\$ – Phoenix Sep 16 '15 at 22:06
  • \$\begingroup\$ The formula in line 10 is wrong. It formula should be =(B7/B8) * B9. Line 8 is the result of B1+B7, not B1 + B6 as the table shows. \$\endgroup\$ – Peter Bennett Sep 16 '15 at 22:11
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You are making a mistake during the calculation of the voltage divider, you need (R2/Rtotal)*5V instead of (R1/Rtotal)*5V, since the Vab is across R2 (Rparallel).

enter image description here

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  • \$\begingroup\$ I see. Can you please explain why it needs to be like that? I have trouble wrapping my head around it. It just doesn't seem to make sense to me. \$\endgroup\$ – Phoenix Sep 16 '15 at 22:28
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    \$\begingroup\$ In a voltage divider, the voltage divides across the resistors in proportion to their resistances. To make the situation easier to visualize, lets replace your parallel resistors with a 2K resistor. Then the total resistance is 3K. The voltage across the 1K resistor will be 1/3 of the supply voltage, and across the 2K resistor we will have 2/3 of the supply voltage. \$\endgroup\$ – Peter Bennett Sep 16 '15 at 22:56

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