# Resistor bridge calculations ?

Discussion in 'Homework Help' started by curry87, Dec 22, 2010.

1. ### curry87 Thread Starter Member

May 30, 2010
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0
Need some help to solve the following resistor problems.
First for resistor map 1(first figure)

How do i calculate the current going though r5 ?
So far i know the voltage between:
vr13 is 5.5v
vr24 is 1.7v

For resistor map 2 (second figure)
What steps are reqired to calculate the current going though r5 or r6 or r7 and the equivalent resistance?

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Feb 17, 2009
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3. ### Jony130 AAC Fanatic!

Feb 17, 2009
3,995
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And of course you could use nodal analysis.

(Vcc - Va) /R1 = (Va - Vb) / R5 + Va/ R3 (1)

(Vcc - Vb) / R2 + (Va - Vb) / R5 - Vb/ R4 = 0 (2)

IR5 = (Va - VB)/ R5

IR5 = 393.30633uA

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4. ### curry87 Thread Starter Member

May 30, 2010
101
0
A little lost in the calculations for va and vb on figure 1 can you please break them down and explain exactly how to calculate them step by step my electrical theory knowledge is limited?

If i find the voltage at node a and node b using the voltage divider calculation (r2/r1+r2)*vcc then subtracting b from a and dividing that by r5 why doesnt it come up with the same answer as using nodal analysis to find current though R5 ?

5. ### Jony130 AAC Fanatic!

Feb 17, 2009
3,995
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OK, let's try something simpler Thévenin's theorem

http://en.wikipedia.org/wiki/Thévenin's_theorem

So first step is to find Vth ( Va - Vb ) without R5

Va = Vcc * R3 / (R1 + R3) = 5.53846154V

Vb = Vcc * R4 / (R2 + R4) = 1.7027027V

So Vth = ( Va - Vb) = 3.83575884V

The next step is to find Rth looking into Va; Vb

Rth = R1||R3 + R2||R4 = 3.07692308KΩ + 5.67567568KΩ = 8.75259876KΩ

So finally we end with this simple diagram

IR5 = Vth/(Rth + R5) = 392uA

And if you add R6 and R7 there is now problem becaues those resistor are connect parallel to R1 and R7 parallel to R4.

So when finding Vht then
R1 = R1||R6
R4 = R4||R7

And of course you can directly determine IR5 from nodal analysis

I use Mathematica software to do all the math for me.

6. ### curry87 Thread Starter Member

May 30, 2010
101
0
How do you find Ir1,Ir2 when r5 is connected ive tried (vcc-va)/r1 and (vcc-vb)/r2 but doesn't give me the values that the simulation comes up with ?

Is IT= ir1 + ir2 or ir3 +ir4 ?

7. ### Jony130 AAC Fanatic!

Feb 17, 2009
3,995
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Witch equation you use to find Va and Vb ?

8. ### curry87 Thread Starter Member

May 30, 2010
101
0
The voltage divider eq for r1,r3 then r2,r4 .

9. ### Jony130 AAC Fanatic!

Feb 17, 2009
3,995
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You must understand that this equations

Va = Vcc * R3 / (R1 + R3) = 5.53846154V

Vb = Vcc * R4 / (R2 + R4) = 1.7027027V

Holds only when R5 is disconnect form the circuit.
And I use this equations only for find Thevenin equivalent circuit (Vth).
Becaues there is no simply way to find IR5 current.

And I give you equations for Va, Vb and I5 when R5 is connect to the circuit.
http://images.elektroda.net/54_1293045055.png

And of course you could use Δ-Y transform to find Va and Vb

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