Why Does Superposition Give Different Results in PSPICE?

[jesuslovesu]

Never mind, did superposition incorrectly.

Homework Statement

http://img258.imageshack.us/img258/9206/jesus2gn2.th.png

Find Vload using superposition and thevenin's equivalent

Homework Equations

The Attempt at a Solution

Well for network on the left of Rload, I found Vth = 5 V and Rth = 15.1 kohms
For the network on the right of Rload, I found Vth = 7.5 V and Rth = 8150 ohms.

So using superposition I would do:
1) 5 V * 10k / (10k+15.1k) = 1.99 V on Rload
2) 7.5 V * 10k / (10k + 8150) = 4.13 V
Adding those I get 6.12 V.

However, when I simulate this in PSPICE I get 4.33 V.
I also checked each side and I get 1.99 V on the left (when I disconnect the right) and I get 4.13 V on the right (when I disconnect the left).

What am I doing wrong, I can't even remotely see where that 4.33 V comes from.

 

[kamerling]

When you calculate the voltage on the load due to one voltage source, you can't just remove the others, but need to replace them with their internal resistance.

for 1) you'd get 5 v * (10K // 8150) / ((10K // 8150) + 15.1k)
(A // B) is the value of the resistance of A parallel with B, or AB/(A+B)

 

[Moetasim]

It seems like you have made a mistake in your calculations using superposition. Double check your equations and make sure you are taking into account the voltage divider rule correctly. Also, make sure you are using the correct values for Rth and Vth for each network when calculating the voltage across Rload. Additionally, it may be helpful to redraw the circuit and label all the nodes and components to make sure you are keeping track of everything accurately. If you are still getting different results, try asking a classmate or your instructor for help.