Solve Thevenin & Diodes Homework with Ideal Diode

[roeb]

Homework Statement

http://img444.imageshack.us/img444/5601/scan0001my.th.jpg

Hey, I'm trying to solve this problem using thevenin's method. As you can see in method 1 I was able to solve for the correct output voltage of 7.5 volts. When I try Thevenin's method I get 4 volts. I have a feeling that I'm missing something simple. Does anyone see what I've done wrong? Note that this is an ideal diode so there isn't a voltage drop over the diode.

Thanks,
roeb

Homework Equations

The Attempt at a Solution

 

[Adjuster]

roeb,

You don't have the correct value for the Thevenin equivalent resistance. Remember that to calculate this, after removing the load, replace voltage sources with wires (and if there are any current sources, replace them with breaks).

With the 15V source in your problem replaced with a wire, what kind of network do the two resistors present? I suspect that, because the resistors are in series to form a potential divider, you have seen them as being in series for calculating the Thevenin resistance. Not so!