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!

 

[Mene]

Hi Roeb,

I see that you have correctly solved for the output voltage using the first method. However, when using Thevenin's method, you must take into account the effect of the ideal diode on the circuit. Since the diode has a voltage drop of 0 volts, it essentially acts as a short circuit when it is forward-biased. This means that the 10kΩ resistor in parallel with the diode will have no effect on the circuit.

To solve for the Thevenin equivalent circuit, you must replace the diode with a short circuit and calculate the voltage across the 10kΩ resistor. This will give you the Thevenin voltage, which in this case is 5 volts. Then, you can calculate the Thevenin resistance by considering the 20kΩ and 10kΩ resistors in parallel, giving a value of 6.67kΩ.

Using these values, you can then solve for the output voltage using the Thevenin equivalent circuit, which should give you the correct answer of 7.5 volts. I hope this helps and good luck with your homework!