Using Ideal and Constant Voltage Drop Models

[NBaumbick]

Homework Statement

I am practicing diode circuit analysis for an ideal diode model and constant voltage drop model.I have an initial circuit shown here:

Which I then redraw to look like this:

I want to make sure my process in redrawing this was correct. When I attempt to get current through the two resistors, if I assume the diode is on in the ideal instance, I obtain a value for I1 but wouldn't that end up short circuiting the path to the second resistor giving a result of 0Amps for I2?

 

[berkeman]

Homework Statement

I am practicing diode circuit analysis for an ideal diode model and constant voltage drop model.I have an initial circuit shown here:
View attachment 212044
Which I then redraw to look like this:
View attachment 212045
I want to make sure my process in redrawing this was correct. When I attempt to get current through the two resistors, if I assume the diode is on in the ideal instance, I obtain a value for I1 but wouldn't that end up short circuiting the path to the second resistor giving a result of 0Amps for I2?

Welcome to the pf.

I didn't follow your re-draw part, but basically the diode will clamp/limit the voltage cross its parallel resistor to whatever you are given for Vf of the diode (0.6V? 0.7V?)...

 

[NBaumbick]

Welcome to the pf.

I didn't follow your re-draw part, but basically the diode will clamp/limit the voltage cross its parallel resistor to whatever you are given for Vf of the diode (0.6V? 0.7V?)...

I'll use 0.7V for the constant voltage drop method but I have to be able to analyze a circuit like this using the ideal diode model as well which is where I am having trouble.

 

[berkeman]

You have chosen 0.7V for your initial ideal diode equations. Vf of a diode depends on If for a real diode.

I guess the super-ideal diode situation assumes 0V=Vf. Can you show your work for that assumption?

 

[NBaumbick]

I can try but it is basically still the picture representation. If I assume the diode to be on it is replaced with a short circuit and considering both paths are going to ground, doesn't this basically make a jumper wire effect?

 

[NBaumbick]

I can try but it is basically still the picture representation. If I assume the diode to be on it is replaced with a short circuit and considering both paths are going to ground, doesn't this basically make a jumper wire effect?

 

[berkeman]

Yes, if the diode Vf is 0V, then the one resistor is shrted and you are left with only one resistor.

 

[NBaumbick]

Ok, it seemed suspicious that a practice problem should want the current when the resistor is shorted. Thank you for affirming.

 

[berkeman]

Welcome. And in the more practical circuit analysis problem, you would use the Vf versus If curve published in the diode datasheet to write the equation to figure out the currents and voltages. Using the Vf=0V ideal diode assumption just makes that solution easier to get to.