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frixis • Feb 2, 2009

# Diodes questions, assumptions required, urgent help

the Question
this is actually an example in my book (sedra and smith, 5th ed) which i fail to understand completely!!!
the question is :
Assuming the diodes to be ideal find V and I in the circuits.
I obviously went through the example but it totally escapes me. They've first assumed that D1 is conducting and proceeded from there. i'm lost.
i tried to assume that d1 is not conducting and proceeded from there but i didnt know how to check my answer at all.
please help. i'm soooooooooooooo lost and its like a bug in my system. i think i understand basic diodes but this assumption stuff is really confusing.
thankyou
Norm • Feb 3, 2009
First of all, it is better if you give the chapter no. and the example number because a lot of people probably own a copy of S&S here.

As for the assumption part, it simply says the diodes in the example behave exactly as the ideal diode conduction graph predicts it will. It will start conducting after 0.6V and it will immediately cut-off any conductance below that point. Just calculate how much voltage drop across each diode the circuit generates and you will determine which ones are open and which ones are closed. Then you can determine how much voltage is passing through a resistor, capacitor or inductor. I'm guessing it's a simple circuit question.
hey frixis you didnt got your ans,man,havenbt you posted in other forums too...
frixis • Feb 4, 2009
yeah i got the answer from my teacher finally.
he said:
first spot which diode has the highest potential across it then assume that the other one does not exist and calculate the voltage at the node.
so for a D2 has the highest voltage
and then u assume d2 is on. and then u calculate the voltage at the node where d1 is connected. that will tell me whether or not d1 is on. if it is then i do it all again if it isnt then i'm good.