# Diode Problem to find out Current

Discussion in 'Homework Help' started by Abhinavrajan, Aug 9, 2016.

1. ### Abhinavrajan Thread Starter Member

Aug 7, 2016
81
1
I am unable the solve this problem.
Could someone help to find out which diode is ON and which is OFF and tell me how to find out ?
I am unable to write the KVL.

And help to find the current "I"

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2. ### WBahn Moderator

Mar 31, 2012
20,219
5,744
As in your other diode thread (and in which you showed exactly zero effort), we are going to ask you to show YOUR best effort to attempt to work YOUR homework problems. This is NOT the Homework Done For You forum.

We need to see YOUR attempt so that we can tailor our assistance to match where YOU are having problems.

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3. ### RBR1317 Active Member

Nov 13, 2010
397
77
Before one can calculate the current it must be decided which of the diodes are conducting. Since these are ideal diodes they can be considered as switches. Look at the voltage at the junction of the two resistors. Close one diode switch and open the other. What is the voltage at the junction? Would it be enough to reverse-bias the open diode? Then do the opposite: open the first diode switch and close the other. What is the voltage at the junction then? Would it be enough to reverse-bias the now open diode? If neither diode is able to generate sufficient voltage at the resistor junction to reverse-bias the other diode, then both diodes must be conducting. So decide which diode(s) are conducting, replace them in the circuit with a closed or open switch as appropriate, and calculate the current in the circuit.

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4. ### Abhinavrajan Thread Starter Member

Aug 7, 2016
81
1

I am not asking to solve the problem.
I am just unclear with the KVL for this loop.
I just need the assistance i.e. How to approach the problem. That's all.

In my previous thread involving the simple diode problem, I got the answer as 5.5 long back. It was due to some grading problem with the site, It was wrongly graded. Then, I chose to post in the forum where I can get help after a day.

5. ### crutschow Expert

Mar 14, 2008
16,505
4,439
You first solve it assuming the left diode off and the right diode is on.
Then you solve it assuming the left diode is on and the right diode is off.
Both calculations are very simple and can be done by inspection.
Now you look at the voltages generated for each solution and see which diode(s) is actually forward biased and which (if any) is reverse biased.

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6. ### WBahn Moderator

Mar 31, 2012
20,219
5,744
As others have already suggested, you analyze diode problems such as this by setting up multiple slightly modified problems each with assignments regarding which diodes are one and which diodes are off. With only two diodes, there are only four possible sets of conditions for the diode states.

There are several approaches you can use.

One is to simply analyze each of the possible conditions, replacing an ON diode with a short circuit (replace it with a wire) and an OFF diode with an open circuit (just remove it). After each analysis consider if the results are consistent with the choices made for that condition, namely that no current is flowing in the reverse direction through any of the ON diodes and the voltage across all of the OFF diodes is negative. As soon as you have found a circuit that satisfies these requirements for all of the diodes, you have found the solution and can stop.

With a bit of practice you will be able to estimate which diodes are ON and which are OFF pretty accurately. So take your best guess and run the analysis. If any of the choices are wrong, then run the analysis again switching your assumptions for the diodes that were wrong. Keep doing this until you get the right solution.

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