# Homework Help: Determining two diodes' currents

1. Jun 18, 2014

### JasonHathaway

1. The problem statement, all variables and given/known data

Assuming that D1 and D2 are ideals, determine ID1 and ID2 according to the table below.

2. Relevant equations

Circuit Analyzing methods (KVL, etc...)

3. The attempt at a solution

I've confirmed my solutions for the first three cases with final answers of the book I use and also with Multisim.

But for the fourth, while replacing the diodes with "on-switches", I used superposition to calculate the currents. I've ended up with ID1=ID1'+ID1''=12+9=21 mA, and ID2=-9 mA.

My answer is correct for ID2, but that not the case for ID1 (The answer should be 12 mA). Looks like the effect of the 9V source on ID1 was ignored. Why is that?

2. Jun 18, 2014

### Simon Bridge

Because there is no reason for it to have an effect - the diodes are basically replaced with short circuits.
The D1 short basically isolates the two halves from each other.

3. Jun 19, 2014

### JasonHathaway

My circuit analyzing again :S

Thanks :)

4. Jun 19, 2014

### Staff: Mentor

This is indeed a weird question. For the circuit shown, there are not 4 possible states, there is only one! For the options to make sense, either the "diodes" need to be SCRs or something similar (with gating arrangements not shown), or each diode needs an associated series switch---but in the schematic this is clearly missing.

I had no idea what to make of the exercise, not knowing what could be meant by a diode being off even though it may be forward-biased.

Now I can see what was intended, but this abomination reflects badly on the examiner. I'd grade him Epic Fail!

5. Jun 19, 2014

### Simon Bridge

The exercise is on the "assumed states" method for analyzing diode circuits.

6. Jun 20, 2014

### Staff: Mentor

Last edited: Jun 20, 2014
7. Jun 20, 2014

### phinds

My thoughts exactly