# Sample exam problems (diodes)

1. Sep 29, 2013

### asdf12312

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

2. Relevant equations

3. The attempt at a solution
My instructor gave us some sample problems he said might be on the exam, I tried working them out but I was having difficulty with 3 of them:

For the 1st problem, when there is positive value for voltage, I got the 1st diode as off (open circuit) and the 2nd diode as on (short circuit). This would mean Vout is always 0 for positive voltage. When the voltage becomes negative, the 1st diode becomes a short and the 2nd becomes open circuit. So 20||20=10 and Vo=(0.5)Vin using voltage division. Graph for Vout would be 0 when the 5V peak sine wave is positive curve, and a curve of half the amplitude when it is a negative curve (min=-2.5).

For the 2nd problem, I was used to working with 1K ohm resistors so the 500 ohm kinda confused me. But if the value of resistor doesn't matter, I find that diode starts conducting when Vi=2.5 and conducts fully when Vi=2.7 (+2V). This would mean graph is a linear line (Vo=Vi) until Vi=2.7 when it becomes a horizontal line. For the -3V, diode starts conducting when Vi=-2.5 and conducts fully when Vi=-2.3. Graph would look similar to the +2V one.

For the 3rd one, I had a zener diode for another problem so I am assuming there is positive value at the negative end. Also VZ=VZ0 because rz is very small. For +20V, I got current flowing through diodes B, C, D. the voltage is 20-2Vd-Vz=10.4V. For +5V, I got a negative voltage after the voltage drop across zener diode so I found no current would flow through the loop. For -5V, I got a positive voltage after the voltage gain in zener diode, so zero current again. For the last one (-20V), I found current flowing through doides A, C, E if I assume zener diodes can conduct voltage in both ways. Then the voltage is -20+Vz+2Vd=-10.4V.

2. Sep 29, 2013

### rude man

(a) is A-OK.

For (b), the resistance certainly does matter.
When Vin = 0 what is the current flowing thru the lower diode? You have to try both Vd = 0.5V and Vd = 0.7V and pick the right one.

Then, run Vi from -5V to +5V to get the output. What is Vout when Vin = -1.9V? When Vin = -2.1V?

Does the top diode ever turn on?

You gt (c) right also.

Why would the zener not conduct current the same way for a negative 20V input as for a positive 20V input? The zener current is in the same direction for both +20 and - 20V inputs.

3. Sep 29, 2013

### asdf12312

(b) I don't really understand this type of circuit, usually with the previous problems i had only 1 diode and voltage battery. When vi=0, the current through lower diode would be (0-Vd+3) diovided by resistance. I'm not really sure what you mean by choosing Vd=0.5 or 0.7 though. I get I=5 or 4.6mA depending on this.

When Vi=-1.9 and -2.1, current has to flow through bottom diode only. I think vout would be -2.3V for both because that's when the diode finishes conducting.

am I doing this right so far? Maybe the vout graph has to include both values then, for both the +2 and -3V sources. So I should graph both for vi=-5 to +5V. But I think my only problem is what vout would be at Vin=0V, -2.3V or just 0V.

also, for (c), would I be right in thinking that if I graphed VR, it would look like the sin wave input except with amplitude of 10.4V? And V0 would be a constant value instead of a curve, +/- 9.6V depending on the sine value.

Last edited: Sep 29, 2013
4. Sep 29, 2013

### rude man

If the input is -1.9V, and the voltage drop across the bottom diode is 0.5V, what would be the current?

And what about the top diode? What input voltage would turn it on?

5. Sep 29, 2013

### rude man

Remember what the problem stated: if i < 1 mA, Vd = 0.5 and if i > 1 mA then Vd = 0.7v. So which do you think you should choose?

6. Sep 29, 2013

### asdf12312

I see now. If input is -1.9V, the current through the bottom diode is (-1.9-Vd+3)/500=1.2mA. But this is >1mA..so I should choose the bottom diode Vd=0.7V right? For the top diode using a similar problem I was assigned I found it starts conducting at 2.5V.

7. Sep 30, 2013

### rude man

For the top diode: what's the highest voltage the anode can assume?