# Voltage across diode

1. Sep 15, 2013

### anhnha

I want to ask about the bold part. In the case, what is the voltage across the diode pair, zero or voltage source (Vin) when it is in positive half?

2. Sep 15, 2013

### meBigGuy

I=E/R and division by zero is undefined. That's the cop-out that I'd use.

Basically, you are asking what happens when you short an ideal voltage source. No need for a diode.

How about an ideal current source in series with an ideal voltage source?

The answer is : sorry, does not compute! Qualifies as a paradox.

3. Sep 15, 2013

### anhnha

Thank you.
Yup.
Consider a circuit including an ideal voltage source, V and an ideal wire is connected across two ends of the source.
Let's call the voltage between two ends of the ideal wire v.
$$v = \lim_{R \rightarrow 0} i.R = \lim_{R \rightarrow 0} \frac{V}{R} . R = V$$

(by apply L'Hospital's Rule for 0/0)
Can you tell me why this is a paradox?

Last edited: Sep 15, 2013
4. Sep 15, 2013

### meBigGuy

No idea how I came to that conclusion. I should go back and edit it out. LOL

Applying limits is the only valid approach for dealing with infinity, but I can't help you with whether this is a valid application of L'Hospital's Rule.

5. Sep 15, 2013

### meBigGuy

Actually, thinking about it a bit, I think you nailed it. In the limit it makes sense.

6. Sep 21, 2013

### cabraham

A current source connected to a voltage source is no paradoxat all. One will deliver power to the other. Connect a 1 volt source to a 1 amp source in a loop. The relative polarity determines which which one delivers and which one receives power.
Claude

7. Sep 21, 2013

### meBigGuy

didn't I just say that I was wrong? Which doesn't seem to be something you are capable of.