Voltage Across Diode Pair: Ideal Source | Vin Positive Half

[anhnha]

I am reading an old thread and because it is too old I can't post reply.
Here is the link: https://www.physicsforums.com/showthread.php?t=449044

If the diodes are ideal, there is no reverse leakage current, & no forward voltage drop. If the ac source is an ideal current source, the voltage across the diode pair is always zero, since 1 of the 2 diodes is forward biased.

If the ac source is an ideal voltage source, you get a paradox. A zero resistance perfect voltage source is loaded by a perfect diode with zero voltage drop, resulting in the current ramping up towards infinity.

Claude

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?

 

[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.

 

[anhnha]

Thank you.

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

Yup.
I have just thought about this. Don't know if it is correct or not.
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)

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

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

Can you tell me why this is a paradox?

 

[meBigGuy]

Can you tell me why this is a paradox?

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.

 

[meBigGuy]

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

 

[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

 

[meBigGuy]

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