Output voltage in circuit with diodes

[Jahnavi]

Homework Statement

Homework Equations

The Attempt at a Solution

Since both D1 and D2 are forward biased for the positive half cycle for the voltage input , current will flow through them . Both the diodes offer very little resistance , but do they act like connecting wires when forward biased ?

I get option 3) i.e zero output because I feel potential drops across the two diodes will be equal . Am I correct ?

Since they are different diodes , could they offer small but different resistance to the current OR do we assume that both offer negligible resistance ?

 

[rude man]

Since both D1 and D2 are forward biased for the positive half cycle for the voltage input , current will flow through them . Both the diodes offer very little resistance , but do they act like connecting wires when forward biased ?

Yes

EDIT: I made a mistake, thinking this is a standard full-wave rectifier, but it isn't.
In a f/w rectifier, D1 and D3 (or D2 and D4) are reversed.

 

[cnh1995]

Am I correct ?

I think so.

When forward-biased, replace the diodes with closed switches and when reverse-biased, replace them with open switches.
In both the cases, you get V_o=0V.

 

[Jahnavi]

No.

Are you saying there will be a non zero output ?

 

[Jahnavi]

When forward-biased, replace the diodes with closed switches

My basic doubt is that will there be equal potential drops across D1 and D2 when they are forward biased considering they are not similar ( or are they ? With numbers just distinguishing them )

 

[berkeman]

Vo(t) will be zero, except for a certain initial condition that is not specified in the problem. Can you see the issue? Look carefully at everything that is labeled in the drawing...

 

[rude man]

Are you saying there will be a non zero output ?

No. The output is zero since the two current paths D1-D3 and D2-D4 cancel each other (assuming matched diodes).

 

[CWatters]

+1 Symmetry makes it zero volts.

 

[berkeman]

makes it zero volts.

Except for when?

(even when the diodes are ideal and symmetric, there is an issue...)

 

[berkeman]

Spoiler

The initial value Vo(0) is not given. If Vo(0) = 0, then it will remain zero. But if Vo(0) = 1V, it will decay to zero based on the value of the resistance...

 

[rude man]

Ooh yeah, there is a bigger problem! Namely, unless V₀ is very small (< 1V or so) some diodes are going to burn out and all will be damaged!

 

[berkeman]

Yeah, there's that too. Pretty strange way to define Vin(t), IMO.

 

[rude man]

Yeah, there's that too. Pretty strange way to define Vin(t), IMO.

Pretty strange circuit too! Wonder if they meant it to be a full-wave rectifier ...?

 

[Jahnavi]

No. The output is zero since the two current paths D1-D3 and D2-D4 cancel each other (assuming matched diodes).

Suppose first half of input voltage is positive , then all four diodes are forward biased. Current will divide between D1 and D2 . D1 current will flow through D4 and D2 will flow through D3 . Is this wrong ?

Please help me understand your reasoning . How is D1- D3 a current path ?

 

[berkeman]

Suppose first half of input voltage is positive , then all four diodes are forward biased. Current will divide between D1 and D2 . D1 current will flow through D4 and D2 will flow through D3 . Is this wrong ?

That is correct.

 

[Jahnavi]

That is correct.

But since D1 and D2 are different diodes , wouldn't the resistance offered by them and hence potential drops across them different .If that is true , there will be a potential difference across the middle resistor R ?

 

[berkeman]

But since D1 and D2 are different diodes , wouldn't the resistance offered by them and hence potential drops across them different .If that is true , there will be a potential difference across the middle resistor R ?

That is correct. So either the problem statement or the context of the problem (your professor's instructions and teachings in class) would need to tell you whether to use an ideal model for the diodes, or use a datasheet model (and which datasheet to use).

What have you been told to do for this problem?

And did you understand my comment about the strange definition of Vin(t)? Even if they are non-ideal 1N4148 diodes and you are told to use their datasheet to work the problem, if Vo(0) = 0V, it doesn't matter. Can you say why?

 

[Jahnavi]

This is a high school level problem so I am not sure whether the problem setter has some implicit assumptions in his mind .

I think they are implicitly assuming that all ideal diodes even if different offer negligible resistance ?

 

[berkeman]

This is a high school level problem so I am not sure whether the problem setter has some implicit assumptions in his mind .

I think they are implicitly assuming that all ideal diodes even if different offer negligible resistance ?

Yes, at that level, they are probably assuming ideal diodes (0V = Vf, and infinite reverse impedance).

Did you understand my issue with the Vin(t) definition?

 

[Jahnavi]

Did you understand my issue with the Vin(t) definition?

I need some time to think on this

Yes, at that level, they are probably assuming ideal diodes (0V = Vf, and infinite reverse impedance).

So if all the diodes are forward biased , can we treat them as connecting wires ?

But then where does the input voltage drop , I mean across which component ?

Is this situation similar to as if a battery is connected across a wire of negligible resistance ?

 

[berkeman]

So if all the diodes are forward biased , can we treat them as connecting wires ?

But then where does the input voltage drop , I mean across which component ?

Is this situation similar to as if a battery is connected across a wire of negligible resistance ?

You are asking good questions!

Yes, dealing with ideal diodes is a pain, because zero voltage drop across them is non-physical, and leads to strange situations in some circuits. IMO, it's better to just think of the Diode Equation (check Wikipedia) when thinking about diodes. That gives you about 0.6V for Vf across them, with just Is as the reverse leakage current.

But even with real diode models in this problem, as mentioned by @rude man any input waveform over about 2x 0.6V peak will be clipped, and deliver power to the diodes potentially resulting in their destruction.

 

[berkeman]

just think of the Diode Equation (check Wikipedia)

https://en.wikipedia.org/wiki/Shockley_diode_equation

 

[Jahnavi]

That gives you about 0.6V for Vf across them,

This is the potential barrier voltage . But when a voltage is applied across the diode , this barrier potential is nullified . Isn't it ? It is this external applied voltage which we call as the potential across the diode ?

 

[CWatters]

To me this looks like an obviously trick question. They want you to rush in, think it's a rectifier circuit and answer 1 when it's not. Answer 3 (zero) or 4 (none of these) are both valid answers depending on the assumptions you make about the diodes, the source etc.. So it's a poor question. I'd still answer zero.

 

[berkeman]

This is the potential barrier voltage . But when a voltage is applied across the diode , this barrier potential is nullified . Isn't it ? It is this external applied voltage which we call as the potential across the diode ?

Did you read the wikipedia page about the Diode Equation that I linked to in post #22?

 

[Jahnavi]

Thanks everyone for contributing in this thread .

 

[berkeman]

So if all the diodes are forward biased , can we treat them as connecting wires ?

But then where does the input voltage drop , I mean across which component ?

This circuit is pretty goofy, so I wouldn't spend too much time analyzing it. In most simple diode circuits, you can often start by assuming ideal diode behavior (short circuit in forward conduction, and open circuit when reverse-biased). But in this circuit, that would short out the voltage source if there were any voltage source. But also in this circuit, the Vin(t) is identically equal to zero anyway, so it doesn't really matter.