Understanding Basic Diode Questions

[needhelp171]

My understanding is that a diode conducts current if the voltage at its anode is larger than the voltage at its cathode. I don't understand this.

1) Once the diode begins conducting current, the voltages at the anode and the cathode are identical (the diode serves as an open wire). Therefore, by its own rule, the diode ought to stop conducting current.

2) I heard that the voltage at the anode must be at least 0.7 volts higher than the voltage at the cathode. Is that true, even for an ideal diode? I know the constant 0.7 may differ, but I'm just looking for a general principle.

 

[cnh1995]

1) Once the diode begins conducting current, the voltages at the anode and the cathode are identical (the diode serves as an open wire). Therefore, by its own rule, the diode ought to stop conducting current.

An ideal diode acts as a short circuit when it conducts, hence there is no voltage drop across it. It is same as an ideal wire.

2) I heard that the voltage at the anode must be at least 0.7 volts higher than the voltage at the cathode. Is that true, even for an ideal diode? I know the constant 0.7 may differ, but I'm just looking for a general principle.

This is not true for an ideal diode.

 

[needhelp171]

Oops -- meant to say short wire, not open wire!

And yeah...if it is a short wire, then the voltage before and after the diode is exactly the same. But the physical properties of a diode demand that it only conduct current if the voltage before the diode is greater than the voltage after -- not equal.

In that sense, then when the voltage at the anode exceeds the voltage at the cathode, shouldn't the diode frantically alternate between conducting current and not conducting current, and have undefined behavior?

 

[cnh1995]

Ideal diode behavior depends on the direction of current. If the current is from A to K, it conducts, otherwise it doesn't. It's like a valve.

But the physical properties of a diode demand that it only conduct current if the voltage before the diode is greater than the voltage after

That is for practical diode.

 

[LvW]

The famous exponential equation from W. Shockley answers everything - without any interpretation or simplification. If you want - you can simply calculate the resistances (static or dynamic) for any forward voltage and/or any current. So - what is the problem?

 

[Merlin3189]

Accepting that LvW has the complete answer, I'd just like to try to help you a bit with understanding.

... Once the diode begins conducting current, the voltages at the anode and the cathode are identical (the diode serves as an open wire).

That's wrong - on both counts.
Would you say, once a resistor starts conducting the voltages at each end are the same? (I hope not.)
Diodes do not behave like a piece of wire (if you mean by that, a zero resistance conductor, which of course normal wires are not.) You can talk to LvW about how they do behave, but i hope he might not mind if I say that the PD across the diode does not decease when it conducts. As with a resistor (albeit in a different mathematical way) the PD increases with current.

I heard that the voltage at the anode must be at least 0.7 volts higher than the voltage at the cathode. Is that true, even for an ideal diode? I know the constant 0.7 may differ, but I'm just looking for a general principle.

I'd have said, you can say what you like for an ideal diode - just depends on what you ideally want it to do. But mostly when we talk about an ideal diode there is no forward voltage drop nor resistance: it justs conducts in the forward direction and does not conduct in the reverse direction.
Since no real diodes behave like that, we may use other ideal models. I think that is what you have in mind: it does not conduct up to 0.7V, then it conducts with no further increase in PD, but no drop in PD either - just fixed at 0.7V.
Real diodes don't do that either. It is just a different ideal model.

Some real diodes conduct something like LvW's Shockley equation (maybe all afaik, I'm just hedging my bets because I know LvW of old!) That means (if I understand it correctly - just looking at the equation as a mathematical expression) that the current is exponentially related to the PD when it is forward biased, which in turn would mean (and I'm getting on very soft ground here) that it conducts for all PDs except 0. But for small PDs the current it conducts is tiny. Then around this magic voltage 0.7V (or whatever it is, as you say) over a relatively small change in PD (a few 100 mV) the current increases drasticaly from microamps to amps. At that point you can get large changes in current for small changes in voltage as if you had a small resistor in series with a 0.7V source.
(Purely a personal opinion: I think a Si diode is conducting enough for me to take an interest around 0.5V and could be 1.0V by the time it is conducting amps. But I still accept the 0.7 model most of the time.)

Your general principle is correct - you never get rid of the 0.7V (or whichever). Below that it is a poor or non conductor. Above it conducts well with a small resistance and the 0.7V drop.

I'm not disagreeing with anyone about reading about the Shockley equation. But I would say that for me, it is helpful to look at a graph of current against PD for a diode (preferably a real one if you can find one, but if not, even an idealised or calculated one will give you an idea of what diodes actually do.)

 

[needhelp171]

One person says the diode behaves as an open wire while conducting, the other says it doesn't.

You say, for a diode, there's a voltage drop across it when it is conducting current?? The other guy said the exact opposite...?

 

[Merlin3189]

All I can say is, either do the tests yourself, or look at datasheets like this Fairchild 1N400x They make the things, so they should know what they do.
Look at page 2, Electrical Characteristics, and see what is said for V_F
Or page 3, fig. 2, Forward Characteristics.

The 1N4148 which is a small signal diode (rather than power diode like 1N400x) is here 1N4148
and you'll find similar (but different) data. Again, look for "forward characteristics".

But most of all I would say, do the experiment. It's not hard and then you'll know what happens, rather than what's supposed to happen.

Edit: and just BTW, I wonder why you want to know?

 

[needhelp171]

I'm taking a course and I'm having some trouble understanding the concepts. I'm sure, in practice, there will be resistance, but I want to know the behavior of an ideal diode. Sorry if that wasn't clear.

 

[Merlin3189]

The famous exponential equation from W. Shockley answers everything -... - you can simply calculate the resistances (static or dynamic) for any forward voltage and/or any current. ...

I don't know if OP has tried this yet, but I found it not quite so simple. I can't work out the values of n, V_T and I_S.
I used WikiP to find V_T as 25 to 28mV, n as 1 and I_S as 10^-12, and I tried with another equation VD=VT * ln((ID-IS)/IS) using the same V_T and trying various values of IS of 10^-12 to 10^-15
I eventually got some graphs like those in textbooks, but I've no idea whether they are valid, because I just changed values until the graph looked like what was wanted. But generally the graphs showed conduction well before 0.5V (though I don't know what is the smallest current that counts as conduction.)

If you happen to know a link where accurate values are given, I'd be interested, so that I can try again with some valid data.

... I want to know the behavior of an ideal diode. ...

IMO that's a more difficult question than how a real diode behaves! It seems to me to depend on whose ideal we are talking about. I know my ideals are different from some others, so that would not help you. I would think you need to know the ideals of whoever is teaching you or whoever is going to test you.

It may even depend on context. If someone says, a diode behaves like an ideal 0.7V source in series with an ideal diode, then they must mean that their ideal diode does not include the 0.7V. If someone talks about a 0.7V drop with an ideal diode, then their ideal includes the 0.7V. I have come across both these in common use. You might have to deduce or guess the ideal model they were using.

But I think you are better off understanding diodes and knowing how they behave in as much detail as you can. Then you can choose to model it in the best way for any particular circumstance. Or if someone else talks of an ideal model, you should be able to work out what they are thinking of.

 

[cnh1995]

Voltage across diode comes into picture in case of practical diode. Suppose you have a diode in series with a resistor and battery. Polarity of the battery will decide the dirrection of current. If that direction happens to be from A to K in the diode, the diode conducts, otherwise it doesn't. For practical diodes, the exponential diode equation is applicable and 0.7V drop is accounted for.

 

[LvW]

One person says the diode behaves as an open wire while conducting, the other says it doesn't.
You say, for a diode, there's a voltage drop across it when it is conducting current?? The other guy said the exact opposite...?

Needhelp171 - you got two conflicting answers. What is the consequence? Open your mind and/or read some additional resources in order to arrive at a conclusion/answer by yourself. You never should blindly believe any answer within such a forum - and, surprisingly, this is true also for textbooks and other knowledge sources. Always make use of your own brain!
In post#5, the name "Shockley" was mentioned. Why didn`t you perform a short Google-search with the keyword "Shockley"? You will see an equation describing the current-voltage relation for a pn diode and you will see a corresponding graph, which tells you everything. So you could find the answer by yourself (voltage drop yes/no ?).

Here is a typical example: In case you are interested to UNDERSTAND how a bipolar transistor works, you will be confronted with two different explanations: Some books say that the collector current will be determined/controlled by the base CURRENT and some other books state that it is the base-emitter VOLTAGE.
Don`t ask me why several decades after invention of the transistor two different "explanations" still exists. It is really mysterious.
At this place, I do not intend to inform you about my position, but I recommend to you the following:
Try to find out which book or article (a) makes a simple statement only without any attempt to justify the claim or (b) which text contains examples and/or verifications to support/proove the given explanation. This will help you to find the correct answer.