Welcome back to electronics, this is Dr.Ferri.
This lesson will cover the ideal diode model.
In the previous lesson remember, we introduced some simple diode models,
we introduced the idea of the diode itself and the characteristics, and then we
wanted to come up with simple models that are easier to use than circuits.
So, the lesson objective is to go over, in particular, the ideal diode model and
its operation.
And then to describe how we can use that to analyze a DC circuit.
In the ideal diode model, it has these characteristics.
It has two places, there are two states,
we'll call them, the ideal the on state, where the diode is conducting.
And the off state where it's not conducting.
The actual curve looks like this.
So in the forward bias, again, the current is going in the forward direction,
in the reverse bias is only until this point the two things are,
that we should note is that the current is very small, very negligible and
that the voltage is negative.
So that's we call the verse, reverse biode, bias.
The ideal diode model does not deal with the breakdown region.
So, we want to figure out what these are.
If I want to replace the diode with something easier to analyze in our
actual circuit, what do I analyze, what, what do I put in there, what components?
Well, let's think about the ideal diode, off state.
Well, the current is zero.
With if the current as zero.
[SOUND] If the current is zero,
that is equivalent to an open circuit.
So that's easy.
When it, when the diode is in the reverse bias region and
it's off, we can replace the diode with an open circuit.
Now, what about when it's in the on region?
Now that's a little bit more difficult to, to explain.
So what we're going to do, is look at a typical resistor.
A look at a typical resistor, it has these characteristics, remember.
And this slope is 1 over R.
If I let R go to zero.
This slope goes straight up and down, and
that's where we get the same, behavior as the ideal line.
So, in the on region, it's equivalent to having a, oh a short circuit.
Because when the resistor goes to zero, that is
a short circuit, or maybe just, you can say, it's just a plain wire.
So, when the, when the diode is on, and
the current is flowing, you re, can replace the diode with a short circuit.
Or, as i said, just a wire.
So to summarize this model, we've got two possible states,
on when it's conducting right here, off when it's not conducting.
And in the on state, when the diode is, is is,
when the, current is positive, we replace the diode in our circuit with a short.
When it's off or blocking, we replace the diode with an open circuit.
Let's take a look at an example.
Now in this particular example, with the polarity of the source right here,
like this, the current is going to want to flow in this direction.
It's going to want to flow the direction of the diode.
So, it will flow.
In this case, I've reversed the polarity, and so
the current's going to want to flow in this direction.
But it can't, because of the diode blocks it.
So in this case, this would be a case where the diode was on.
In this case the diode is off.
[SOUND] So when the diode is on, I can replace the circuit.
With an equivalent circuit, where I've replaced the diode with a short.
I still have this current flowing, i sub d.
And, this resistor there.
So, in this case, i sub d would be equal to 10 over R amps.
And it's positive.
Now, this case the diode is off.
So when the diode is off, I replace it with an open circuit.
And I keep my polarity of my voltage drop there.
So I can each, actually use that to do a KVL around here, for example.
If I did a KVL around here.
I would have, 10 plus V sub D plus V sub R equals zero.
And in this case, there's no current flow, so V sub R would be equal to zero.
So, I can use standard circuit analysis methods
to be able to work with this circuit, once I've made the substitutions.
Again, with the diode off, I replace with an open.
Diode on, I replace it with a short.
So, in summary, we've shown that diodes act as a short or
an open depending on the bias, forward bias versus reverse bias.
In our next lesson, we will give a very systematic way of
handling circuits that have multiple ideal diodes.
See you online at forums.
Thank you.