Diodes
Diodes
Some Diode Properties
Thinking About Diodes
Using Diodes
More Complex Circuits
What If I Want A Better Model?
Problems
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Diodes

        Diodes are different and useful electrical components.  Diodes are used in many applications like the following.

        Here are the goals for this lesson unit.
Diode Properties

        Diodes have the following characteristics.

        In general, diodes tend to permit current flow in one direction, but tend to inhibit current flow in the opposite direction.  The graph below shows how current can depend upon voltage for a diode.

Note the following.

        The circuit symbol for a diode is designed to remind you that current flows easily through a diode in one direction.  The circuit symbol for a diode is shown below together with common conventions for current through the diode and voltage across the diode.


Thinking About Diodes

        Diodes are a little schizophrenic.

This schizophrenic behavior gives us a way to think through what happens in many diode circuits.

        We're going to adopt a simplified model for the diode.  Instead of the actual voltage-current curve for the diode shown in the thin, lighter red, curved line below, we're going to imagine that the diode has the voltage-current curve shown in the thicker, dark red lines below.

        The approximate voltage-current curve gives us one way to analyze circuits that contain diodes, and to account for their schizophrenic behavior.

        Now, consider this kind of simplified model for the diode.         So, this idealized model for the diode is sometimes an open circuit, and somtimes a closed circuit - truly schizophrenic!  This model for the diode is often referred to as the ideal diode model.

Using Diodes

        Now, let's examine a simple diode circuit.  Remember what we know about ideal diodes.  We will assume that the diode is ideal for the sake of argument.

        Now, let's look at a simple diode circuit.

It's just a diode and a resistor operating on an input voltage.  We would like to determine how the output voltage depends upon the input voltage.  We know something about the circuit.

        We can start to figure out what happens in this circuit by examining what happens in the circuit in the two situations.         Let's assume that the diode is ON.  If the diode is ON, then, we can consider it so be a short circuit.   Here is the circuit with the diode and symbols for the diode voltage and current.

We've replaced the diode with a short circuit below.

Since it's now a short circuit, Vd has to be zero.  Let's think this through.

        Let's assume the diode is OFF.  Then, the diode can be replaced by an open circuit.  Here's the equivalent circuit.

        All of the above is consistent.  We have examined all the possibilities for the diode (ON and OFF) and what we get is consistent so we must have a good prediction of how the diode works in this circuit.

        What can we conclude here?



What If The Circuit Is More Complex?

        If the circuit is more complex, then we still need to remember that every diode can be ON or it could be OFF.  Here's a circuit with two diodes.

There are four combinations of diode states that can occur in this circuit.  Let us examine all four possibilities.  Here are the four combinations with each diode replaced by either a short circuit or an open circuit, depending upon whether we assume the diode is ON or OFF.

        To determine how this circuit works, you'll have to check every possibility.  We will start with the first case.  In this situation, we have:

In this case, both diodes are OFF.

        Now, consider the second case.  Here is the equivalent circuit for the second case
        Now, examine the third case.

This case is exactly the same as the second case except that the two diodes are reversed.  The same argument we used for the second case works here with 1s and 2s interchanged, so we conclude:

        Finally, we get to the last case.
        We can summarize what happens in this circuit with a few simple statements.

What If I Want A Better Diode Model?

        We've been operating on the assumption that the diodes all act like our ideal model which has no voltage drop in the forward direction - when current flows.  The ideal model, and a theoretical voltage-current curve are shown below.

This is the model we've been working with.  A better - but still not exact model - is shown below.  You can see the model by clicking the small red button at the bottom right of the graph.

        This, new and improved - but not perfect - model can be modelled in terms of the first model we used - the ideal diode.  (It's not a perfect model of the diode because - as you can see - the two straight lines do not model the "corner" in the curve to perfection.)  A circuit model that gives the better voltage current curve is shown below - within the dotted lines around the circuit model.

The diode inside the model is ideal, in the sense that it has no forward drop across it when current flows through it.  The source in series with the ideal diode serves to account for the forward voltage drop - assumed constant in this model.  Note that the added voltage source serves to oppose the flow of curent until the voltage applied to the diode exceeds the threshold voltage, V,.  In the model above, the threshold voltage is 0.8v.

        There are still better models for diodes.  The diode has a nonlinear capacitance associated with it, for example.  You might want a more detailed model for the diode if you were using a simulation program and you wanted the results to be as exact as possible.  There are lots of other effects that could be modelled.  However, that's a topic for another lesson, another day.  That's it for this lesson.

        However, before you leave this lesson, be assured that the model we now have, and even the ideal diode model can often be used to predict performance of circuits with diodes, and they can help you understand those circuits.



Problems
Diode Problem 1.1 - The Diode Rectifier
Diode Problem 1.2 - A Modified Diode Rectifier
Diode Problem 1.3 - A Two-Diode Circuit
Diode Problem 1.4 - A Voltage Limiter

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Copyright 2002 E. J. Mastascusa