Diodes
Diodes
Some Diode Properties
Using Diodes
More Complex Circuits
What If I Want A Better Model?
Problems
You are at:  Elements - Diodes - Introduction to Diodes
Diodes

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

• Converting AC power from the 60Hz line into DC power for radios, televisions, telephone answering machines, computers, and many other electronic devices.
Here are the goals for this lesson unit.
• Given a circuit with a diode,
• Be able to use a simplified model of a diode to predict when current flows through the diode, and when it does not.
• Be able to use information about current flowing to predict other behavior in a circuit.

Diode Properties

Diodes have the following characteristics.

• Diodes are two terminal devices like resistors and capacitors.  They don't have many terminals like transistors or integrated circuits.
• In diodes current is directly related to voltage, like in a resistor.  They're not like capacitors where current is related to the time derivative of  voltage or inductors where the derivative of current is related to voltage.
• In diodes the current is not linearly related to voltage, like in a resistor.
• Diodes only consume power.  They don't produce power like a battery. They are said to be passive devices.
• Diodes are nonlinear, two terminal, passive electrical devices.
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.

• When the voltage across the diode is positive, a lot of current can flow once the voltage becomes large enough.
• When the voltage across the diode is negative, virtually no current flows.
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.

Diodes are a little schizophrenic.

• Sometimes they let a lot of current flow through them,
• Sometimes they permit hardly any current flow through them.
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.

• When current is flowing, this approximate model predicts no voltage across the diode.  In this situation, we say that the diode is ON.
• When the voltage across the diode is negative, this approximate model predicts no current flowing through the diode.  In this situation, we say that the diode is OFF.
Now, consider this kind of simplified model for the diode.
• When the diode is ON, it has no voltage across it so it acts like a short circuit!  When the diode is ON, the current through the diode is positive, and the voltage across the diode is zero.
• When the diode is OFF, current is zero, so it acts like an open circuit!  When the diode is OFF, the voltage across the diode is negative, and the current through the diode is zero.
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.

• When the diode is ON, it has no voltage across it so it acts like a short circuit!
• When the diode is OFF, current is zero, so it acts like an open circuit!
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.

• When the diode is ON, the voltage across it is zero because it acts like a short circuit.
• When the diode is OFF, the current through it is zero because it acts like an open circuit.
• We have one or the other of these two situations.  It can't be both ways, and it has to be one or the other.  That gives us a strategy that will let use figure out what happens in circuits with diodes.
We can start to figure out what happens in this circuit by examining what happens in the circuit in the two situations.
• We can assume that the diode is ON and check whether that assumption is consistent with what else we know - KCL, KVL and the diode.
• We can assume that the diode is OFF and check whether that assumption is consistent with what else we know - KCL, KVL and the diode.
• We are using the method of contradiction to solve this problem.  Click here for a short note on the method of contradiction.
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.

• The diode is ON and the voltage across it is zero.
• The current through the diode, Id, must be postive.  It can't be negative.  Current through a diode can never be negative.
• The current through the diode, Id, is Vin/R, (use Ohm's Law) so you cannot have a negative input voltage.
• That means that our assumption that the diode is ON has to be false for negative input voltages.
• The diode is ON for Vin > 0.
• The diode is OFF for Vin < 0.
Let's assume the diode is OFF.  Then, the diode can be replaced by an open circuit.  Here's the equivalent circuit.

• The diode is OFF and the current through it is zero.
• The voltage across the diode, Vd, must be negative.  It can't be positive.
• The voltage across the diode, Vd, is just Vin, (use KVL) so you cannot have a positive input voltage.
• A positive input voltage is inconsistent with the assumption the diode is OFF.
• The diode is OFF for Vin < 0.
• The diode is ON for Vin > 0.
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?

• If the input voltage is positive, current flows through the diode, and the output voltage is equal to the input voltage.
• If the input voltage is negative, no current flows through the diode, and the output voltage is zero.

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.

• D1 OFF, D2 OFF
• D1 OFF, D2 ON
• D1 ON, D2 OFF
• D1 ON, D2 ON

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:

• D1 OFF, D2 OFF

In this case, both diodes are OFF.

• Since both diodes are OFF, there is no current though either diode.  Consequently, there is no current through the resistor and Vout = 0.
• If Vout = 0, we have enough information to compute the voltage across each diode assuming that we know the input voltages.
• We can write KVL around either of two loops, and each loop will contain just one diode.
• Around the first loop we have:
• VD1 = V1 - Vout = V1
• Since the voltage across the diode must be negative when there is no current through the diode we must have V1 < 0.
• Around the second loop we have:
• VD2 = V2 - Vout = V2
• Since the voltage across the diode must be negative when there is no current through the diode we must have V2 < 0.
• We conclude:
• Vout = 0 when V1 < 0 and V2 < 0.
• In words, the output voltage is zero when both input voltages are negative.
Now, consider the second case.  Here is the equivalent circuit for the second case
• D1 OFF, D2 ON
• Since D2 is ON, it has been replaced by a short circuit, and that makes Vout = V2.
• If D2 is ON, the current must be positive, and that will occur only when V2 > 0.
• If Vout= V2, we have enough information to compute the voltage across D1.
• We can write KVL around the loop that contains the resistor and D1.  Around that loop we have:
• VD1 = V1 - Vout = V1- V2
• Since the voltage across a diode that is OFF must be negative, we have to have V1< V2.
• In words, when V2 is positive and we have V1< V2, the output will be V2.
Now, examine the third case.
• D1 ON, D2 OFF

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:

• In words, when V1 is positive and we have V2< V1, the output will be V1.
Finally, we get to the last case.
• D1 ON, D2 ON
• Since both diodes are ON, both diodes have been replaced by short circuits.
• The output voltage, Vout, is equal to both V1 and V2.
• The only way that can happen is if we have, Vout = V1 = V2.
• In words, when both input voltages are equal, that is what the output voltage becomes.
We can summarize what happens in this circuit with a few simple statements.
• Given the diode circuit:\ below, and assuming that the diodes are ideal,
• When both input voltages are negative the output is zero.
• When either or both input voltages are positive, the output voltage is equal to the larger of the two input voltages.

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