Diode

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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.
  • Converting radio frequency signals into audible signals in radios.

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.

Thinking About Diodes         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.

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