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 V_{in}/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 V_{in} > 0.
The diode is OFF
for V_{in} < 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, V_{d}, must be negative.
It can't be positive.
The voltage across the
diode, V_{d}, is just V_{in}, (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 V_{in} < 0.
The diode is ON
for V_{in} > 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 V_{out}
= 0.
If V_{out}
= 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:
V_{D1}
= V_{1} - V_{out} = V_{1}
Since the voltage across
the diode must be negative when there is no current through the diode we
must have V_{1} < 0.
Around the second loop
we have:
V_{D2}
= V_{2} - V_{out} = V_{2}
Since the voltage across
the diode must be negative when there is no current through the diode we
must have V_{2} < 0.
We conclude:
V_{out}
= 0 when V_{1}
< 0 and V_{2}
< 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 D_{2}
is ON, it has been replaced by a short circuit, and that makes V_{out}
= V_{2}.
If D_{2}
is ON, the current must be positive, and that will occur only when V_{2}
> 0.
If V_{out}=
V_{2}, we have enough information
to compute the voltage across D_{1}.
We can write KVL
around the loop that contains the resistor and D_{1}.
Around that loop we have:
V_{D1}
= V_{1} - V_{out} = V_{1}-
V_{2}
Since the voltage across
a diode that is OFF must be negative, we have to have V_{1}<
V_{2}.
In words, when
V_{2} is positive and we have V_{1}< V_{2},
the output will be V_{2}.
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
V_{1} is positive and we have V_{2}< V_{1},
the output will be V_{1}.
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, V_{out},
is equal to both V_{1} and V_{2}.
The only way that can
happen is if we have, V_{out} = V_{1} = V_{2}.
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.