Video Transcript
In this video, we will be learning
about a process known as current rectification and how it’s achieved in
practice.
Now, let’s start by imagining a
simple circuit consisting of an AC source and a resistor. Now, the AC source will produce a
time-varying sinusoidal potential difference. In other words, if we would just
take a voltmeter in parallel with the resistor so as to measure the potential
difference across the resistor, as time progressed, the voltmeter would show a
sinusoidally varying potential difference across the resistor. And this is why the source is known
as an AC source or alternating current source because it outputs a sinusoidally
varying potential difference. And so, whenever its potential
difference output is positive, there is a resultant current flow in one direction in
the circuit; let’s say in this direction. And then as soon as the potential
difference output becomes negative, the current starts flowing in the opposite
direction. And so, what we’ve got is a
back-and-forth flow of charge in this circuit, depending on what point in the cycle
the output potential difference is.
Now, alternating current sources
are great. They can be used in conjunction
with transformers to step up or down their voltage. And this fact allows us a great
deal of flexibility. However, sometimes, we have
components in circuits that need direct current. For example, some of our laptops or
lightbulbs will run on direct current. But as we can recall, mains
electricity is AC. In that situation, what we need to
do is to take the AC from the mains which looks a little bit like this and somehow
turn it into direct current if we want to power these appliances. By the way, it is worth recalling
the alternating current is defined as an electric current that periodically at
regular intervals reverses its direction. However, a direct current is a
current that flows in one direction only.
So coming back to our circuit
diagram here, we’ve got an alternating potential difference that results in an
alternating current within this circuit. And this alternating potential
difference is similar to what is generated in the mains. So if we’ve got an appliance that
runs on direct current, then we somehow need to convert this alternating potential
difference into a direct source of potential difference. Now, there are two ways to do
this.
The first way is to take all the
negative parts of this potential difference curve and to get rid of them
entirely. In other words, if we still have a
potential difference source that acts kind of like a sinusoid for the first half of
the cycle and then for the half of the cycle where it should be negative it actually
becomes zero and then the cycle repeats again so the positive part stays the same
and the negative part actually becomes zero, then we technically have a direct
potential difference source. It’s no longer an alternating
potential difference source because at some points, the potential difference is zero
and at other points, the potential difference is positive. But at no point is the potential
difference negative. And therefore, if a positive
potential difference generates a current in the circuit in this direction for
example, then at no point will it generate a current in this direction because there
is no negative potential difference at any point.
Now, the process that converts this
kind of potential difference across a component in a circuit to this kind of
potential difference is known as rectification. And more specifically, this kind of
rectification is known as half-wave rectification because originally the voltage
source produces a sinusoidal voltage. And after rectification, we can see
that half of the sinusoidal voltage is reproduced whereas during the other half of
the cycle, the voltage is zero. And so, only half of the initial
sinusoid is reproduced in this case. And hence, it’s called half-wave
rectification. And the point is that if this is
the potential difference across a component in the circuit, then a graph showing the
current for that component will have a similar shape.
And so, if we now plug the current
for that component against time, then the shape of that curve would be the same as
the shape of this curve. Because assuming the component
across which we’re measuring the current behaves like a resistor, we can use Ohm’s
law to remind us that the voltage across a component is directly proportional to the
current through that component. And hence, the shape of the current
curve is going to be the same as the shape of the voltage curve. And because the current for this
component is always either positive or zero, positive or zero, positive or zero, and
so on and so forth, the current for that component is a direct current because there
is no reversing in direction of that current.
So as we’ve seen, we can use
half-wave rectification to take a sinusoidally oscillating voltage and turn it into
a direct voltage source. That’s resulting in a direct
current. But how exactly do we go about
doing this? Well, what we can do is to take our
circuit and stick a diode in series with the current source and the resistor. Now, a diode is an electrical
component that only allows current to pass through it in one direction and does not
allow any current to pass through in the other direction. Therefore, if the diode was not
present in the circuit, then the current through the circuit would look something
like this, whereas when we put the diode in the circuit, it does not allow any
current in the negative direction.
In other words, current can’t flow
this way. But even though the potential
different source is emitting a sinusoidal voltage, the diode prevents any current in
the negative direction. And so, if we measure the potential
difference across the resistor in this circuit, then it would look something like
this because the current through the resistor looks something like this, a rectified
alternating current. And so, putting a diode in series
with the component of the circuit results in half-wave rectification. However, half-wave rectification is
not the only way to go from AC to DC. There is something known as
full-wave rectification which takes a sinusoidal potential difference source and
reverses the polarity of all the negative parts of the cycle.
In other words, a cycle that should
have been doing this is now instead doing this. And we can see that this is called
full-wave rectification because this time even the initially negative parts of the
cycle are present in the rectified voltage just as positive parts of the cycle. We no longer have the potential
difference through half of the cycle being zero. So how do we go about achieving
full-wave rectification? Well, as we can guess, diodes are
involved once again. And in fact, there’s a way to
achieve full-wave rectification with two diodes in the circuit. But this is a little bit more
complicated. The method that we will be looking
at uses a bridge circuit. And this bridge circuit will
consist of four diodes.
Now, the circuit in question looks
something like this. And in the circuit, we can see one,
two, three, and four diodes connected in what’s known as a bridge circuit. Now, this circuit is going to help
us achieve full-wave rectification. And here’s how. Let’s imagine that our AC source is
putting out a sinusoidally varying potential difference as we were expected it
to. Now, during the first half of a
sinusoidal cycle, we can see that the potential difference is positive. And therefore, let’s go by the
convention that a positive potential difference will induce a current in the
clockwise direction in this circuit.
Well, in this case, we can see then
that current is flowing in this direction in the circuit. And it does so until it reaches
this branch point here. Now, when it reaches this branch
point, it can easily travel in this direction. However, the current cannot travel
in this direction because this diode here stops that from happening. Remember that the direction of the
arrow in the diode circuit symbol shows the direction in which conventional current
is allowed to flow through that diode. But anyway, so current can flow
through this branch but not through this branch. And then once again, the current
reaches a branching point. At which point, the current can
easily flow in this direction, but it cannot flow in this direction because there’s
another diode stopping it from going that way. And so, what we have is current
flowing in this direction through our resistor and coming back round this way.
Now, it’s worth noting by the way
that this low curvy part of the circuit shows that the orange wire and the blue wire
are not connected to each other. The orange wire is passing over the
blue wire. And this means that the current
continues to flow in this direction along the circuit. And then upon reaching this
branching point, the current is now about to flow both in this direction and in this
direction. Looking at the current flow in this
direction, we see that it reaches this branch point once again. And we’ve seen what happens at that
branch point already.
Looking at the current flow in this
direction through the diode, we see that it then reaches this branching point, at
which point current flows clockwise once again, around this orange part of the
circuit. And at this point, we’ve looked to
the complete circuit. But the thing to keep in mind is
that during the first half of the cycle, the current through the resistor is in this
direction. Let’s keep this current direction
in mind and think about what happens if we now look at the negative part of the
cycle. That is the potential difference
we’ll try and induce a current flow in the opposite direction in the circuit. Well, in that case, the current
flowing through the circuit will be counterclockwise now. So now, what we’ve got is current
flowing this way along the circuit up and along the orange wire at which point it
reaches a branch point.
Now, at this branch point, current
can go in this direction because the diode direction allows it but not in this
direction. And so, current flows along this
branch and reaches another branch point. At that branch point, current is
allowed to flow in this direction, but not along this diode. And so, current flows along this
blue wire through the resistor and down through the resistor as we’ve drawn it and
comes back round this way. Then, the current continues to flow
and reaches a branch point. At this branch point, current is
allowed to split and flow along both branches because that is allowed by those
diodes. Now, the current in this branch
reaches a branch point here and then is allowed to flow through this diode and also
in this direction, at which point we’ve once again considered the whole circuit.
But here’s the interesting
part. During the positive part of the
cycle, we saw that the current through the resistor was flowing in this
direction. And as we’ve just seen, during the
negative part of the cycle, current is once again flowing in this direction through
the resistor. And so what we’ve successfully
managed to do is to create a circuit in which current is always flowing in the same
direction through the resistor. Now, the magnitude of that current
will change with time depending on the magnitude of the potential difference
generated by the AC source at any given point in time.
So for example, at this point in
time, the magnitude of the potential difference generated is not quite maximum. And so, the current value is not
going to be quite maximum. However, at this point in time, the
potential difference value is maximum. And so, the current through the
circuit is going to be maximum as well. Now, the end result of all of this
is that if we were to place an ammeter in series with the resistor and have the
ammeter measure the current through the resistor, what we’d see as time passes is
that we’ve got a full-wave rectified current through the resistor. And using Ohm’s law once again, we
can see that because the resistor will have a constant resistance value, the
potential difference across that resistor will be directly proportional to the
current through that resistor. And therefore, if we stick a
voltmeter parallel to the resistor to measure the potential difference across it,
what we would see is this: a full-wave rectified potential difference. And so that is another way in which
we can convert AC into DC, an alternating current source resulting in a direct
current through a resistor.
Now, there are still problems with
both of these methods, half-wave rectification and full-wave rectification as
well. For one, normally a DC voltage
would be a smooth flat line. However, rectified potential
differences are definitely not a smooth straight line. And there are ways to get around
this using capacitors for example. However, we won’t be looking at
that here. Instead, let’s get some practice in
understanding current rectification by looking at an example question.
Diagram (a) shows a circuit that
can be used to rectify an alternating current. If the input voltage is that shown
in diagram (b), which of the following graphs shows the output voltage as measured
by the voltmeter in the circuit diagram? [A] Diagram 1 [B] Diagram 2 [C] Diagram 3 [D] Diagram 4
Okay, so in this question, we’ve
got diagram a, which shows the circuit that we’re considering, and diagram b, which
is the input voltage that’s produced by the AC source in this circuit. And as well as this, we’ve been
told that the circuit in diagram a can be used to rectify an alternating
current. Now, based on this information, we
need to work out which one of these four graphs here shows the output voltage as
measured by the voltmeter in the circuit.
Okay, so to answer this question,
let’s first consider what we have in the circuit diagram in diagram a. What we’ve got is an AC source, a
diode, a resistor, and a voltmeter. Now, the AC source, the diode, and
the resistor are in series. And the voltmeter is measuring the
potential difference across the resistor. Now, the AC source is producing a
sinusoidal potential difference across the circuit. And if we had a circuit where there
was no diode, but instead we just had an AC source, the resistor, and the voltmeter,
then the sinusoidally varying potential difference produced by the AC source would
be the same as the potential difference across the resistor. In that situation, we would have a
sinusoidally varying current through the resistor as well.
However, coming back to the circuit
in diagram a, that’s not what we’re going to see because remember we have a
diode. Now, a diode is a circuit component
that only allows current flow through it in one direction. Now, we can choose to say that
anytime the potential difference produced by the AC source is positive, that is
going to generate a current flow in this direction that’s clockwise around the
circuit. And then, we can see that that
current flow is actually allowed to be sustained because current can flow in this
direction through the diode. We can see which direction a diode
allows a current through it by looking at the arrow in that circuit diagram.
And so, anytime the potential
difference is positive produced by the AC source, there is a current in the
clockwise direction in the circuit. Therefore, there is a current
through the resistor in this direction in the circuit. And then for the resistor, we can
recall that the potential difference across the resistor which is the value measured
by the voltmeter is equal to the current through that resistor multiplied by the
resistance of the resistor itself. So the point is that anytime the
potential difference from the source is positive, a current is flowing clockwise
through the circuit which is allowed to flow because of the diode. And so, there is a nonzero current
for the resistor. And if there’s a nonzero current
through the resistor, then the potential difference also is nonzero, where that
potential difference is the voltage measured by the voltmeter. And this is because we’re looking
at Ohm’s law just for the resistor and the resistance of the resistor is a
constant.
Therefore, we can say that the
voltmeter will measure the same potential difference across the resistor as is
produced by the source whenever the source is actually producing a positive
voltage. However, as soon as we look at the
negative portion of the curve, then it’s a whole different story because when the
voltage from the AC source becomes negative, that negative voltage tries to generate
a current in the circuit in the opposite direction to before. In other words, this AC source is
now trying to set up a current in the counterclockwise direction, which would be
fine if it weren’t for the diode because the diode does not allow a current through
it in the opposite direction. And hence, there is no current in
this circuit because there aren’t any other branches for the current to flow through
that don’t end up at the diode at some point.
And so, whenever the potential
difference from the source is negative, there is no current in the circuit. So the current of the resistor is
zero. And then coming back to Ohm’s law
once again, if the current is zero, then the potential difference across the
circuit, which is remember being measured by the voltmeter, is also going to be
zero. So in other words, for the entirety
of the time that the potential difference source is producing a negative voltage,
the voltage measured by the voltmeter itself is going to be zero because there is
zero potential difference across the resistor.
Then, we go back once again to the
positive part of the cycle. And in that situation, once again,
we will have a positive potential difference measured by the voltmeter. And that potential difference will
follow the voltage produced by the AC source. And once again, we return to the
negative part which is going to result in a zero voltage measured by the
voltmeter. And so, all in all, what we expect
to see is something like this when it comes to the potential difference measured by
the voltmeter. And at this point, we can identify
this as a half-wave rectified potential difference.
But more importantly, out of the
four diagrams that we have to choose from, we can see that the first of these
diagrams correctly matches what we’re expecting to see in terms of the voltage
measured by the voltmeter in diagram a.
So now that we’ve looked at an
example question, let’s summarize what we’ve talked about in this lesson.
We firstly recalled that diodes are
circuit components allowing charge flow. That is, there can be a current
through them in one direction, but not in the reverse direction. And this is the circuit
diagram. Secondly, we saw that diodes can be
used in rectifier circuits to convert AC to DC. And finally, we looked at two
different types of rectification, maybe half-wave rectification, where sinusoidal
voltage source is converted into having positive peaks but then a zero voltage,
where the negative half of the cycle would otherwise be. And half-wave rectification can be
done by placing a single diode in series with the AC source.
We also looked at full-wave
rectification, where a sinusoidally varying AC source is modified so that the
positive peaks stay exactly the same, but the negative peaks are mirrored in the
horizontal axis. Or in other words, they take on the
same value, but the opposite polarity to what they normally would have. One way to achieve full-wave
rectification is using a bridge circuit. And that consists of using four
diodes placed in a very specific orientation so that the potential difference across
this part of the circuit, for example, this resistor here, is full-wave
rectified. And so, that is an overview on
current rectification.
