- [Narrator] One common application of diodes is to
build a rectifier, which is an electronic device that
converts from alternating current to direct current.
If you've ever broken apart a wall wart power adapter,
you've probably seen a set of diodes inside.
Those diodes are converting the 120 volts AC,
that comes from a wall outlet into a DC
voltage for a device to use.
Since, an AC signal periodically reverses direction,
it consists of both positive and negative
voltages at different times.
The goal of a rectifier circuit is to remove
all of the negative voltages so that
the output signal is entirely positive.
Now, that can mean simply eliminating the negative voltages
by making the output voltage zero during that time,
or it can mean converting those negative voltage swings
into positive output voltages.
The simplest type of rectifier circuit
is called a half-wave rectifier.
It consists of a single diode placed in series between
an electric load and an AC voltage source.
When the input voltage from the AC source
swings in the positive direction,
the diode will act like a short circuit
and allow current to pass through.
So, the voltage across the load, which is the output of the
half-wave rectifier will be similar to the input voltage.
Now, when the input voltage changes direction
and swings to the negative side,
the diode will act like an open circuit to
block current from passing through it.
Since, the circuit has been effectively broken open,
no current flows through the load.
So, the output voltage across the load will be zero volts.
These two plots illustrate the relationship between an
AC input voltage and the corresponding output
voltage from a half-wave rectifier.
When the input is positive, so is the output.
But when the input is negative, the output goes to zero
because no current flows through the load.
Although, it's common to think of the term DC as referring
to a constant voltage, even though the rectifiers output is
constantly changing, it is still technically direct current
because the voltage is always positive so the current will
only ever flow in one direction.
Now, that explanation of the half-wave rectifier is a bit of
a bit of a simplification because the diode will not
actually turn on and allow current to flow through it until
the input voltage from the AC source is greater
than the forward voltage of the diode.
Also, due to the forward voltage drop across the diode
the output voltage from the half-wave rectifier will
be slightly less than the input voltage.
We lose some of that input voltage to the forward
voltage drop across the diode.
To demonstrate that, I've built a half-wave rectifier
circuit on my breadboard using my function generator
to provide a sign wave as the AC input voltage and using a
10 kilo ohm resistor to serve as the electrical load.
I've connected channel one of my oscilloscope to display
the input voltage in yellow, and channel two to display
the output from the half-wave rectifier,
which is the voltage across the load resistor in blue.
Looking at the scope, I can see that the output signal
only contains the positive parts of the input signal.
Additionally, the positive humps of the output wave form
are slightly smaller than the input signal because I lose
about 0.6 volts of the input voltage as the forward
voltage drop across the diode.
Although, the half-wave rectifier circuit is simple to use,
it's not very efficient for converting from AC to DC
because I lose all of the energy from the negative
half of my AC signal to the diode.
If I want to preserve more of the energy from the input
signal then I should use a full-wave rectifier,
which converts both the positive and the negative parts
of the input AC signal into a pulsating DC output.
The most common full-wave rectifier circuit consists
of four diodes in a wheatstone bridge configuration.
The diodes are oriented so that
when the input signal is positive,
current will flow from the source through this
diode and into the top of the load,
and then current coming out of the load
takes that path through the diode,
and it's returned back to the voltage source.
When the AC signal switches polarity and turns negative,
the current will take a different route through this
diode into the top of the load,
and, then the return current from the load will pass through
the bottom left diode back to the source.
Regardless of whether the input
voltage is positive or negative,
the output of the full-wave rectifier is always positive,
and pushes current in the same direction through the load.
This breadboard circuit is a full-wave rectifier
that uses four diodes to convert the AC signal
from my function generator into a DC voltage
across a 10 kilo ohm load resistor.
On the oscilloscope, I see that the output signal looks
like a completely positive version of the input signal,
and just like with the half-wave rectifier,
the humps of the output waveform are
smaller than the input signal due to the forward
voltage drop across the diodes.
Although, the pulsating output of the rectifier
circuit is technically a direct current signal
since the voltage is always positive,
most of the time when we're converting from AC to DC,
we really want a constant output voltage.
So, to achieve that, most power supplies that convert
from AC to DC use a low-pass filter on the output
of the rectifier to smooth out the bumps
and make the output more steady.
When building rectifier circuits, we need to consider the
amount of power that will be dissipated by the diodes
to choose sturdy enough components.
For the example circuits in this video, I chose the common
1N4148 Signal Diode because I was rectifying
relatively small signals so the diodes
wouldn't need to dissipate much power.
However, if I needed my rectifier circuit to handle
more power, then I should use power diodes like the 1N4001,
which have a much higher maximum current
rating than basic signal diodes.
The 1N4001 power diode is designed to handle up to an
amp of current operating as a rectifier,
but with that capability comes the downside that it requires
a larger forward voltage drop of about one volt.