Welcome to Professor Mad.
Electronics for beginners. Today, we are going to talk about
“how to convert direct current into alternating current”.
Electric current refers to the rate of electrons moving in an electronic circuit.
If, this many electrons are moving through the circuit
at a second , we can say
the electric current of this circuit is 1 Ampere.
Even though the electrons flow from negative terminal to positive terminal,
In Most of the textbooks, Conventional Current assumes that
current flows out of the positive terminal,
through the circuit and into the negative terminal of the source.
This was the convention chosen during the discovery of electricity.
It was not quite accurate. We have unfortunately stuck with this convention
to this day and so current flow from positive to negative
is called, “conventional current flow” and is used when drawing circuit diagrams.
In fact, it makes no difference which way current is flowing as long
as it is used consistently. The direction of current flow
does not affect what the current does. Throughout this video
we use the term Current for the conventional current.
There are two forms of electric current alternating current (AC) and direct current (DC).
One is not better than the other as both are necessary.
Here we will explain each of them and how to convert Direct current
into alternating current.
Let's start with direct current. DC is a linear type of electrical current.
direct current always only flows in one direction. From positive pole to negative pole
Keep in mind that the electrons flow opposite to that
For DC, the positive and negative poles always remain the same.
We can draw a graph for the magnitude of direct current with the time
like this, since the current does not flow
in the opposite direction, the graph only has a positive side.
DC powers appliances with delicate electronic circuits,
that require a simple and steady current flowing in one direction.
Nowadays, this type of current powers cellphones, computers, and most other electronics.
And the current obtained from batteries, solar cells, etc.
Alternating current (AC) is an electric current which periodically
reverses direction and changes its magnitude continuously with time.
AC constantly changes its flow between positive and negative terminals.
This means that electrons also change their flow,
following the negative to positive end as the polarity changes.
We can draw a graph for the magnitude and direction of alternative
current with the time, "like this".
Since the current changes its direction periodically,
the graph has both positive and negative cycles.
The speed at which AC changes polarity and completes several cycles in one second
is called frequency, and it is measured in Hertz.
The most common periodic signal waveforms that are used in Electrical and Electronic Engineering are
the Sinusoidal Waveforms. However,
an alternating AC waveform may not always take the
shape of a smooth shape based around the trigonometric
sine or cosine function. AC waveforms can also take the
shape of either Complex Waves, Square Waves or Triangular Waves
and these are shown in the video. Standard AC found at most of our homes,
is a 120V amplitude pure sine wave with a 60Hz frequency.
However, many appliances and electronics use
direct current (DC), which provides consistent power to the device.
We explain how to convert AC in to DC
in our previous videos. On the other hand
most renewable power generation systems such as solar PV systems and wind
generators involve DC. So it is important to
convert this generated DC into AC for distribution purposes.
In today's video we are going to talk about
How to convert DC to AC, or inverting the DC
Imagine you have a DC battery that can supply direct current
and someone taps you on the shoulder and asks you to produce AC instead.
How would you do it?
If all the current you produce flows out in one direction,
what about disconnecting the wires, the current becomes zero,
then lets flip it back, now we have current
that flows in the opposite direction.
Like wise we can change the wires back and forth so the current direction keeps reversing,
what you end up with is very abrupt changes of current:
all in one direction, all in the other direction,
and back again. Look at the graph of
the current against time, you'll get a square wave.
Although electricity varying in that fashion , technically, its an alternating current.
Instead of swapping the wires , we can do this by using four switches.
Switching your current in given manner on and off, very rapidly,
would give pulses of direct current ; we called this as H-bridge arrangement
—which would do at least half the job.
To make proper AC, you'd need a switching h bridge arrangement
that allowed you to reverse the current completely and do it about 50‐60 times every second.
Visualize yourself swapping your switches back and forth
over 3000 times a minute. That's some neat fingerwork you'd need!
That is why we use the fastest switches in the world,
"Transistors". We can use transistors,
more specifically "Insulated Gate Bipolar Transistors or IGBTs".
The most common IGBT switching frequency ranges from 20 to 50 kHz.
An insulated-gate bipolar transistor (IGBT) is a three-terminal power semiconductor device
primarily used as an electronic switch, IGBT can be switched ‘ON’
and ‘OFF’ by activating the gate. If we make the gate more positive
by applying voltage across the gate, current can flow from the collector to emitter.
So the IGBT in its “ON“state and if we make the gate negative or zero push
the current flow stops and IGBT will remain in “OFF” state.
We can use IGBTs as switches
At its simplest, an inverter consists of what is known as a H-Bridge arrangement.
This circuit illustrates the implementation of a single phase H-Bridge circuit using four IGBTs.
The operation of the H-bridge is straightforward. By closing A and B , and opening C and D at
the same time, current starts to flow through the BULB from left to right.
By opening A and B and closing C and D at the same time,
current starts to flow through the BULB from right to left.
Likewise by turning on and off the IGBT switch pairs reverses the current direction.
That means Alternating current is generated. To work these IGBTs correctly,
accurate control signals must be provided to A, B, C and D. These control signals
are generated by a separate circuit which can control the frequency of the output AC
signal. As an example, we can use a cheap arduino board to supply these controlling signals.
Even though we converted the DC to AC square wave , it's not at all like the AC sine wave supplied
to our homes, which varies much more smoothly. To avoid damaging the IGBTs at short circuits,
when changing polarity it is necessary to turn off one set of IGBT before turning on the next.
During the transition, all the IGBTs must go off at one point.
We normally use a Diode with an IGBT to provide a necessary path for inductive
current in order to limit potential voltage build up during the transition period.
We can use a capacitor to smooth out any variation in the d.c. supply.
AC square waves are very abrupt and can end up damaging some delicate electronics in your home.
To solve this, manufacturers invented rectification devices that modify the wave,
creating what is known as a modified sine wave (MSW).
As the name suggests, modified sine wave is not really a true sine wave at all. It is simply a
stepped wave, like the image shown above. While MSW is better than a square wave,
it is still not a pure sine wave, which is required to power certain delicate electronics.
In a modified square wave design as shown above, the square waveform shape basically remains the
same but the size of each section of the wave-form is appropriately dimensioned so that its average
value matches closely to an AC waveform’s average value. As you can see there's a proportionate
amount of gap or null areas between each square blocks, these gaps ultimately help to shape up
these square waves into sinewave like output Further developments in the industry allowed
for the creation of rectification circuits, giving way to the creation of pure sine waves.
So as we describe there are 3 major types of inverters. square wave, modified sine
wave and pure sine wave. Square wave type is the most basic and cheapest one and the pure
sine wave is the most complex and costly one. You can select one type according to your requirement.
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