This video is going to be about AC to DC power supplies. As an example, we will take a look
at the principles involved in designing a power supply that can convert regular AC mains
voltages to 5V DC. In order to get a better understanding of everything, we will also
use some circuit simulations that will help us visualize everything. When it comes to
power supplies, there are two main types: linear and switching.
This is what a linear power supply looks like while this is a compact switch mode power
supply that is used as a cell phone charger. Immediately, you will notice that the linear
power supply is bulkier in comparison. It is also much heavier at 262g as opposed to
48 grams for the switch mode power supply. Here are the ratings for both.
The linear power supply requires an input of 230V, and as we will see later, this is
important. It also gives us two outputs, one is 9.6V DC as indicated by this symbol and
the other is 3V AC indicated by this symbol. We are looking at an output of around 3W which
is very low given the size of the unit but this one is designed for a guitar pedal which
means that the emphasis is in producing a very low noise output. The SMPS version on
the other hand, can work between 100-240V and it also produces a much higher output
of 5W, which is 5V DC at 1A.
Linear power supplies are the older variants but they are still very common, particularly
in audio gear as they produce a much cleaner output. In comparison, switch mode power supplies
are newer, they are a bit more complex but they have the advantage of being compact,
lightweight and much more efficient. They do require additional filtering depending
on the final application. Let’s think about the things we would need to design a 5V linear
power supply. If you have worked with electronics before, then the first thing that you would
think of using is the 7805-voltage regulator to obtain 5V.
This is kind of suitable however; it requires a DC input voltage and it can only handle
a max voltage of 35V DC whereas we will be using the AC mains voltage as the input that
is much higher. Because of this, we need to add a couple of more sections or stages which
will first reduce the AC voltage to something that’s more suitable and they will then
convert this to DC which can be used by the 7805 regulator. In total, we end up with four
stages which are as follows: a transformer, diodes, a filtering capacitor and the final
voltage regulation stage. Let’s start by taking a look at the transformer.
At a simplest level, a transformer has three parts: a coil at the input which is called
the primary, a core which is made up of a number of steel or iron plates and a coil
at the output which is called the secondary. The two coils themselves are electrically
isolated, however, because of the way the transformer is constructed, they are magnetically
coupled which is what we want. What this really means is that when we apply an AC voltage
to the primary coil, we will get an AC voltage at the secondary coil.
Let’s connect the primary to the AC mains, I will use 230V for this example which is
common for most parts of the world, however, some regions like North America use 120V and
as we will see, the input voltage matters. Let’s assume that the primary has 500 turns
and the secondary has 100 turns. Now, we can calculate the output voltage by multiplying
the input voltage with the ratio of the turns present in the secondary coil to that in the
primary coil and this gives us an output voltage of 46V. This is much higher than the max 35V
that the 7805 can handle which means that we need to reduce it further. We could lower
this to 35V and use that as the input, but for a 5V supply this is too high and it will
not be very efficient. Ideally, you want to be as close to the final output as possible
for maximum efficiency.
The 7805 requires a minimum input voltage of 7.5V for proper operation this means that
an input voltage of around 9V would be ideal.
Going back to our transformer setup, since we cannot change the AC input voltage, we
can only change the turns ratio. By changing the number of primary turns to 2500, we can
obtain an output voltage of about 9.2V which is more suitable for our needs.
Just a side note before we continue to the next bit. Transformers can be used to convert
between different AC voltages. In our example, we are stepping down the voltage from 230V
to 9.2V, however, you can also do the opposite and step-up the voltage by adjusting the turns
ratio depending on your needs.
One more thing to keep in mind is that the output voltage at the secondary is directly
dependent on the voltage that is applied to the primary, this means that we need to make
sure that we have enough headroom for any minor voltage fluctuations.
If we had applied 120V to the primary instead of 230V, then the secondary voltage would
have been 4.8V, half of what we need and the 7805-voltage regulator would not work very
well. There is a lot to talk about when it comes to transformers, however, that is not
the topic of interest today so we are going to move on to the next stage which is converting
the 9.2V AC voltage to DC.
Here we have a 9.2V AC voltage source that is connected to a 100-kilo ohm resistor which
acts like a load. This allows us to plot the voltage across the resistor so that we can
visualize it. There is nothing much happening here as the output voltage follows the input
voltage as can be seen in the waveforms. Watch what happens when I add a diode in series.
The green colour indicates that we are in the positive AC cycle while the red indicates
we are in the negative.
The output waveform is only present for the positive cycle. This is because diodes only
conduct when their anode is at a higher potential when compared to their cathodes. Hence, it
only conducts during the positive AC cycle and does not conduct during the negative.
This type of a circuit is called a half-wave rectifier circuit. You can also see that the
RMS or effective voltage is about half the peak which is 8.8V. The difference between
the input and output peak voltage is because the diode develops a voltage drop across it
when it conducts. This is a start, but the output is nowhere close to what a steady DC
output should look like, which is a straight line like this.
In order to improve the output further, we need to use a total of 4 diodes which are
connected as shown on screen. I will explain the circuit in a bit, but first take a look
at the output. You can see that we now have a positive voltage across the load in both
the cycles which is much better. The RMS voltage has also increased a bit which again is a
positive thing. Let’s now analyse the circuit. We have the same 9.2V AC input voltage that
is represented in this waveform. We have a positive cycle and then we have a negative
cycle. I am going to pause the simulation during the positive cycle so that we can take
a look at what the diodes are doing.
During the positive cycle, the AC voltage is applied to diodes D1 and D4, however, only
diode D1 can conduct as the voltage is applied to its anode. This means that current can
flow through D1 and then through the load. The other end of the load is connected to
diodes D3 and D4. However, current can only flow through a standard diode from its anode
to its cathode. Which means that only D3 will conduct and this allows the current to return
to the source.
Let’s take a look at what happens during the negative cycle. Remember that the polarity
is reversed, which means that this top half that is indicated by red is negative, when
compared to the bottom half. In other words, current can flow from the positive terminal
to reach diodes D2 and D3. However, only D2 will conduct as it is forward biased. Current
will flow through the load in the same direction as before and it will reach diodes D3 and
D4. Once again, only D4 will conduct and this allows the current to return to the source,
completing the circuit.
As you can see, the current alternates between the diode pairs and this allows it flow in
the same direction each time. This circuit is called a full-bridge rectifier circuit
and this is the second section of the power supply. All we need to do now is try to make
the output a bit smoother to resemble a steady DC voltage. To do that, we need to use a capacitor
to give us something like this.
One component is all it takes to make a difference. The output is much smoother now and looks
very similar to a steady DC voltage. It is common to use higher value capacitors like
a 1000uF or even more depending on the application. Notice that the output voltage is about 8.3V
DC while the input is 9.2V AC, this is because at any given time, two diodes will be conducting
and since there is a voltage drop of around 0.7V across each one, you can expect the output
to be 1.4V lower than the input. To an extent, the voltage drop does depend on the current
flowing through the diode, so that is something to keep in mind.
The final stage is very simple. Instead of using a 100k resistor as the load, we can
connect the output to a 7805-voltage regulator which will then give us a steady 5V output.
You will need to add a couple of filtering capacitors to it and it is always a good idea
to refer to the datasheet to obtain the recommend values. You would generally also add a 0.1uF
ceramic capacitor to take care of high frequency noise.
And that is how linear DC power supplies work in general. We have four different stages
that are used in combination to convert AC mains voltage to whatever DC value you require.
There are variants to these types of power supplies but hopefully the information contained
in this video will help you understand how they work. When it comes to transformers,
there are variants which have three output terminals instead of two and these are also
called taps as seen here.
You can use the two end terminals and ignore the middle one, that will work fine. However,
these types of transformers are mainly designed for use in circuits that require both positive
and negative voltages. As an example, the centre tap can be used as the ground or reference
and by doing this, we can obtain a positive voltage of let’s say 24V between the centre
and the top. While we can obtain a -24V supply between the centre and the bottom taps. If
you were to measure the voltage between the two end taps, then you would read a value
of 48V in this case. Keep in mind that voltage is nothing but the potential difference between
any two terminals. The actual voltage values will depend on the ratio of the turns, but
this is a basic principle behind them.
It is also common to have a circuit like this which is a two-diode full bridge rectifier
with the centre tap as the common reference. Remember that the two outputs are going to
be out of phase, meaning when one is positive, the other will be negative. This means that
the top diode will conduct during one cycle and at this time, the bottom diode will be
OFF as it is reverse biased. In the next cycle, the bottom diode will conduct while the top
will be switched OFF. This way, we can rectify the AC input on both cycles.
So that is about it for this video on linear power supplies. It’s a basic but pretty
useful topic particularly when you are getting started with electronics. Thanks for watching,
and I will see you in the next one!