All electronic devices have one thing in common – they all need power supply. Think of electronic devices like TV, music system, computers, radio, mobile phone etc, they all need some source of power. However the electronic circuitry inside such devices requires low voltage DC power supply. The power that we get from the wall outlet is 230 V 50 Hz (In India). So a need arises to convert high voltage AC into low voltage DC. In this section we shall discuss the block diagram of power supply in detail.
Let us start with the block diagram of power supply system given below. There are total five blocks in the diagram, which we shall discuss one by one in their respective order.
1. Transformer :
A transformer is a device which transforms high voltage AC into low voltage AC or vice versa. Our goal is to convert high voltage AC into low voltage DC. So there is absolutely no reason to use step-up transformer. The transformer that is used in power supply is step-down transformer, which steps down the input AC voltage. The magnitude by which transformer steps down the voltage depends on the turns ratio of primary and secondary winding. Observe the magnitude of sinusoidal signal before the transformer block. Its magnitude is quite high as compared to that of the signal after the transformer block diagram. This indicates that that the signal was stepped down by the transformer. There arises an obvious question as to why transformer is used in this system. The main reason why we use transformer in the system are as follows.
- We want to reduce the voltage level which we get from the AC mains. Transformer can do the job of reducing the voltage level in a simple and efficient manner.
- The diodes used in the rectifier block cannot handle such a high level of voltage from the AC mains. So the voltage is first stepped down by the transformer and the reduced voltage is applied to the rectifier section.
2. Rectifier :
Before understanding what are rectifiers and how they are used in power supplies, let us first understand the meaning of the word “rectification”. Merriam-Webster dictionary defines the word rectification as -“to correct” or “to set right” or “to correct by removing errors”. Let us try to understand why the word “rectifier” or “rectification” is used in the context of electronics. A sinusoidal signal, as we know, has both positive and negative values. Because of the symmetry on both sides, the average value of sinusoidal signal VDC turns out to be zero. We certainly are interested in nonzero average value (or do you want zero average voltage as your final output?). We can get nonzero average voltage by removing the negative half cycle of the sinusoidal signal. The circuitry which does this job is called rectifier as it corrects the signal by removing the negative side of the signal. Rectifiers are basically of two types
- Half wave rectifiers
- Full wave rectifiers
The function of half wave rectifier and full wave rectifier is shown below. Full wave rectifiers are the most commonly used rectifiers in power supply. Speaking of full wave rectifiers in simple language, it simply allows the positive half cycle to pass through and inverts the negative half cycle to positive half cycle. This is shown above in the block diagram of power supply and its function is shown below. 3. Filters :
The output after being processed by full wave rectifier is not a pure DC. The output is a pulsating DC. The output contains large fluctuations in voltages. This is quite apparent from the block of full wave rectifier shown above. The power supply that we intend to design must not have any variation in output voltage. The voltage that we get from full wave rectifier fluctuates between 0 V and Vpeak, and hence it contains AC components. These AC components needs to be filtered out so as to obtain DC voltage. This is where filters come into picture. Filters, as the name suggests, filters out any AC component present and provides DC as the output. However, the output from the filter is still not a pure DC but filters removes the AC component in the voltage to a considerable extent. This increases the average DC value of the output voltage. Now a question must arise as to how we can make a filter and which components are required to make a filter. Although it not the goal of this section to study filters in detail, it must suffice to know that filters used in power supplies can be made simply by using capacitors. We leave the design of capacitive filter to some other section. Typical input and output voltage of filters used in power supply is shown below. As shown in the figure above, the output voltage from the filter contains voltage ripples. This output is not a pure DC, however considerable amount of AC component is filtered out by the filter. The effectiveness of the filter to remove the AC component is indicated by the ripple factor. Smaller the ripple factor, better the filter.
4. Regulator :
Many power supplies which are not intended to power sophisticated equipments contains circuits only upto filter section. Ordinary voltage adapter used to charge small batteries in toys is one such example. The output from the filter is directly applied to power up the circuit. However when there is a need to provide very good quality power supply, a regulator is also added in the circuit.
What is a regulator and why is it used? This must be an obvious question that should arise in our mind. A regulator is an electrical or electronic device which maintains a constant output voltage irrespective of changes in load current, input voltage or temperature. That is the basic definition of regulator.
One question still remains – What is the need of regulator and why is it used? To understand the reason, consider the case when there is a voltage fluctuation in AC mains. You can refer to the block diagram of the power supply given at the top of the page. There is at transformer connected with the power supply mains. If there is a fluctuation in AC mains, there will be fluctuation in the output of transformer too. The simple reason for this is that the transformer only steps down whatever is given at its input. It can’t decide for itself – “Hey, there is a fluctuation in AC mains, I must adjust my output”. The turns ratio which determines the factor by which transformer steps down the voltage is same irrespective of changes in input voltage. The output of transformer is only as good as its input. This output is passed on to rectifier which again rectifies whatever is applied to it. Filter tries to eliminate the AC components as it is designed to do. However the average DC voltage may not be the same as it was intended to be. In such a condition, a voltage regulator comes into picture. Other case where a need of regulator arises is when there is a change in load current. For changes in load current, there is a change in output voltage. In order to maintain constant voltage irrespective of changes in load current, voltage regulator is used.
Thus by connecting all the necessary blocks as shown in the beginning of this page, we can make a good quality power supply. Such power supply can be used to power sophisticated electronic gadgets. For any doubts or questions, feel free to leave a comment below.