DC Power Supply- an IntroductionIn this section we are going to study how the AC mains supply is converted into the DC supply required for operating many of the common electronic equipments. As you all may be aware almost all of the electronic equipments require DC power supply for their operation. Even those equipments to which we provide AC mains supply, convert it internally into DC supply to power the electronic circuits So, almost all electronic circuits require DC power supply and we have AC supply commonly available in our homes, offices etc. Now, if we somehow convert the AC mains supply to DC, then we can run our equipments using this converted DC supply. The process of converting the AC mains supply to DC supply is called “rectification” and the circuit used for this purpose is called “rectifier”. Using the rectifier circuit and some other electronic components one can make a power supply to provide DC power to our electronic equipments. Sections in a power supply Power Supply Let us now see that are those components that together with the rectifier make a complete power supply. • Step Down Transformer • Rectifier Circuit • Filter Circuit e. To remove these pulses from the DC supply and to make it a clean DC supply. But the DC output from the filter circuit changes according to change in the load value or according to change in the input AC mains voltage. this DC supply will contain small amount of pulses. . Rectifier Circuit In the next section. To keep this DC output constant irrespective of change in input AC mains voltage and the load. The job of this rectifier is to convert this AC supply into DC. only the voltage is reduced. a circuit known as regulator circuit is used.• Regulator Circuit Step Down Transformer The step down transformer is used to reduce or step down the mains AC supply voltage to a low value. this reduced AC voltage is fed to a rectifier circuit. The output of the regulator will be a constant DC voltage. Let us now see each of these block in detail their types. The output from the step down transformer is still in the AC form. This regulator is the last block in the power supply. The output of the rectifier will be a DC supply. Regulator Circuit This final DC output when given to equipment must provide a constant DC supply. Filter Circuit. It is the job of this filter circuit to convert this DC with pulses into a pure DC. i. this pulsating DC supply is next fed to a filter circuit. which can be used to power the required equipment. how they can be made using various electronic devices etc. but it will be a pulsating DC supply. so it will not conduct. Three most common type of rectifier circuits are • Half wave rectifier • Full wave rectifier • Full wave bridge rectifier Let us now see how these rectifiers convert the AC supply into DC supply. and the current flows through the diode to the load at the output. diode is forward biased. negative half cycle of input AC voltage. no current flows through the load. the mains AC supply is reduced using a step down transformer. Half Wave Rectifier As shown in the figure. As the diode does not conduct. During this time there will be no output voltage. half wave rectifier is made of a diode. First. . During the other half cycle. the reduced AC supply is converted into DC by this rectifier circuit.Rectifier circuit After stepping down the A C mains supply. This flow of current through load will generate output voltage in the load. This reduced AC supply is fed to the rectifier circuit. the diode is reverse biased. Forward biasing of diode makes it conduct. In this rectifier during the positive half cycle of AC. Full Wave Rectifier The circuit for full wave rectifier is shown in the figure 3. you can see that in the full wave rectifier circuit shown in the figure 3. This will reverse bias the D2 diode. in the second half. a positive DC output is available. Now. in both the half cycles of AC. the output of this rectifier is not a pure DC voltage. in a full wave rectifier. As both the half cycles are rectified in the full wave rectifier. the secondary of the transformer is center tapped. the current will flow through the Load in one direct ion only. the efficiency of full wave rectifier is double than the efficiency of half wave rectifier. half wave rectification. negative half of the AC cycle. At this time the diode D2 remains reverse biased and current will not flows through it. Also. a negative half cycle is induced in the upper half and a positive half cycle is induced in the lower half of the secondary winding. it is pulsating DC output normally cheap battery eliminators and DC adaptors use this type of single diode. The diodes are arranged in such a manner that in both cycles of AC. i.e. In this circuit. As a result of this the current will flow through diode D2 and Load.As only the half cycle of the AC is rectified and converted into DC voltage. as you can see in the figure 2. Positive half cycle in the upper half of the secondary will forward bias the D1 diode and current will flow through D1 and Load to ground. both the cycles positive and negative cycles of AC is rectified. Because of this. during the positive half cycle of AC at the input of transformer will induce a positive half cycle in the upper half of the secondary winding and at the same time a negative half cycle is induced in the lower half of the secondary winding. because two equal and opposite currents flow through the secondary winding does not . Also. in a full wave rectifier small size transformer can be used. So. As you can see in this figure. this rectification is called half wave rectification. During this time. D to C through Load and from point C to B through D3. point A is positive and B is negative. This reduces cost.allow the transformer core to get saturated. in the secondary winding. Full Wave Bridge Rectifier In the full wave rectifier. As you can see in the figure. So. Because of this the transformer has to produce twice the voltage required. During the positive half cycle of input AC signal. a full wave bridge rectifier does not require a specially tapped transformer. this arrangement of diodes in the secondary is called “bridge arrangement”. weight and size of the power supply. . at any time only the half of the secondary windings are used in the circuit. During the negative half cycle of input AC signal. Let us see the working principle of this rectifier. D3 and D4 remain reversed biased. D to C through Load and from point C to A through D1. in the secondary winding. point B is positive and A is negative. This makes diode D1 & D2 forward biased and the current flows from point B to point D through D2. D1 and D2 remain reversed biased. If the output requirement is 10V DC. In this rectifier four diodes are connected across the secondary winding as you can see in the figure 4. the output is taken from a center tapping at the secondary of the transfer. During this time. This makes diode D4 & D3 forward biased and the current flows from point A to point D through D4. A full wave bridge rectifier solves this problem by using a special arrangement of diodes as shown in the figure 4. then you will need a transformer which provides 20V DC at its output winding. the coil will stop the AC pulses. it is called pulsating DC voltage as it in the form of half AC pulses. Depending upon these components. The DC supply we receive from the battery is pure DC supply. direction of current through load is same.Note that for both half cycles of AC. To get pure DC supply from pulsating DC voltage these AC pulses needs to be removed/reduced from the rectified DC output. the capacitor must be of Electrolytic type having proper value and voltage rating. We know that capacitors allow the AC signal to pass and coils stop the AC signal. Filter circuits are used for this purpose. the capacitor will bypass the AC signal and when a coil is used in series to pulsating DC signal. the filters are classified as • • • • Capacitor filter Inductive filter (Choke/Coil) LC filter Pie filter Capacitor Filter When a capacitor is connected between the output terminals as shown in the figure 5. These properties of capacitor and coil are the reason behind the filter circuit being made of coil or capacitor or combination of both. Filter Circuit The output from the rectifier is not pure DC voltage. which will pass the DC signal while stopping AC pulse. When a capacitor is used in parallel to pulsating DC signal. this bridge rectifier rectifies full AC wave and produces DC output from the AC input using the full secondary winding. So. which will pass the AC pulse while stopping DC signal. To use as a filter. from point D to C. To make a filter circuit we need a device. . or a device. without any AC pulses. the capacitor acts as a filter. when the rectifier output is given to the coil. iron core inductor or choke/coil can be connected in series with the rectifier output to make a filter. the capacitor blocks DC and allows some part of the AC to pass through it. The property o9f the coil is it passes dc supply and block AC supply. at the output terminal. So. Inductive Filter (Choke/Coil) Other than capacitor. some part of the ac ripple is blocked by the coil and the complete dc passes through it to the output terminal. it is opposites of the capacitor. .We know that one major property of capacitor is it blocks DC supply and allows AC supply to go through it. This gives a DC output with reduced ripple. So. when the DC supply with AC ripple. is passed through the capacitor. For them a combination of the two methods are used. first a capacitor filters the rectifier output. LC filter is a combination of capacitor and choke is used. PI filter In this method of filtration one choke and two capacitors are used. These are called combination filters and one of them is LC filter. but sophisticated electronic equipments require a perfect dc supply. Above two methods may be good enough for many electronic items. the output still contains some ripple signal. As you can see in the figure.Combination filters Filtering the rectifier output using only one component is not enough. . This rectified signal is next given to the choke for the further filtration and finally another capacitor is used to remove any remaining ripple from rectified signal. LC filter As shown in the figure. This circuit gives better filtration than the capacitor filter or the choke filter is used alone. In this method the ripple which is not removed by one filter will get removed by the next filter. This further because of its resembles to the Greek alphabet pi is known as PI filter. .This further will provide almost pure dc supply at the output.