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power supply
Posted Date: 27 Apr 2008 Resource Type: Articles/Knowledge Sharing Category: Education
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Posted By: C.Agnes Member Level: Silver
Rating:  Points: 6
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POWER SUPPLY:
All electronic circuits need dc power supply either from battery or power back units. It may not be economical and convenient to depend upon battery power supply. Many electronic equipment contains circuits which convert the ac supply into dc voltage at the required level. The unit containing these circuits is called the Linear Mode Power Supply (LPS). In the absence of ac main supply the dc supply from battery can be converted into required ac voltage which may be used by computer and other electronic systems for their operation. Also, in certain applications dc to dc conversion is required. Such a power supply unit that converts dc into ac or dc is called Switched Mode Power Supply (SMPS).
1. Linear Mode Power Supply: ac/dc power supply converter
2. Switched Mode Power Supply:
a) dc/dc power supply converter
b) dc/ac power supply converter
Linear Mode Power Supply
An ac/dc power supply converts ac mains (230V, 50Hz) into required dc voltages (and is found in all mains operable system). The basic building blocks of the linear power supply are shown in figure 5.
Figure : Basic building block of Linear Mode Power Supply
WORKING PRINCIPLE:
AC Input:
The AC supply is given to the transformer. Here we use 230 V AC power as input.
Transformer:
The ac voltage, typically 220Vrms, is connected to a transformer. Here we are using a step down transformer, which steps that ac voltage down to the level of the desired dc output. The potential transformer in our circuit diagram will step down the power supply voltage (0-230V) to (0-5V) and (0-12V) level. Then the secondary of the potential transformer will be connected to the precision rectifier.
Figure : Circuit diagram (Power supply)
Working principle of Transformer:
The transformer works on the principle of faradays law of electromagnetic inductions.
The core is built up of thin laminations insulated from each other in order to reduce eddy current loss. The winding are unguarded from each other and also from the care. The winding connected to the load is called the secondary winding. The high voltage winding encloses the low voltage.
Let us say that transformer has N1 turns in its primary winding and N2 turns in its secondary winding. The primary winding is connected to a sinusoidal voltage of magnitude V1 at a frequency FH2. A working flux is set up in magnetic core. The working flux is sinusoidal as the applied voltage is sinusoidal. When these fluxes link the primary and the secondary winding, EMF is induced in them. The EMF induced in this is called the self-induced EMF and that induced in the secondary is the mutually induced EMF. These voltages will have sinusoidal waveform and the same frequency as that of the applied voltage. The currents, which flow in the primary and secondary circuits, are respectively I1 and I2.
Rectifier:
Then the secondary of the potential transformer will be connected to the precision rectifier. It is circuit in which employs four diodes to convert AC voltage into pulsating DC voltage. The advantage of using precision rectifier is it will give peak voltage output as DC, rest of the circuits will give only RMS output.
Bridge rectifier:
When four diodes are connected as shown in figure 6, the circuit is called as bridge rectifier. The input to the circuit is applied to the diagonally opposite corners of the network, and the output is taken from the remaining two corners.
Let us assume that the transformer is working properly and there is a positive potential, at point A and a negative potential at point B. The positive potential at point A will forward bias D3 and reverse bias D4.
The negative potential at point B will forward bias D1 and reverse D2. At this time D3 and D1 are forward biased and will allow current flow to pass through them. D4 and D2 are reverse biased and will block current flow.
The path for current flow is from point B through D1, through D3, through the secondary of the transformer back to point B
One-half cycle later, the polarity across the secondary of the transformer reversing, forward biasing D2 and D4 and reverse biasing D1 and D3. Current flow will now be from point A through D4, through D2, through the secondary of T1, and back to point A. The current flow through load is always in the same direction. In flowing through load this current develops a voltage.
One advantage of a bridge rectifier over a conventional full-wave rectifier is that with a given transformer the bridge rectifier produces a voltage output that is nearly twice that of the conventional full-wave circuit.
Filter:
A full-wave rectified voltage from the bridge rectifier is initially filtered by a simple capacitor filter to produce a dc voltage. This resulting dc voltage usually has some ripple or ac voltage variation.
Voltage Regulators:
A regulator circuit removes the ripples in the filter output and also remains the same dc value even if the input dc voltage varies or the load connected to the output dc voltage changes. This voltage regulation is usually obtained using one of the popular voltage regulator IC units.
Voltage regulators comprise a class of widely used ICs. Regulator IC units contain the circuitry for reference source, comparator amplifier, control device, and overload protection all in a single IC. The internal construction of the IC is somewhat different from that described for discrete voltage regulator circuits. The external operation is much the same. IC units provide regulation of either a fixed positive voltage, a fixed negative voltage, or an adjustably set voltage. The regulators can be selected for operation with load currents from hundreds of milli amperes to tens of amperes, corresponding to power ratings from milli watts to tens of watts.
Three-terminal voltage regulators:
Figure 7 shows the basic connection of a three-terminal voltage regulator IC to a load. For a selected regulator, IC device specifications list a voltage range over which the input voltage can vary to maintain a regulated output voltage over a range of load current. The specifications also list the amount of output voltage change resulting from a change in load current (load regulation) or in input voltage (line regulation).
Figure : Voltage regulator
The series 78 regulators provide fixed regulated voltages from 5 to 24 V. Figure 7 shows how one such IC, a 7812, is connected to provide voltage regulation with output from this unit of +12V dc. An unregulated input voltage Vi is filtered by capacitor C1 and connected to the IC’s IN terminal. The IC’s OUT terminal provides a regulated + 12V which is filtered by capacitor C2 (mostly for any high-frequency noise). The third IC terminal is connected to ground (GND). The input voltage may vary over some permissible voltage range and the output load may vary over some acceptable range. Then the output voltage remains constant within specified voltage variation limits. These limitations are spelled out in the manufacturer’s specification sheets. A table of positive voltage regulated ICs are provided in table 1.
Table 1: Positive Voltage Regulators in 78XX series
IC Part Output Voltage (V) Minimum Vi (V)
7805 +5 7.3
7806 +6 8.3
7808 +8 10.5
7810 +10 12.5
7812 +12 14.6
7815 +15 17.7
7818 +18 21.0
7824 +24 27.1
A fixed three-terminal voltage regulator has an unregulated dc input voltage, Vi, applied to one input terminal, a regulated dc output voltage, Vo, from a second terminal, with the third terminal connected to ground.
The series 78 regulators provide fixed positive regulated voltages from 5 to 24 volts. Similarly, the series 79 regulators provide fixed negative regulated voltages from 5 to 24 volts.
DC Output:
The required DC output voltage is obtained from this block.
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