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Friday, 18 January 2019

Power Supply Characteristics

Any power supply ideally must have the following characteristics

1. The magnitude of the output must be presented accurately and precisely.
2. The ripple content of the output must be zero.
3. The output voltage must have high stability irrespective of load current and line voltage changes.
4. The source impedance must be zero.
5. It should be portable.

Sources of DC Voltage: 

The following is the list of DC voltage sources and shows the application of the type of source:


Source
Application
Primary cells (Dry cell)
For use in low power applications like radios, Calculators, Multimeters, Tape recorders, etc.
Secondary cells (Wet Batteries)
For use as backup batteries for PA amplifiers, Inverters, UPS etc
AIkaline batteries
Low power sources mostly with portable instruments.
Nickel Cadmium cells
For large load currents.
Lithium Sulphur dioxide
For large load current requirement
Lithium iodine cells
For large load current requirement
DC Voltage standard
For calibration of instruments
The Laboratory Power supply
To conduct laboratory experiments
Line operated Power supplies
For low medium and High power applications
Uninterrupted Power supplies
Computer centers, telephone exchanges, transmitters, Microwave stations, repeaters etc


Power supply:

The commonly used power supply, with electronic circuits is the line operated power supply. In this the mains voltage is utilized in a transformer or directly, to work a rectifier. The rectified output, will be filtered using a filter circuit, that eliminates the ripple content of the rectified output. The output of the power supply will be pulsating unidirectional current. The block diagram of the power supply is shown in Figure.
Block Diagram of a line operated power supply

A generalized block diagram of the power supply stage is shown in Figure. The transformer isolates the load from the mains power supply ground. It provides either a step up voltage or step down voltage in its secondary. Taps or center tap or both can be provided at the secondary of the transformer.

A rectifier converts the alternating current to pulsating unidirectional current. Mostly Full wave rectifier, or bridge rectifier circuits are used as rectifier circuits.

The alternating component of the rectified voltage is called the ripple voltage. The ripple should not be present and it should be minimized. A filter circuit is used to filter off the ripple contained in the output of the rectifier.

The bleeder resistor is connected across the output terminals of the rectifier, to present a light load on the power supply. It helps in maintaining constant DC voltage in the output of the rectifier. Further it can be of a potential divider type to tap the different voltages required as per the circuit conditions.

The magnitude of the output voltage and the amount of current that can be drawn from the power supply depend on the design of the power supply and the demand of the circuit.

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