Power Supplies, Physics tutorial

Alternating current power supply:

The mains electricity supply is supplied at 220 volts AC, though in some parts of world in the past, mains electricity was supplied as Direct Current while it was supplied as Alternating Current in others.

Major problem with direct current is that transformers can't be utilized for voltage increment or reduction. With Alternating Current, long distance transmission at very high voltages makes it possible to transmit energy sustaining minimal transmission losses.

In the Alternating Current Power Supply, mains voltage is supplied through electricity outlet at 220 volts 50 Hertz. If equipment to be powered directly at 220 volts AC then it should pass through transformer to either increase or decrease the voltage.

Alternating Current from public utility company is usually not a pure sinusoidal waveform at 220 volts AC but suffers from a number of distortions to pure waveform. Supply may have impurities like electrical noise that may take place within any band of electromagnetic spectrum able to transmit by power grid.

Frequency of alternating current frequently deviates from 50 hertz as power grid frequency is influenced by loading. The common occurrence is the situation where voltage output is higher or lower than 220 volts; and occasionally there is total absence of electricity supply or power outage.

Other forms of electrical noise comprise spikes and other transients that may be during to inductive or capacitive (reactive) loading of AC circuit.

Basic direct current power supply:

In mains-supplied electronic systems AC input voltage should be converted in Direct Current voltage with correct value and degree of stabilization. In these basic configurations peak voltage across load is equal to peak value of Alternating Current voltage supplied by the transformer's secondary winding.

Above is the basic half wave rectifier while illustration below is full wave rectifier circuit that uses centre tapped transformer both for isolation and for voltage transformation.

Output ripple produced by the simple rectifier circuits is too high for most applications being only satisfactory for such applications as driving small motors or lamps. Output waveform is deeply enhanced when filter capacitor is added after rectifier diodes.

This circuit is the full wave rectifier and utilized only two rectifier diodes as it is designed around the centre tapped transformer. Functional equivalent of this circuit that doesn't use centre tapped transformer should use bridge rectifier circuit comprising four rectifier diodes to get full wave rectification.

1429_Full Wave Bridge Rectifier with Filter Capacitor.jpg

Output voltage waveform is enhanced significantly with full wave rectifier incorporating filter capacitor. Addition of the filter capacitor generates continuous voltage curve where in region stated by line b-c, filter capacitor wholly supplies load current as rectifier diode is reverse biased in this region. The slope of this line increases as the load current increases bringing point c lower and increasing the ripple.

146_Half Wave Rectifier Filter Waveform.jpg

When no load current is drawn from supply DC output voltage is equal to peak value of rectified Alternating Current voltage. It will be simple for you to comprehend that value of voltage ripple obtained is directly proportional to load current and inversely proportional to filter capacitor value.

Linear power supply:

Voltage output generated by unregulated power supply is subject to loading load and variations in supply voltage and for critical electronics applications, linear regulator are utilized to set voltage to precise value that is stabilized against fluctuations in input voltage and load.

Regulators decrease ripple and noise in output while they frequently give current limiting that protects power supply and joined circuit from overload.

Broad classification of power supplies that are utilized in electronic devices are Linear Power Supplies and Switching Power Supplies. Linear Power Supplies have the major benefit of simplicity of design while major drawback is their size as their current handling capacity increases.

Adaptable linear power supplies are common laboratory and service shop test equipment, permitting output voltage to be adjusted over a range. For instance, bench power supply utilized by circuit designers may be adaptable up to 30 volts and up to 5 amperes output. Some can be driven by the external signal, for instance, for applications needing a pulsed output.

Switching power supply:

These power supplies don't work on same principle as Linear Power Supplies as generally, Alternating Current input at mains voltage, is rectified directly to get the Direct Current voltage without using mains transformer. Direct current voltage is then rapidly switched on and off by electronic switching circuitry at high frequency - resulting in the small, light weight, and inexpensive power supply.

Commercial switching power supplies are always regulated and when the properly insulated high-frequency transformer is utilized, output will be electrically isolated from mains voltage that is vital for operator safety.

It is convenient modular device which converts mains input AC voltage to DC voltages required by personal computer and ever since introduction of IBM PC/XT, many Personal Computer kinds have emerged that differ by their structure, form factors, connectors and volt/amp ratings. All invariably rely on Switching Power Supply Units for their operation. The standard Personal Computer Power Supply Unit produces +5V, +3.3V, +12V1, +12V2, -12V and standby 5V. Additional DC-DC converters step down 12V to CPU core voltage and other low voltages required for motherboard components, peripherals, floppy drive, and serial ATA connectors.

Power supplies for computers uses switching mode technology and are energy star compliant. This means that computers use less than 10% of rated power in standby mode while in the active mode efficiency of cheap models used to be 60-70%.

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