All electronic circuits need a power source to work.
For electronic circuits made up of transistors and/or ICs, this power source must be a DC voltage of a specific value.
A battery is a common DC voltage source for some types of electronic equipment especially portables like cell phones and iPods.
Most non-portable equipment uses power supplies that operate from the AC power line but produce one or more DC outputs.
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Power Supply Characteristics
The input is the 120 volt 60 Hz AC power line. Alternately, the input may be 240 volt AC.
The power supply converts the AC into DC and provides one or more DC output voltages.
Some modern electronic circuits need two or more different voltages.
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Components of a Power Supply
Main circuits in most power supplies.
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Power Supply Working
The AC line is first passed through a low pass filter of the form.
This eliminates noise on the AC line from bothering the power supply circuits and prevents unwanted signals from the power supply from being transferred back into the AC line where they might interfere with other equipment.
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Transformer
A transformer is commonly used to step the input AC voltage level down or up. Most electronic circuits operate from voltages lower than the AC line voltage so the transformer normally steps the voltage down by its turns ratio to a desired lower level.
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Rectifier
The rectifier converts the AC sine wave into a pulsating DC wave.
There are several forms of rectifiers used but all are made up of diodes.
Rectifier types and operation will be covered later.
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Filter
The rectifier produces a DC output but it is pulsating rather than a constant steady value over time like that from a battery.
A filter is used to remove the pulsations and create a constant output.
The most common filter is a large capacitor.
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Regulator
The regulator is a circuit that helps maintain a fixed or constant output voltage.
Changes in the load or the AC line voltage will cause the output voltage to vary.
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DC-DC Converter
Most modern power supplies also contain one or more DC-DC converters
Modern electronics often demand different voltages to function.
A DC-DC converter changes one DC voltage to another, higher or lower DC voltage.
A DC-DC converter is used with a power supply to prevent the need for a second AC-DC supply.
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Bridge Rectifier
Another widely used rectifier is the bridge rectifier. It uses four diodes.
This is called a full wave rectifier as it produces an output pulse for each half cycle of the input sine wave.
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Ripple
A small variation occurs in the DC because the capacitor discharges a small amount between the positive and negative pulses. Then it recharges. This variation is called ripple.
The ripple can be reduced further by making the capacitor larger.