DC Power Supply
The majority of electric appliances (e.g. TV, radios, stereo systems, computers, VCR, CD players, lab equipment, etc.) and circuits require a DC source for operation. Dry cells and batteries are a form of DC source. Their advantage is that these are portable, (i.e. they can easily be taken from one place to the other) and are ripple-free (i.e. the output received from these does not contain any AC contents at all), however, their voltages are low, they have to be changed frequently and are costly compared to traditional DC power supplies. Since the biggest and cheapest source of power is domestic AC supply, therefore, conversion of alternating voltages (usually 230-volt RMS) to DC voltages (normally low values), proves more valuable. This process of converting AC voltage to DC voltage is called rectification, and it is completed with the help of the following devices.
- Voltage Regulator Circuit
Thus, if the aforementioned 3 elements are combined together, a DC power supply comes into being. In other words, a circuit inside some electronic device, which converts AC input voltage to nearly a pure DC output voltage and which supplies power consistently for operating a circuit or system, is called a power supply. Remember, the power supply is of the following 2 types:
Unregulated Power Supply
It is a power supply wherein DC terminal voltages are affected or changed excessively as a result of a change of load. Its terminal voltages lessen as soon as load or load current increases.
Regulated Power Supply
It is a DC power supply, the DC terminal voltages of which remain nearly consistent despite variations in load current. An unregulated power supply can be converted to a regulated power supply by including a voltage regulating circuit.
DC Power Supply Circuit
The block diagram shape of a DC power supply circuit has been depicted in figure1. A complete power supply unit basically consists following 5 parts.
Its work is to step up or step-down AC voltage according to the requirements. Thus, solid-state gadgets or circuits, (which operate on a DC power supply), change voltage levels as per requirements. Normally, a step-down transformer is used for this purpose, through which AC voltage taken from an ordinary power supply, is stepped down to the input level of the rectifier. Another advantage of the transformer is that it provides isolation from the supply line i.e. instead of connecting electronic items directly with the supply line, they are affixed with the secondary of a transformer. Thus, the transformer mutually isolates the supply line and electronic devices, which is very important from a safety point of view.
It is a circuit, in which one or more than one diode has been mounted in order to convert AC voltage to a pulsating DC voltage. The selection of a rectifier depends on the load attached to the power supply. However, for greater DC output voltage and efficiency purposes, mostly bridge rectifiers are used. However, in case there is no possibility of some damage due to ripples on output, half-wave rectifiers can also be used.
The function of this part of the circuit element is the removal of ripples or AC contents from the output voltage supplied through the rectifier. Practically, no filter can be ripple free like a DC battery or which can provide output voltage devoid of any AC contents. However, through a careful selection of a filter and increasing the number of its sections, ripple values found in the output of a rectifier circuit can be minimized substantially, so that the power supply operates in an excellent manner. Remember, selecting the values of an inductor and capacitor inside a filter, depends on the calculation of its load current and voltage.
The basic function of a voltage regulator is to keep the terminal voltages of a DC supply always constant, irrespective of any changes in load or AC input voltages received by the transformer.
Normally, Zener diodes and transistors are used for the purpose of voltage regulation. It has to be inculcated in mind that the achievement of 100% constant voltage through a voltage regulator is practically unattainable. However, negligible changes in the majority of tasks can be acceptable. Since electronic gadgets are not capable of sustaining too many variations in power supply, therefore electronic instruments and circuits can relatively safely be operated via nearly constant output from a regulator circuit.
Its function is the provision of multiple DC voltages to diverse electronics circuits according to their requirements. Voltage dividers consist of a series of several resisters placed parallel to voltage regulators’ output terminals, due to which they function at a variety of DC levels. Thus, there is no need to provide different DC supplies to different electronic circuits.
Remember, from the list of devices mentioned above, only the transformer and rectifier are essential for the conversion of AC to DC. Filters, voltage regulators, and voltage dividers are just required to improve the output of a DC power supply. Though, apart from battery charging and operating small DC motors, these components are essential for a number of applications.
Solid State Power Supply
A complete solid-state power has been illustrated in figure2. From left to right, it consists: of a transformer with a current limiting resistor R1, a rectifier diode capable of full-wave rectification, a pie type filter, a transistor series voltage regulator, and a voltage divider.
As is apparent from the diagram, unregulated AC voltages are supplied on the full-wave rectifier’s input through a transformer. Rectifier’s output goes to CLC or a pie filter, which it passes on to a voltage regulator composed of one transistor. Thus, regulated DC supply parallel to voltage divider resistance RB, starts. Practically, the output is approximately ripple-free.
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