What are power sources?

Power sources or power supplies are electronic circuits composed of different electronic components, that convert an alternating current (AC) into direct current (DC). Power supply devices are commonly known as AC-DC or AC to DC converters. Power sources are often known as power supply units (PSU) and are used almost in every electronic device that uses a DC power supply. Armature radio transmitters, receivers, and uninterruptible power supply (UPS) are some examples of power supply units. The requirement of UPS is as a crucial power supply unit for computers, which prevents data from getting corrupted in case of sudden power failure, it also prevents various components used in the computer motherboard from getting damaged. The similarity between these units is that all of them use AC as the main power source, which requires the power chord to plug into the wall power socket.

In the case of personal computers and other electronic devices such as projectors, and desktop 3D printers, all of them uses a switched-mode power supply (SMPS), these power supply units converts the AC power into an ATX (advanced technology extended) specification, that requires the output voltage from the power supply unit to be from 5 V to 12 V.

Here in this article, a brief introduction has been provided regarding the power supply unit and its major components that aids AC-DC conversion with a high degree of efficiency.

The different components of a power supply device are outlined here in this article.


A transformer is a device that works on the principle of mutual induction. The mutual induction states that a current-carrying conductor induces a magnetic field around itself, if a second conductor is brought adjacent to the first conductor, a similar magnetic field is generated that induces a current flow in the second conductor.

A transformer consists of two coil windings, the primary winding, and secondary winding, when an electric current is passed through the primary winding, it generates the same current into the secondary winding without changing the frequency of the AC. The primary windings and secondary windings do not have physical contact with each other.

The most important use of a transformer is to step up the voltage (low-voltage to high-voltage) and step down the voltage (high-voltage to low-voltage).

Internal architecture of a transformer
CC BY-SA 3.0 | Image credits: https://en.wikipedia.org | BillC


A rectifier is a device that converts the AC power supply into a DC supply. The AC is a fluctuating sinusoidal wave, the fluctuations are removed by the rectifier and make the current unidirectional. There are various kinds of components that are used in rectifier circuits like wet chemical cells, mercury-arc valves, silicon-based semiconductor switches, etc. The diodes are one of the most popular and commonly used components in a rectifier circuit. The process of AC rectification is basically done in two ways, they are:

  1. Half-wave rectification: In this type of rectification, the AC is converted into a pulsating DC voltage. During the positive cycle of the AC, the diode acts as a forward bias and allows a natural flow of current, but during the negative cycles of the AC, the diode behaves as a reverse bias and limits the negative cycles to pass the diode. Thus, the output is a current signal consisting of only positive cycles.
  2. Full-wave rectification: A full-wave rectifier circuit converts the entire input signal into a DC output. This type of circuit makes use of two diodes. During the positive half cycle of the AC, the first diode acts as a forward bias allowing only the positive cycles. During the negative half cycle of the AC, the second diode acts as a reversed bias, thus limiting the negative cycles and allowing the positive cycles. Thus a signal is received having continuous positive pulses.


The requirement of the filter is to block the noises and disturbances present in the input AC, which is also known as the ripple component. The DC output from the rectifier is smoothened by the use of a filter. The filter circuits are so designed, that it transforms the pulsating DC output into a smooth unidirectional DC output. The two of the most commonly used filters are capacitance filter and resistor-capacitor filter. The capacitance filter is a simple filter and provides good efficiency. The major function of the resistor-capacitor filter is to attenuate the AC by allowing most of the DC signal to pass through it. The resistor-capacitor filter can be a low pass filter as well as a high pass filter. It limits some of the AC frequencies and allows the required frequencies to pass through.

This circuit is the final circuit in the power source requirement, as the output from the PSU is a filtered and regulated DC signal.

Power factor

The power factor of an electrical circuit depends on the AC power system. The power factor can be stated as the ratio between the real power absorbed by the load and the apparent power that flows in the electrical circuit. Real power represents the capacity of the electrical current to do specific work and is given by the product of voltage and current. Whereas apparent current is the root mean square (RMS) voltage multiplied by RMS current. power factor value of less than 1 signifies that voltage and current are out of phase.

The concept of power factor is an important requirement in an electrical power system. Loads having low magnitude power factors demand more current consumption, this increases the requirement of using thicker wires to overcome the resistance effects, thus increasing the cost.

To overcome the drawbacks, the power correction factor is used. With the power correction factor, the efficiency of the distributions system is improved. For instance, inductors motors are linear load devices, the low power factor is improved by using a passive network of inductors, or sometimes capacitors are also used. Rectifiers, on the other hand, operate on non-linear loads and generally distorts the current drawn into them, this distortion can be overcome by using an active or passive power correction factor.

Context and Applications

This topic is extensively taught in many graduate and postgraduate degree courses of:

  • Bachelors of Technology in Electrical Engineering
  • Bachelors of Technology in Electrical and Electronics Engineering
  • Bachelors of Technology in Electronics and Communication Engineering
  • Bachelors of Technology in Mechanical Engineering
  • Masters in Technology (Power Systems)

Practice Problems

Q1. What is the requirement of a filter circuit in a PSU?

  1. It smoothens the DC output signal.
  2. It converts AC into DC.
  3. It increases the input voltage.
  4. None of these

Answer: Option a

Explanation: The requirement of a filter circuit in a PSU is to smoothen the DC output signal.

Q2. Which of the following parameter depends on the AC power system?

  1. Power line signal
  2. Power line signal and power factor
  3. Power factor
  4. None of these

Answer: Option c

Explanation: The power factor is one such parameter that depends on the AC power system.

Q3. What is the work of a step-down transformer?

  1. It converts low-voltage to high-voltage
  2. It converts high-voltage to low-voltage
  3. It converts low-power signal to high-power signal
  4. None of these

Answer: Option b

Explanation: The work of a step-down transformer is to convert high-voltage into low-voltage.

Q4. Which of the following converts an input AC signal into pulsating DC output signal?

  1. Limited power source
  2. Linear power supplies
  3. Half-wave rectifier
  4. Transformer

Answer: Option c

Explanation: A half-wave rectifier converts the input AC power source into a pulsating DC output.

Q5. What is the full form of UPS?

  1. Uninterruptible power supply
  2. Uninterrupted power supply
  3. Uninterrupted power source
  4. Uninterruptible power source

Answer: Option a

Explanation: The abbreviation for UPS is an uninterruptible power supply.

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