Design and Selection of Components in Power Supply

The power supply is one of the most important parts of any electronic device. It is responsible for providing the correct amount of power to the electronic components in the device. The design of the power supply must take into account the power requirements of the electronic components, the efficiency of the power supply, the cost of the power supply, and the size of the power supply.

A power supply is a device that supplies electrical energy to one or more electric loads. The term is most commonly used to refer to devices that convert one form of electrical energy to another, though it may also refer to devices that convert energy from another form, such as solar panels and wind turbines.

The three main categories of power supplies:

  1. uninterruptible power supplies (UPS)
  2. standby power supplies (SPS)
  3. off-line power supplies (OPF). 

UPS provides backup power when the mains power fails, while SPS provides power to devices that do not require continuous power, such as computers. OPF is used when there is no mains power.

There are two main types of power supplies:

  1. linear power supplies
  2. switch-mode power supplies (SMPS).

Linear power supplies use a linear regulator to convert the input voltage to the desired output voltage. SMPS use a switch to regulate the input voltage. SMPS are more efficient than linear power supplies, but they are more complex and expensive.

The selection of a power supply depends on a number of factors, including the type of load, the required output voltage and current, efficiency, size, and cost.

Regulated Power Supply

  • A regulated power supply is an electrical device that supplies power to an electronic device or circuit. The power supply regulates the voltage and current to the device or circuit.
  • There are three main parts to a regulated power supply: the transformer, the rectifier, and the regulator.
  • The transformer steps down the voltage from the AC power source to the level needed by the device or circuit. The rectifier converts the AC voltage to DC voltage. The regulator controls the voltage and current going to the device or circuit.


  • Works on faradays Law of mutual Induction.
  • It states that when a change takes place in the magnetic flux which is linked with a circuit, an electromotive force current will induce in the circuit.
  • Primary and secondary coils are present.
  • Used to step up or step down the supply voltages.
  • It can provide isolation from the supply line.
  • Prevents unwanted electrical noise signals on the power line.


  • Converts sinusoidal ac voltage into either positive or negative pulsating dc.
  • P-N junction diodes can be used for rectification purposes.
  • It needs one, two, or four diodes.
  • Rectifiers can be half-wave or full-wave ( center tap or bridge type).
  • The output voltage has pulsating nature.
  • Filter circuits are used to reduce ac components from the output voltage of the rectifier.

Transformer + Rectifier + Smoothing (filter)

The transformer+rectifier+smoothing block is a common circuit used to provide power to electronic devices. The transformer steps up the voltage from the AC power source, the rectifier converts the AC to DC, and the smoothing filter produces a smooth DC output.

Transformer + Rectifier + Smoothing + Regulator (voltage regulator)

A transformer is an electrical device that converts alternating current (AC) from one voltage level to another. A rectifier is an electrical device that converts alternating current (AC) to direct current (DC). A smoothing capacitor is an electrical device that helps to smooth out the ripples in a direct current (DC) signal. A voltage regulator is an electrical device that helps to maintain a constant voltage level.

Voltage regulator

A voltage regulator is an electrical regulator designed to automatically maintain a constant voltage level. It is often used in electronic devices to protect against power surges and overvoltage.

  • Can be constructed from the Zener diode/discrete transistors/integrated circuits.
  • The main function is to maintain constant output dc voltage.
  • If there are any ac ripples that are not removed by the filter then the voltage regulator removes them.

Regulated Power Supply

A regulated power supply is an electronic device that supplies electric energy to an electrical load. The main purpose of a regulated power supply is to maintain a constant output voltage level, regardless of changes in the load current or input voltage level.

Important characteristics of power supply are as follows.

  • The required output dc voltage.
  • The regulation
  • The average and peak currents in the diode.
  • The peak inverse voltage of each diode.
  • The ripple factor

Component Requirements:

  1. Transformer 12-0 1  No.
  2. Diode 1N4007  4 Nos.
  3. Capacitor 1000µf/ 25V 1 Nos.
  4. Capacitor 1µf/ 25V 1 Nos.
  5. Voltage Regulator IC 7805 1  No. 

Why is Resistance Required?

Fig. (a) shows that if we connect the lead of 2.5 V directly to the 10 V battery without resistance then it will damage.

Fig.(b) shows the lead with resistance.

In order for a current to flow in a circuit, there must be a potential difference, or voltage, across the circuit. The potential difference can be provided by a battery or other power source. The current will flow from the point of highest potential to the point of lowest potential. If there were no resistance in the circuit, the current would flow unchecked and could potentially damage the components in the circuit. The resistance acts to limit the current and protect the circuit.

Effect Of Resistance On LED

  1. The current through the LED is 915mA while the absolute maximum is  20.0mA therefore the LED blows.
  2. Current through the LED is61.9mA while the recommended maximum is 20.0mA the usable lifetime of the LED may be reduced. 
  3.  The resistor limited the current through the LED. Therefore the LED is working at its optimum.


  1. A resistor is a two-terminal electronic component that resists the flow of electrons through it.
  2. The produced voltage is proportional to the electric current through it.
  3. In accordance with Georg Simon Ohms law:

                                   V= IR

  1. Resistors are used in amplifiers as loads for active devices.

  2. They act as filters when used in combination with capacitors.

6 . Resistor has its resistance value in ohms and power 

dissipating capacity in watts.

How Resistance  is works

  • Resistance in the circuit works the same as a rock in a creek.
  • It reduces the flow of electrons just like the rocks reduce the flow of water.
  • The figure shows how the flow of electrons reduces.

What Is Resistance

  1. If we make an analogy to water flow in pipes, the resistance is bigger when the pipe is thinner, so water flow is decreased.
  2. Resistance is an electrical quantity that measures how the device and material reduce the electrical current flow through it.
  3. Resistance is the quantity that is measured in ohms.


  • There are as many types as there are many applications.
  • Employed in amplifiers as loads for active devices, in bias networks, and as feedback elements.
  • Establish time constants when used in combination with capacitors.
  • To reduce voltage they are used in power circuits.
  • Pull up and pull down resistors in logic circuits.
  • Each resistor has two main characteristics that are its resistance value in ohms and its power dissipating capacity in watts.

Types Of Resistor

Fixed Resistor

  • Carbon Composition
  • Film Type
    • Carbon Film
    • Metal Film
  • Wire wound Resistor

Variable Resistor or adjustable resistors.s

  • Potentiometer
  • Preset
  • Trimmer

Special Resistors 

Special Resistors are designed to perform a specific function in specific applications. There are many different types of Special Resistors such as; Surface Mount Resistors, Chip Resistors, Thick Film Resistors, Wire Wound Resistors, Carbon Film Resistors, Metal Film Resistors, Fusible Resistors, Non-Inductive Resistors, High Voltage Resistors, High-Temperature Resistors, Flameproof Resistors, Precision Resistors, and many more.

Special Resistors are used in a variety of applications such as; automotive, aerospace, military, medical, and many more. It includes LDRs(light-dependent Resistors), varistors, or VDRs (voltage-dependent resistors).

Types Of Resistor – Carbon Composition

    • They are fixed-value resistors.
    • Consist of a solid cylindrical resistive element with embedded wire leads or metal end caps to which the lead wires are attached.
  • The body is protected by paint or plastic.
  • Composition of carbon deposit on it.
  • Tolerance: ± 5- ± 20%
  • Power rating 1/4W - 2 W

Types Of Resistors – Film type

A film resistor is a resistor where the resistive element is made of a thin film of material deposited on an insulating substrate. They are made by vapor deposition of

 the resistive material onto the substrate. The deposition can be done by any of a number of processes, including sputtering, evaporation, and chemical vapor


Film Of Metal/Composition Deposit On It.

Tolerance:- ± 1 -  ± 3%

Power Rating 1/4w - 2 W

Types Of Resistor - Wire Wound

A resistor is an electrical component that limits or regulates the flow of electrical current in an electronic circuit. Resistors can also be used to provide a specific voltage level for an active device such as a transistor. A wire-wound resistor is a type of resistor where the resistive element is made of a long wire wound around a central core. The main advantage of a wire-wound resistor is its ability to dissipate large amounts of heat without damage.

Nichrome wires are wounded on the china tube

Wattage Rating

A resistors wattage rating indicates the maximum amount of power that the resistor can safely dissipate.

Table – Colour Coding Of Resistor

The table below shows the color coding of resistor bands. 

| Band | Colour | 


  1. | Black | 
  2. | Brown | 
  3. | Red | 
  4. | Orange | 
  5. | Yellow | 
  6. | Green | 
  7. | Blue | 
  8. | Violet | 
  9. | Grey | 
  10. | White |

Colour Coding Of Resistance Bands

1   0 X 10+5% Ω

Brown Black  Yellow Gold

Purple = 10 

lbs Green = 15 

lbs Blue = 20 

lbs Black = 25 lbs

  1. The first band  is the first significant figure of component value
  2. The second band is the second significant figure
  3. The third band is the decimal multiplier
  4. Fourth if present indicates tolerance of value in percent (no color means 20%)