Understanding Power supplies

Types of Power Supply

There are many types of power supply. Most are designed to convert high voltage AC mains electricity to a suitable low voltage supply for electronics circuits and other devices.
A power supply can by broken down into a series of blocks, each of which performs a particular function.

For example a 5V regulated supply:

understanding power supply
Each of the blocks is described in more detail below:
  • Transformer – steps down high voltage AC mains to low voltage AC.
  • Rectifier – converts AC to DC, but the DC output is varying.
  • Smoothing – smooths the DC from varying greatly to a small ripple.
  • Regulator – eliminates ripple by setting DC output to a fixed voltage.

Power supplies made from these blocks are described below with a circuit diagram and a graph of their output:

  • Transformer only
  • Transformer + Rectifier
  • Transformer + Rectifier + Smoothing
  • Transformer + Rectifier + Smoothing + Regulator

Dual Supplies

Some electronic circuits require a power supply with positive and negative outputs as well as zero volts (0V). This is called a ‘dual supply’ because it is like two ordinary supplies connected together as shown in the diagram.

Dual supplies have three outputs, for example a ±9V supply has +9V, 0V and -9V outputs.

Transformer only

The low voltage AC output is suitable for lamps, heaters and special AC motors. It is not suitable for electronic circuits unless they include a rectifier and a smoothing capacitor.

Transformer + Rectifier

The varying DC output is suitable for lamps, heaters and standard motors. It is not suitable for electronic circuits unless they include a smoothing capacitor.

Transformer + Rectifier + Smoothing

The smooth DC output has a small ripple. It is suitable for most electronic circuits.

Transformer + Rectifier + Smoothing + Regulator

The regulated DC output is very smooth with no ripple. It is suitable for all electronic circuits.

Transformer

transformer circuit symbol
Transformers convert AC electricity from one voltage to another with little loss of power. Transformers work only with AC and this is one of the reasons why mains electricity is AC.

Step-up transformers increase voltage, step-down transformers reduce voltage. Most power supplies use a step-down transformer to reduce the dangerously high mains voltage (230V in UK) to a safer low voltage.
The input coil is called the primary and the output coil is
called the secondary. There is no electrical connection between the two coils, instead they are linked by an alternating magnetic field created in the soft-iron core of the transformer. The two lines in the middle of the circuit symbol represent the core.
Transformers waste very little power so the power out is (almost) equal to the power in. Note that as voltage is stepped down current is stepped up.

transformerThe ratio of the number of turns on each coil, called the turns ratio, determines the ratio of the voltages. A step-down transformer has a large number of turns on its primary (input) coil which is connected to the high voltage mains supply, and a small number of turns on its secondary (output) coil to give a low output voltage.

  turns ratio = Vp  = Np    and power out = power in
Vs Ns Vs × Is = Vp × Ip
Vp = primary (input) voltage
Np = number of turns on primary coil
Ip  = primary (input) current
Vs = secondary (output) voltage
Ns = number of turns on secondary coil
Is  = secondary (output) current
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