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.
- 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
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.
Transformer + Rectifier
Transformer + Rectifier + Smoothing
Transformer + Rectifier + Smoothing + Regulator
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.
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.
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