Working of adaptor

Adaptors are devices which convert the AC supply to DC low voltage suitable for the devices. Few of the devices were we require adaptors are the video games, small DC motors, etc. It is very interesting to find what is actually inside an adaptor and how does it convert AC voltage (220v in India) to DC voltage in range of 0 -12v.

All the adapter have 3 important parts inside them:

• Step down Transformer: The step down transformer steps down AC voltage to a small value say 12v.


• Full Wave Rectifier: The full wave rectifier flips the negative portion of the sinusoidal secondary voltage to positive side. It has four diode connected in the fashion as shown in the circuit diagram. The input terminals are connected to the point 1 and 3 while the output is obtained from 2, 4. Rectifier uses the property of the diodes that they allow the flow of current through them only in one direction. When a positive cycle of current flows, the current leaves from D1 and return from D3. D2 and D4 do not allow the flow of current. When the negative cycle arrives then D2 and D4 are conducting and D3 and D1 become non-conducting. What we observe is that the current always leaves through point 2  and return from point 4 or we can say that 2 is always at higher potential than 4. This means that the current waveform for the negative cycles must have got reversed. The waveform we obtain in CRO confirms this.

• Capacitive Filter: The current waveform which we obtained is though unidirectional but sinusoidal in nature which is not proper for a DC application. To convert it close to an ideal DC we need to filter out it out. For thiswe generally use a capacitor of large value. Capacitor has a property of storing the energy (or we can say getting charged) when its terminal voltage is less than the supply voltage and getting discharge (or act as voltage source) when the source voltage drops below its terminal voltage. Let us understand this by studying the voltage waveform. In the rising side of the voltage waveform, the capacitor gets charged upto maximum voltage Vm. On the sloping down side, as the voltage becomes less than its terminal voltage, it expands its energy, thus preventing such a fast fall in the voltage. Voltage would have fallen by a very small amount by the time next cycle starts and it gets charged again. Thus we get an average voltage very close to DC voltage across its terminal.