A two diode circuit is able to provide full wave rectification when used with a centre tapped transformer.
This two diode format for the full wave rectifier uses a centre tapped transformer and was widely used when thermionic valves / vacuum tubes were used. Today with semiconductor diode bridge rectifiers, the circuit is not as commonly seen.
Two diode full wave rectifier circuit
The basic full wave rectifier circuit can be seen to use two diodes and a centre tapped transformer.
This circuit is very easy to implement, although it does need a centre tapped transformer.
The current flow within the circuit can be seen from the diagram below. This is useful in seeing how the circuit operates and how it is not as efficient in terms of usage of the transformer as circuits such as the bridge full wave rectifier.
Looking at the circuit it can be seen that on one half of the cycle, current passes in one half of the transformer and passes through the diode. The other diode is reverse biased and does not conductor. Then for the other half of the cycle, the other side of the circuit comes in to play.
Issues with this form of full wave rectifier
Looking at the current flow diagram, it can be seen that current each half of the secondary winding is only used of half the cycle. This makes for very inefficient use of the transformer in terms of cost and resources.
- Transformer output voltage half what it could be: Utilising a centre tap in the transformer means that only half the full voltage across the two halves of the wind together can be utilised.
- Increased heating losses: As a result of the way in which the two diode full wave rectifier circuit operates, each half of the transformer is used for half of the time. This means that the current through each winding is twice what it would be if a true half wave rectifier such as a bridge rectifier were used. As heating losses are equal to the square of the current times the resistance, this means that there is four times the heat dissipated for half the time. Over the complete cycle this means that there is twice the heating loss of an equivalent full wave bridge rectifier circuit..
- Increased transformer cost: Each half of the secondary winding needs to be able to provide the full voltage and also at a high current level. This means that the transformer will be significantly more costly than one requiring a standard secondary without the centre tap.
As a result of the points noted above, to create a full wave bridge rectifier using the two diode full wave rectifier system would require a transformer √2 times the size of the one needed for the bridge rectifier. This would cost more as well as being heavier and more bulky. With bridge rectifiers now costing very little, this is the preferred option for most applications.
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