Two Diode Full Wave Rectifier Circuit

A two diode version of a full wave rectifier circuit can be used on a number of occasions to make use of both halves of an alternating waveform.

Diode Rectifier Circuits Include:
Diode rectifier circuits Half wave rectifier Full wave rectifier Two diode full wave rectifier Full wave bridge rectifier Synchronous rectifier

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. It saved on the number of diodes and hence the number of valves / tubes required, thereby saving considerable cost - additional valves / tubes added significant amounts to the production costs.

Today with semiconductor diode bridge rectifiers, the circuit is not as commonly seen as bridge rectifiers only require a single winding on the transformer, and the cost of the centre tap with additional windings outweighs the cost of the two additional diodes required for a bridge rectifier.

Two diode full wave rectifier circuit

The electronic circuit design for the two diode full wave rectifier uses a centre tapped transformer with the centre tap taken to the ground or 0V line and then one diode is connected to each outer connection of the transformer. The two anodes or two cathodes are connected together dependent upon the polarity of the required output.

The basic full wave rectifier circuit design can be seen in the diagram.

Full wave rectifier using two diodes and a centre tapped transformer

This circuit is very easy to implement, although it does need a centre tapped transformer. There are only two electronic components, i.e. the transformer and two diodes and these are easily wired in the circuit.

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.

Current flow in a full wave rectifier using two diodes and a centre tapped transformer — Full wave rectifier using two diodes: current flow

This electronic circuit design for a full wave rectifier operates so that the two diodes are connected to a single load resistance and then each diode taking it in turn to supply current to the load, one for each half of the cycle.

Looking in a little more depth, it can be seen that on one half of the cycle, current passes in one half of the transformer and passes through the first 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 with the second diode conducting and the first diode being reverse biassed.

Diode peak inverse voltage rating

One important issue when looking at electronic components that make up the two diode version of the bridge rectifier circuit is the peak inverse voltage rating required for the diodes that are used.

Diodes require 2 x √2 transformer RMS voltage, and preferably more to accommodate transients. — Diodes require 2 x √2 transformer RMS voltage, and preferably more to accommodate transients

Looking at the circuit, one diode will conduct over one half of the cycle, and the other diode over the other half of the cycle.

Taking the case where the diode D1 is conducting, as the voltage from its half of the secondary starts to rise, so does the voltage in the other half. One diode, in this case will conduct, and the other diode, D2 will be reverse biassed.

The voltage will rise to a voltage Vp, the peak voltage from the transformer which is √2 times the RMS voltage. If a capacitor is placed across the load to act as smoothing, this voltage will be maintained, even if it has a small amount of ripple.

With the peak voltage across the load at the peak of the cycle, the diode D2 will see this potential at one end of the diode, and at the other, it will see the peak voltage in the other sense from its half of the transformer. In other words it must block twice the peak voltage, i.e. √2 times the RMS output voltage of the transformer.

As this circuit will often be connected to mains of the line power source, there could be transients that are seen on top of this by the diode. It is therefore wise to increase the peak inverse voltage rating, PIV for the diode by a good margin. Often diodes with a PIV rating of at least four times the peak voltage from the transformer are chosen.

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.
Diodes require high PIV rating: As seen in the paragraph above the peak inverse voltage, PIV for the diodes must be twice the peak voltage from the transformer, and to allow margin to cover transients, it should be more. The bridge rectifier full wave rectifier requires diodes that have only half the PIV rating.

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.