Bridge rectifiersPresented to:

Sir Abbas

Presented by:Shabana Hafeez

Definition

A type of full-wave rectifier with four diodes that converts both the positive and negative cycles of AC voltage to DC.

 This is the type of single phase rectifier which uses four individual rectifying diodes connected in a closed loop "bridge" configuration to produce the desired output.

Diagram of three bridge diodes

ExplanationA diode bridge or bridge rectifier

(occasionally called a Greatz bridge) It is an arrangement of four diodes

connected in a bridge circuit that provide the same polarity of output voltage for any polarity of the input voltage.

When it is used in its most common application, for conversion of altertaning current (AC) input into direct current (DC) output, it is known as a bridge rectifier.

The Diode Bridge Rectifier

Essential feature

The essential feature of this arrangement is that for both polarities of the voltage at the bridge input the polarity of the output is constant.

Second most essential feature is that  it does not require a special centre tapped transformer, thereby reducing its size and cost.

 It does not require a special centre tapped transformer, thereby reducing its size and cost.

Basic operation

When the input connected at the left corner of the diamond is positive with respect to the one connected at the right hand corner, current flows to the right along the upper colored path to the output, and returns to the input supply via the lower one.

The four diodes labelled D1 to D4 are arranged in "series pairs" with only two diodes conducting current during each half cycle.

The Positive Half-cycleDuring the positive half cycle of the supply,

diodes D1 and D2 conduct in series while diodes D3 and D4 are reverse biased and the current flows through the load

diagrammatically

The Negative Half-cycle

During the negative half cycle of the supply, diodes D3 and D4 conduct in series, but diodes D1 and D2switch of as they are now reverse biased. The current flowing through the load is the same direction as before.

diagrammatically

Development of current and voltageAs the current flowing through the load is

unidirectional, so the voltage developed across the load is also unidirectional the same as for the previous two diode full-wave rectifier, therefore the average DC voltage across the load is 0.637Vmax and the ripple frequency is now twice the supply frequency (e.g. 100Hz for a 50Hz supply).

Over all diagrammatic representation of bridge rectifiers

Graphical Representation

Explanation of these wave forms

In each case, the upper right output remains positive with respect to fhe lower right one.

Since this is true whether the input is AC or DC, this circuit not only produces DC power when supplied with AC power: it also can provide what is sometimes called "reverse polarity protection".

That is, it permits normal functioning when batteries are installed backwards or DC input-power supply wiring "has its wires crossed"(and protects the circuitry it powers against damage that might occur with out this circuit in place).

Cont’dPrior to availability of integrated electronics,

such a bridge rectifier was always constructed from discrete components.

Since about 1950, a single four-terminal component containing the four diodes connected in the bridge configuration became a standard commercial component and is now available with various voltage and current ratings.

Output smoothingFor many applications, especially with single

phase AC where the full-wave bridge serves to convert an AC input into a DC output, the addition of a capacitor may be important because the bridge alone supplies an output voltage fixed polarity but pulsating magnitude (see the figure).

Diagram

Smoothing capacitorThe function of the capacitor attached with

the bridge rectifier for smooting the output wave form is known as a “smoothing capacitor”.

One explanation of 'smoothing'is that the capacitor provides a low impedance path to the AC component of little output! reducing the AC voltage across, and AC current through, the resistive load.

In less technical terms, any drop in the output voltage and current of the bridge tends to be cancelled by loss of charge in the capacitor.

Cont’dThis charge flows out as additional current

through the load.Thus the change of load current and voltage

is reduced relative to what would occur without the capacitor.

Increases of voltage correspondingly store excess charge in the capacitor, thus moderating the change in output voltage / current.

The capacitor and the load resistance have a typical time constant ז=RC where R and C are the capacitance and load resistance respectively.

Cont’dAs long as the load resistor is large enough

so that this time constant is much longer than the time of one ripple cycle, the above configuration will produce a well smoothed DC voltage across the load resistance.

Advantages of bridge rectifiersA diode bridge rectifier is simple to build. It needs no control, no active switches and

just a small number of devices.A bridge rectifier will rectify both halves of a

sine wave and give "continuous output" through 360 degrees of the input.

you don't need a center-tapped transformer to use it.

Disadvantages of bridge diode No controlled input current shape and no

controlled output voltage level.The peak reverse voltage that can be

tolerated is 2x the reverse breakdown of the diodes.

The AC portion and DC portion must be isolated (you cannot make a common point between them.