RECTIFICATION

Normal household power is AC while batteries provide DC, and converting from AC to DC is called rectification. Diodes are used so commonly for this purpose that they are sometimes called rectifiers, although there are other types of rectifying devices.

TYPES OF RECTIFIERS

Rectifier

Half-wave Rectifier Full-wave Rectifier

Centre-tape full-wave rec.

Full-waveBridge rec.

Rectifier

A rectifier is a device which converts a.c. voltage (bi-directional) to pulsating d.c. voltage (Uni-directional).

Any electrical device which offers a low resistance to the current in one direction but a high resistance to the current in the opposite direction is called rectifier.

Half-wave rectifier

Simplest process used to convert ac to dc.

Only lets through positive voltages. Rejects negative voltages

A diode is used to clip the input signal excursions of one polarity to zero.

Half-wave rectification• Only the positive half-

cycle of the AC supply is

allowed to pass

TRANSFORMER

A transformer is a static device which transfers the energy from primary winding to secondary winding through the mutual induction principle, without changing the frequency.The transformer winding to which the supply source is connected is called the primary, while the winding connected to the load is called secondary. If N1,N2 are the number of turns of the primary and secondary of the transformer then α =N2/N1 is called the turns ratio of the transformer.The different types of the transformers are1) Step-Up Transformer2) Step-Down Transformer3) Centre-tapped Transformer

Ideal transformer: VS/VP=NS/NP

Real transformers are ~98% efficient

Half-wave Rectifier with Transformer Coupled Input Voltage

iPriPr

Sec V)NN(secV

iNSecN

Pr

A transformer is often used to couple the ac input voltages from the source to the rectifier circuits. Transformer coupling provides two advantages. First, it allows the source voltage to be stepped up or stepped down as needed. Second, the ac power source is electrically isolated from the rectifier circuit, thus reducing the shock hazard. Basic ac circuit recall that the secondary voltage of a transformer equals the turns ratio( ) times the primary voltage.

If NSec > NPri, the secondary voltage is greater than the primary voltage. If NSec < NPri, the secondary voltage is less than the primary voltage. If NSec = NPri, then VSec = VPri.

Full wave rectifier

• The full wave rectifier is the most commonly used type in DC power supply

• A full wave rectifier allows unidirectional (one way) current through the load during the entire 3600 of the input cycle

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Full wave Centre–Tapped transformer

Rectifier Circuit

Two diodes and a centre-tapped transformer are required.

The full-wave center-tapped rectifier uses two diodes connected to the secondary of a center-tapped transformer, The input voltage is coupled through the transformer to the center-tapped secondary. Half of the total secondary voltage appears between the center tap and each end of the secondary winding. For a positive half-cycle of the input voltage, the polarities on the secondary This condition forward-biases the diode D1 and reverse-biases the diode D2 .The current path is through D1 and the load resistor RL, as indicated.During negative half-cycles, D2 is forward-biased and D1 is reverse-biased.

Full-wave Center-tapped Rectifier

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Operation of the Center–Tapped Transformer Rectifier Circuit

For the positive half of the AC cycle:

For the negative half of the AC cycle:

Full-wave rectificationPositive AND negative half-cycles converted to ALL positive

Peak value of output voltage for the full-wave center-tapped rectifier

)(2 outVp

AGVV

VsceVp

outVp 7.02

)()(

Average value of output for the full-wave center-tapped rectifier

Diode peak inverse voltage for the full-wave center-tapped rectifier

VVPIV outP 7.02 )(

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Full-Wave Rectification:Bridge Network

• The dc level obtained from a sinusoidal input can be improved 100% using a process called full-wave rectification.

• The most familiar network is bridge configuration with 4 diodes.

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Operation of the Bridge Rectifier CircuitFor the positive half of the AC cycle:

For the negative half of the AC cycle:

Bridge rectifier

• Current flow in the bridge

vO vO+ +--

BASIC DC POWER SUPPLYPower supplies provide the necessary power, voltage and current requirements for electronic devices.They usually change ac to dc voltage.For example, 220 volts ac is changed to 12.0 volts dc.• Basic DC power supply converts the standard 220 V, 50Hz AC at wall outlets into a constant dc voltage.

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RectifierAC

220 V, 50 HzFilter Regulator

Load

0

VDC

0

VDC

Filtered voltage

Regulated voltage

One of the most important applications of a diode is in rectifying circuits: used to convert an AC signal into the DC voltage required by most electronics

Power Supplies

Consist of:

1. Transformer – steps ac voltage up or down.

2. Rectifier Diodes – change ac to “bumpy” dc.

3. Filter Network – includes capacitors and inductors, smoothes out the bumps.

4. Voltage Regulator – keeps the voltage constant.

Power Supplies• Transformer:- It changes AC voltage based on the

turns ratio of primary and secondary windings.• Filter:- it eliminates the fluctuation in the rectified

voltage and produces a relatively smooth DC voltage • Regulator:- it is a circuit that maintain a constant DC

voltage for variation in the input line voltage or in the load.

• Load:- It is a circuit or device connected to the output of the power supply and operation from the power supply voltage and current

Smoothed output• Un-smoothed

– Large ripple voltage– Not true DC

• Low-value smoothing capacitor– RloadC small

• High-value smoothing capacitor– RloadC big

Important Concepts• Half Wave Rectifier:

– Simplest.– Hard to filter well.

• Full Wave Center-Tapped Rectifier:– Easier to filter.– Requires transformer with center-tap.– Transformer secondary must be twice intended voltage.

• Full Wave Bridge Rectifier:– Easier to filter (just like full wave).– Center-tap transformer not required.– Transformer secondary same as intended voltage.– Higher parts count.