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Q. What do you mean by half wave rectification and full waverectification?

When a single rectifier unit is placed in series with the load across an ac supply, itconverts alternating voltage into uni-directional pulsating voltage, using one halfcycles of the applied voltage, the other half cycles being suppressed because it

conducts only in one direction. Unless there is an inductance or battery in thecircuit, the current will be zero, therefore, for half the time. This is called half-waverectification.

The no-load output DC voltage of an ideal half wave rectifier is:

Working of a Half wave rectifier

The ac voltage across the secondary winding changes polarities after every halfcycle. During the positive half-cycles of the input ac voltage i.e. when upper end ofthe secondary winding is positive w.r.t. its lower end, the diode is forward biasedand therefore conducts current. The input voltage during the positive half-cycles isdirectly applied to the load resistance RL, making its upper end positive w.r.t. itslower end.During the negative half cycles of the input ac voltage i.e. when the lower end ofthe secondary winding is positive w.r.t. its upper end, the diode is reverse biasedand so does not conduct.

Full Wave Rectifier

A Full Wave Rectifier is a circuit, which converts an ac voltage into a pulsating dcvoltage using both half cycles of the applied ac voltage.

Description of Full Wave Bridge Rectifier

This type of single-phase rectifier uses four individual rectifying diodes connected ina closed loop "bridge" configuration to produce the desired output. The mainadvantage of this bridge circuit is that it does not require a special centre tapped


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transformer, thereby reducing its size and cost. The single secondary winding isconnected to one side of the diode bridge network and the load to the other side asshown below.

The Diode Bridge Rectifier

The four diodes labeled D1 to D4 are arranged in "series pairs" with only two diodesconducting current during each half cycle. During the positive half cycle of thesupply, diodes D1 and D2 conduct in series while diodes D3 and D4 are reverse

biased and the current flows through the load as shown below.

The Positive Half-cycle

During the negative half cycle of the supply, diodes D3 and D4 conduct in series,but diodes D1 and D2 switch "OFF" as they are now reverse biased. The currentflowing through the load is the same direction as before.

The Negative Half-cycle

As the current flowing through the load is unidirectional, so the voltage developedacross 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.


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However in reality, during each half cycle the current flows through two diodesinstead of just one so the amplitude of the output voltage is two voltage drops ( 2 x0.7 = 1.4V ) less than the input VMAX amplitude. The ripple frequency is now twicethe supply frequency (e.g. 100Hz for a 50Hz supply).

Q. What do you mean by peak inverse voltage of a rectifying diode?

Peak inverse voltage is the maximum voltage that a diode can withstand in thereverse direction without breaking down or avalanching. If this voltage is exceededthe diode may be destroyed. Diodes must have a peak inverse voltage rating that ishigher than the maximum voltage that will be applied to them in a givenapplication.

Q. What do you mean by ripple voltage and ripple factor?

Ripple voltages are the small undesired residual periodic variations of the DC(Direct current) at the output stage of the power supplies. These ripples are due toinsufficient suppression of the alternating waveforms within the power supplies.

Large Ripple means less effective filtering Smaller Ripple means more effective filtering.

Ripple factor () may be defined as the ratio of the root mean square (rms) valueof the ripple voltage to the absolute value of the dc component of the outputvoltage, usually expressed as a percentage.

=rms value of AC component/value of DC component

For a full-wave rectifier:

For a half-wave rectification:

where

is the peak-to-peak ripple voltage is the current in the circuit

is the frequency of the ac power

is the capacitance

Q. Describe choke input filter with diagram.


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A filter is used in power supplies to eliminate the AC fluctuation present in theRectification process of a Power Supply. This filter may be a choke input filter orcapacitor input filter.

The LC choke-input filter is used primarily in power supplies where voltageregulation is important and where the output current is relatively high and subjectto varying load conditions.

This filter consists of an input inductor (L1), or filter choke, and an output filter

capacitor (C1). Inductor L1 is placed at the input to the filter and is in series with theoutput of the rectifier circuit. Since the action of an inductor is to oppose anychange in current flow, the inductor tends to keep a constant current flowing to theload throughout the complete cycle of the applied voltage. The reactance of theinductor (XL) reduces the amplitude of ripple voltage without reducing the dc outputvoltage by an appreciable amount.

The shunt capacitor (C1) charges and discharges at the ripple frequency rate, butthe amplitude of the ripple voltage (Er) is relatively small because the inductor (L1)tends to keep a constant current flowing from the rectifier circuit to the load. Inaddition, the reactance of the shunt capacitor (XC) presents a low impedance to theripple component existing at the output of the filter, and thus shunts the ripple

component around the load. The capacitor attempts to hold the output voltagerelatively constant at the average value of the voltage.Q. Describe Capacitor input filter.

A typical capacitor input filter consists of a reservoir capacitor C1, connected acrossthe rectifier output, an inductor L, in series and another filter or smoothingcapacitor, C2, connected across the load, RL. A filter of this sort is designed for useat a particular frequency, generally fixed by the AC line frequency and rectifierconfiguration. The capacitor C1 offers low reactance to the AC component of therectifier output while it offers infinite resistance to the DC component. As a resultthe capacitor shunts an appreciable amount of the AC component while the DCcomponent continues its journey to the inductor L


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1. The inductor L offers high reactance to the AC component but it offers almostzero resistance to the DC component. As a result the DC component flowsthrough the inductor while the AC component is blocked.

2. The capacitor C2 bypasses the AC component which the inductor had failedto block. As a result only the DC component appears across the load RL.

Advantages: More output voltage & Ripple less output

Disadvantages: Large in size and weight & High cost

Q. Does low ripple voltage indicate good or bad filtering?Q. Is a full-wave rectifier output easier to filter than that of a half-waverectifier?Q. What is AC/DC converter and battery eliminator?AN AC/DC converter is something that turns outlet-type electricity into battery type

electricity.

A battery eliminator is a device powered by an electrical source otherthan abattery, which then converts the source to a suitable DC voltage that may be usedby a second device designed to be powered by batteries. A battery eliminatoreliminates the need to replace batteries but may remove the advantage ofportability.

The battery eliminator operates through the use of power conversion, andcan step voltages up or down in addition to converting alternating current to directcurrent. The device should include specifications that provide information about itscapacity and the type of power it can handle. It also shows the types of

conversions it does; a nine volt battery eliminator, for example, may just replacenine volt batteries, while others may have adjustable settings for differentvoltages.

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