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A presentation of eSyst.org

LINEAR POWER SUPPLY

Block diagram and functions of a transformer, rectifier, filter, voltage regulator and voltage divider.

Types of rectifier, filter and regulator circuits


Power Supply

All electronic circuits need a power source to work.

For electronic circuits made up of transistors and/or ICs, this power source must be a DC voltage of a specific value.

A battery is a common DC voltage source for some types of electronic equipment especially portables like cell phones and iPods.

Most non-portable equipment uses power supplies that operate from the AC power line but produce one or more DC outputs.


Power Supply Characteristics The input is the 120 volt 60 Hz AC

power line.

The power supply converts the AC into DC and provides one or more DC output voltages.

Some modern electronic circuits need two or more different voltages.

A good example of a modern power supply is the one inside a PC that furnishes 12, 5, 3.3 and 1.2 volts.


Components of a Power Supply

Main circuits in most power supplies.


Transformer

A transformer is commonly used to step the input AC voltage level down or up. Most electronic circuits operate from voltages lower than the AC line voltage so the transformer normally steps the voltage down by its turns ratio to a desired lower level.

For example, a transformer with a turns ratio of 10 to 1 would convert the 120 volt 60 Hz input sine wave into a 12 volt sine wave.


Rectifier

The rectifier converts the AC sine wave into a pulsating DC wave.

There are several forms of rectifiers used but all are made up of diodes.

Rectifier types and operation will be covered later.


Filter

The rectifier produces a DC output but it is pulsating rather than a constant steady value over time like that from a battery.

A filter is used to remove the pulsations and create a constant output.

The most common filter is a large capacitor.


Regulator

The regulator is a circuit that helps maintain a fixed or constant output voltage.

Changes in the load or the AC line voltage will cause the output voltage to vary.

Most electronic circuits cannot withstand the variations since they are designed to work properly with a fixed voltage.

The regulator fixes the output voltage to the desired level then maintains that value despite any output or input variations.


How Rectifiers WorkThe simplest form of rectifier is

the half wave rectifier shown. Only the transformer, rectifier

diode, and load (RL) are shown without the filter and other components.

The half wave rectifier produces one sine pulse for each cycle of the input sine wave.

When the sine wave goes positive, the anode of the diode goes positive causing the diode to be forward biased. The diode conducts and acts like a closed switch letting the positive pulse of the sine wave to appear across the load resistor.


How Rectifiers Work (continued)

When the sine wave goes negative, the diode anode will be negative so the diode will be reverse biased and no current will flow.

No negative voltage will appear across the load. The load voltage will be zero during the time of the negative half cycle.

See the waveforms that show the positive pulses across the load. These pulses need to be converted to a constant DC.


Bridge Rectifier

Another widely used rectifier is the bridge rectifier. It uses four diodes.

This is called a full wave rectifier as it produces an output pulse for each half cycle of the input sine wave.

On the positive half cycle of the input sine wave, diodes D1 and D2 are forward biased so act as closed switches appearing in series with the load.

On the negative half cycle, diode D1 and D2 are reverse biased and diodes D3 and D4 are forward biased so current flows through the load in the same direction.


How the Filter WorksA large capacitor is connected

across the load resistor. This capacitor filters the pulses into a more constant DC.

When the diode conducts, the capacitor charges up to the peak of the sine wave.

Then when the sine voltage drops, the charge on the capacitor remains. Since the capacitor is large it forms a long time constant with the load resistor. The capacitor slowly discharges into the load maintaining a more constant output.

The next positive pulse comes along recharging the capacitor and the process continues.


The Regulator

Most regulators are ICs .

These are feedback control circuits that actually monitor the output voltage to detect variations.

If the output varies, for whatever reason, the regulator circuit automatically adjusts the output back to the set value.

Regulators hold the output to the desired value.

Since ripple represents changes in the output, the regulator also compensates for these variations producing a near constant DC output.


RC pi Filter


ii) RC pi Filter

•C1 performs the same function that it did

in the single capacitor filter. It is used to

reduce the percentage of ripple to a

relatively low value.

•C2 offers infinite impedance (resistance)

to the dc component of the output

voltage. Thus, the dc voltage is passed to

the load, but reduced in value by the

amount of the voltage drop across R2.

However, R2 is generally small compared

to the load resistance. Therefore, the drop

in the dc voltage by R2 is not a drawback.

•Component values are designed so that

the resistance of R2 is much greater than

the reactance of C2 at the ripple

frequency.

RC pi Filter• C2 offers very low impedance to the ac ripple frequency. Thus, the

ac ripple senses a voltage divider consisting of R2 and C2 between the output of the rectifier and ground. Therefore, most of the ripple voltage is dropped across R2.

• The RC filter has some disadvantages, however. First, the voltage drop across R2 takes voltage away from the load. Second, power is wasted in R2, R1 and is dissipated in the form of unwanted heat.

• The input capacitor (C1) has the greatest pulsating voltage applied to it and is the most susceptible to voltage surges. As a result, it is frequently subject to voltage breakdown and shorting. The shunt capacitor (C1 and C2) in the filter circuit is not subject to voltage surges because of the protection offered by the series filter resistor.


Definition of Ripple

The amount of ripple factor of the full wave rectified signal is smaller than the half wave signal and provides a better filtered signal.

The amount of ripple factor of the full wave rectified signal is smaller than the half wave signal and provides a better filtered signal.

THE CHOKE INPUT (L-C FILTER)•As is known, in an inductor filter, ripple increases with RL but decreases in

a capacitor filter.

•The combination of L and C filter makes the ripple independent of RL

a) Shows the filter circuit. b) The voltage variation.

CLC or Pi Filter•The LC input filter is one of the most commonly

used filters.

•The input capacitor C1 is selected to offer very low

reactance to the ripple frequency. Hence, major part

of filtering is done by C1. Most of the remaining

ripple is removed by the combined action of L and

C2.

•L is a large value iron-core inductor (choke.) It has

a high value of inductance and, therefore, a high

value of XL, which offers a high reactance to the

ripple frequency. At the same time, C2 offers a very

low reactance to the ac ripple. L and C2 form an ac

voltage divider and, because the reactance of L is

much higher than that of C2, most of the ripple

voltage is dropped across L. Only a slight trace of

the ripple appears across C2 and the load.

• Aside from the voltage divider effect, the inductor improves filtering in another way. You should recall that an inductor resists changes in the magnitude of the current flowing through it. Consequently, when the inductor is placed in series with the load, the inductor tends to hold the current steady. This, in turn, helps to hold the voltage across the load constant.

• Generally, this resistance is very low and the dc voltage drop across the coil is minimal. Thus, the LC filter overcomes the disadvantages of the RC filter.

• The LC filter has two disadvantages. The first is cost. The LC filter is more expensive than the RC filter because its iron-core choke costs more than the resistor of the RC filter. The second disadvantage is size, since the iron-core choke is bulky and heavy. Thus, the LC filter may be unsuitable for some applications but is still one of the most widely used.


a)ZENER DIODE AS VOLTAGE REGULATOR

•A zener shunt regulator is that

the diode dissipation is too

large in some application.

b) SERIAL TRANSISTOR


Q = 50•The way to reduce the diode zener shunt power

dissipation is called an amplifier zener regulator.

Transistor Q1 is the series control element.

•Zener diode DZ provides the reference voltage

Operation

•If the output voltage decreases, the increased base-

emitter voltage causes transistor Q1 to conduct more.

•Thereby raising the output voltage, maintaining the output

constant.

•If the output increases, the decreased base-emitter

voltage causes transistor Q1 to conduct less, reducing the

output voltage maintaining the output constant.

A simple regulator consists of a sampling circuit, an error amplifier, a conduction element, and a voltage reference element.

The sampling regulator circuit (voltage divider) monitors the output voltage by feeding sample voltage back to the error amplifier.

The reference voltage element (zener diode) acts to maintain a constant reference voltage that used by the error amplifier.

The error amplifier’s output is then fed to the current-control element (transistor), which used to control the load current.

Negative-feedback voltage regulator.


Operation

•Transistor Q1 acts like an emitter follower.

Transistor Q1 provides voltage gain in a

negative-feedback loop.

•Suppose the load voltage tries to increase. The

feedback voltage VF will increase. Since the

emitter voltage Q1 is held constant by the Zener

diode, more collector current flows through Q1

and through R3.

•This reduces the current throughQ1 and R3.

The higher voltage at the base of Q2 increases

the emitter voltage of Q2, and this almost

completely offsets the original decrease in load

voltage.

•Therefore, any attempted change in load

voltage is compensated by negative feedback.


Voltage regulator

•The positive voltage regulator LM78 “xx” and

negative voltage regulator LM79xx digits represent

the output voltage such as 7805 (5V), 7806 (6 V),

7909 (-9V) etc.

•Can handle a maximum output current of 1.5A if

properly heat-sink.

•To remove unwanted input or output spikes/noise,

capacitors can be attached to the regulator’s input

and output terminals, as shown on figure above.

SIMPLE POWER SUPPLY

In SummaryAll electronic circuits and equipment need a power supply, usually

one that supplies are very specific DC voltage.

A battery is a near perfect DC supply but it is used mainly in portable applications.

Most equipment uses an AC to DC power supply.

In most AC to DC supplies, the 120 volt AC line is first filtered then stepped up or down to the desired voltage level then rectified into pulsating DC, then filtered to a constant DC. A regulator holds the output to a desired level. A DC-DC converter may also be used to generate another DC voltage.

The two most common rectifiers are the single diode half wave rectifier and the four diode full wave bridge rectifier.

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Business

presentation of

load voltage

constant output voltage

voltage regulator

half wave rectifier

ac sine wave

rectifier diode

negative voltage