2c Power Supply (voltage Regulator) 45174i

POWER SUPPLY (VOLTAGE REGULATOR)

DC power supply block diagram

Output waveform at each block

POWER SUPPLY (VOLTAGE REGULATOR) • Power supply consist of filters, rectifiers and voltage regulator. • Starting with ac voltage, a steady dc voltage is obtained by rectifying the ac voltage then filtering to a dc level and finally regulating to obtain a fixed dc voltage. • The regulation normally are using IC voltage regulator unit which remains the same dc voltage even the input varies.

TRANSFORMER • The transformer will function to steps down or up the ac voltage to the level for the desired dc output.

Example: Input at primary transformer 115V ac, use a transformer 1:3, voltage at secondary transformer = 345V ac (step up transformer)

TRANSFORMER • Basic transformer consists two windings. Its called primary and secondary. The i/p of transformer is connected to the primary and the o/p is taken from the secondary. • Three types of transformer: 1. step up transformer 2. step down transformer 3. isolation transformer Isolation transformer provide an output voltage that is equal to the input voltage. -use to electrically isolate the power supply from ac power line and protect the power supply and the person working at line voltage.

TRANSFORMER • Transformer ratio is known as the turn ratio of the component. The turn ratio of a transformer is equal to the voltage ratio of the component given as following: Ns = Vs = Ip Np Is Vp Where: Vp = primary (input) voltage Np = number of turns on primary coil Vs = secondary (output) voltage Ns = number of turns on secondary coil

Ip = primary (input) current Is = secondary (output) current

RECTIFIER • A rectifier is a diode circuit that converts the ac to what is called pulsating dc. Its converts the ac signal to dc signal. • Rectifier produce unidirectional waveform (full wave) • There are two circuit which can produce full waveform – center tap transformer and bridge circuit.

i – bridge circuit

ii - Center tap transformer

FILTER • A simple capacitor filter will filter the full waveform to produce a dc voltage. • This resulting dc voltage usually has some ripple or ac voltage variation.

FILTER

RC filter

Single capacitor

LC filter

Others types of filter: • L filter • π – type filter

FILTER • The single capacitor filter are the most basic filter used in voltage regulator. The capacitor is simply connected in the parallel with the load resistance. • The output waveform are known as ripple voltage, Vr. Voltage ripple Vr(p-p)

FILTER The component of the output voltages are: 1)

DC voltage – derive from an ac source after rectifying and filtering will have some variation (ripple) Vrms = Vp/√2 Vdc = Vp – ½ Vr(pp)

2)

The ripple voltage, Vr Vr (pp) = Idct /C = Vp/RfC

Note: the smaller the ac variation (ripple) with respect to dc level the better the filter circuit operation. Ripple factor = ripple voltage, Vr(pp) / DC voltage, Vdc

FILTER Reduction of Ripple Vi Vr(p-p) Vm

Vdc

t T1

T2

T3 T1: C charge fast through the very low resistance of the diode when it conducts until Vm (peak voltage) is reached. T2: C discharge slowly through the load, where the rectified voltage drops below Vm.It charges again when the next cycle comes. T1 = RDC T3 = Capacitor charging time T2 = RLC RD
FILTER Effect of The Capacitor Filter

The effect of capacitor are: i. AC ripple is practically eliminated from the output ii. DC output voltage is maintained during the entire cycle of ac input iii. C charges through the diode and discharges thtough the load, so C is effectively the source of dc voltage. iv. The average (dc) voltage is closer to Vm and the amount of ripple is smaller v. Larger dc voltage where smaller ripple

FILTER Ripple Frequency

• In the half wave rectifier the dc output has a half ripple frequency as to the ac frequency input because only one half of each cycle of the ac input is used to produce dc output. • In the fullwave rectifier the dc output has a double ripple frequency as to the ac input because both cycles of the ac input are used. • A higher ripple frequency is easier to filter, therefore the fullwave rectifier offers the advantages of better filtering. • For light load, there is very small ripple factor hence such capacitor filter provide large dc voltage. • For heavy load, the capacitor filter will produce a lower value of dc voltage due to big ripple factor.

VOLTAGE REGULATOR • A voltage regulator is used to maintain a constant output voltage. • A voltage regulator also can use dc input to provide a dc voltage that not only has much less ripple voltage but remain the same dc value even the dc input voltage varies or the load connected to the output dc voltage changes.