BASIC ELECTRONICS BASIC ELECTRONICSRectifier.ppt
- 3. 3 Full wave Center–Tapped transformer Rectifier Circuit Two diodes and a center-tapped transformer are required.
- 4. 4 Operation of the Center–Tapped Transformer Rectifier Circuit For the positive half cycle For the negative half cycle
- 6. 6 Operation of the Bridge Rectifier Circuit For the positive half of the AC cycle For the negative half of the AC cycle
- 7. 7
- 8. SMOOTHING A capacitor can be used to filter (remove the voltage variation) the output voltage. As the voltage grows the capacitor charges up, and as the voltage falls the capacitor discharges through the resistor. If the capacitance is large enough the voltage will not fall a lot before the capacitor is charged up once more. In this way the output voltage is smoothened.
- 9. 9 Capacitor Filter • A half-wave rectifier with a capacitor filter is shown in Figure . RL represents the equivalent resistance of a load. We will use the half-wave rectifier to illustrate the principle, and then expand the concept to full-wave rectification. • During the positive first quarter-cycle of the input, the diode is forward-biased , allowing the capacitor to charge to within 0.7 V of the input peak.
- 10. 10 • When the input begins to decrease below its peak, as shown in part (b), the capacitor retains its charge and the diode becomes reverse- biased. During the remaining part of the cycle, the capacitor can discharge only through the load resistance at a rate determined by the RLC time constant, which is normally long compared to the period of the input.
- 11. 11 • The larger the time constant, the less the capacitor will discharge. During the first quarter of the next cycle, the diode will again become forward-biased when the input voltage exceeds the capacitor voltage by approximately 0.7 V. • The variation in the output voltage due to the charging and discharging is called the ripple voltage. • The smaller the ripple, the better the filtering action
- 12. 12 • For a given input frequency, the output frequency of a full-wave rectifier is twice that of a half-wave rectifier. This makes a full-wave rectifier easier to filter. • The full-wave rectified voltage has a smaller ripple than does a half-wave voltage for the same load resistance and capacitor values.
- 13. 13 Transformer Voltage: A transformer's required secondary A.C. voltage varies greatly with the type of rectifier chosen and filter arrangement. All A.C. voltage references are R.M.S. Don‘t forget to take into account losses (not included in this guide), especially diode voltage drop. Leave an adequate safety margin for D.C. regulator voltage requirements and minimum operating line voltage Transformer Current Ratings: A transformer's A.C. current rating needs to be recalculated from the D.C. load current. The required current varies with type of rectifier chosen and filter type. Use the formulas below as a guide, shown for common D.C. supplies. Included in the formulas higher peak to peak capacitor charging current in the filter. Rectifier Selection Notes: When selecting rectifiers remember, average current in a full wave circuit is .5 x I D.C. per diode. In a half wave circuit, average current is equal to I D.C. per diode. A rating at least twice the output current is recommended to cover turn on surge. In full wave circuits, the reverse voltage rating should be in excess of 1.4 x V A.C. In half wave circuits, the reverse voltage rating should be in excess of 2.8 x V A.C.
- 14. 14 Capacitor Selection Notes: When choosing capacitor voltage, allowances should be made for D.C. voltage rise due to transformer regulation. Remember, R.M.S. ripple current in a filter capacitor can be 2 to 3 times D.C. load current. Capacitor life is greatly increased by reducing it's temperature via less R.M.S. current or reduced ambient temperature.
- 15. 15
- 17. Marking RMS reverse voltage in Volts Current in Amps 1N4001 35 1 1N4002 70 1 1N4003 140 1 1N4004 280 1 1N4005 420 1 1N4006 560 1 1N4007 700 1 1N5400 100 3 1N5401 200 3 1N5402 300 3 1N5404 525 3 1N5406 800 3 1N5407 1000 3 1N5408 1200 3 17
- 18. 18 S.No Description Half Wave F.W C.P F.W. Bridge 1 2 DC Output voltage 0.318 Vs(peak) 3
- 20. 20 Ripple factor The pulsating output of a rectifier consists of d.c. component and a.c. component ( also known as ripple). The a.c. component is undesirable and account for the pulsations in the rectifier output. The effectiveness of a rectifier depends upon the magnitude of a.c. component in the output : the smaller this component, the more effective is the rectifier. “The ratio of rms value of a.c. component to the d.c. component in the rectifier output is known as ripple factor”
- 21. 21 Ripple factor for Half-wave rectification By definition the effective (ie rms) value of total load current is given by Idc = Im/╓
- 22. 22
- 23. 23 Types of Filters 1. Capacitor Filter (C-Filter) 2. Inductor Filter 3. Choke Input Filter (LC-filter) 4. PiFilter (-filter) Capacitor Filter( C-filter)
- 24. 24 When the Input signal rises from o to a the diode is forward biased therefore it starts conducting since the capacitor acts as a short circuit for ac signal it gets charged up to the peak of the input signal and the dc component flows through the load RL. When the input signal fall from a to b the diode gets reverse biased . This is mainly because of the voltage across the capacitor obtained during the period o to a is more when comapared to Vi. Therefore there is no conduction of current through the diode. Now the charged capacitor acts as a battery and it starts discharging through the load RL. Mean while the input signal passes through b,c,d section. When the signal reaches the point d the diode is still reverse biased since the capacitor voltage is more than the input voltage. When the signal reaches point e, the input voltage can be expected to be more than the capacitor voltage. When the input signal moves from e to f the capacitor gets charged to its peak value again. The diode gets reverse biased and the capacitor starts discharging. The final output across RL is shown in Fig.
- 25. 25 This type of filter is also called choke filter. It consists of an inductor L which is inserted between the rectifier and the load resistance RL. The rectifier contains A.C components as well as D.C components. When the output passes through the inductor, it offers a high resistance to the A.C component and no resistance to D.C components. Therefore, A.C components of the rectified output is blocked and only D.C components reached at the load. Inductor Filter
- 26. 26 LC Filter In inductor filter, the ripple factor is directly proportional to the load resistance. On the other hand in a capacitor filter, it is varying inversely with the load resistance. Hence if we combine the inductor filter with the capacitor the ripple factor will become almost independent of the load filter. It is also known as inductor input filter, choke input filter, L input or LC-section. In this circuit a choke is connected in series with the load. It offers high resistances to the AC components and allows DC component to flow through the load. The capacitor across the load is connected in parallel which filter out any AC component flowing through the choke. In this way the reppls are rectified and a smooth DC is provided through the load.
- 27. 27 CLC or Pie Filter It consists of one inductor and two capacitor connected across its each end. The three components are arranged in shape of Greek letter Pi. It is also called capacitor input Pi filter. The input capacitor C1 is selected to offer very low reactance to the repel frequency hence major parts of filtering is done by C1. Most of the remaining repels are removed by the combining action of L and C2. This circuit gives much better filter then LC filter. However C1 is still directly connected across the supply and would need high pulse of current if load current is large. This filter is used for the low current equipment’s.
- 28. 28
- 29. 29 Power Diodes of largest power rating are required to conduct several kilo amps of current in the forward direction with very little power loss while blocking several kilo volts in the reverse direction. Large blocking voltage requires wide depletion layer in order to restrict the maximum electric field strength below the “impact ionization” level. Space charge density in the depletion layer should also be low in order to yield a wide depletion layer for a given maximum Electric fields strength. These two requirements will be satisfied in a lightly doped p-n junction diode of sufficient width to accommodate the required depletion layer. Such a construction, however, will result in a device with high resistively in the forward direction. Consequently, the power loss at the required rated current will be unacceptably high. On the other hand if forward resistance (and hence power loss) is reduced by increasing the doping level, reverse break down voltage will reduce. This apparent contradiction in the requirements of a power diode is resolved by introducing a lightly doped “drift layer” of required thickness between two heavily doped p and n layers
- 30. 30 Fig shows the circuit symbol and the photograph of a typical power diode respectively