RECTIFIERS.pptxhugugghhihigigigjgjgigjgjgigi

Ggvhchigcucuckghchctuocutoctuoctuocuocuto

1. RECTIFIERS

2. Rectifier is an electrical device that converts alternating current (AC) into direct current (DC). This process is known as rectification. The electrical device that converts direct current (DC) into alternating current (AC) is known as an inverter.

3. Types of rectifiers a .Thermionic valves: b. Metal valve rectifiers Thermionic valves As the name implies, these are devices which allow electron flow in one direction only and work using heat. i. Diode and ii. Triode valves

4. Diode Valves A diode consists of an evacuated glass tube into which are sealed two separate electrodes. Works as one way valves. The cathode, or filament, is so constructed that as current flows through it, a space charge of electrons develops around it as a result of the thermal effect of the current (thermionic emission). Zener diode- which will also allow The current to flow in reverse direction, when Voltage reaches a particular threshold The anode, or plate, is the other electrode. When positive, it attracts electrons across the valve.

5. Electrons can pass only from cathode to anode, as there is no space charge around the cold anode. Consequently, the thermionic valve is a device which allows electrons to flow in one direction but not in the reverse direction.
In order to reduce the time lag prior to thermionic emission taking place, the cathode or filament may be heated by a separate heating circuit or coated with thorium oxide which releases electrons at a comparatively low temperature.

6. Triode Valves The triode valve works on exactly the same principle as the diode valve but has a third electrode (the grid) placed between the cathode and the anode. It is possible, using an external circuit, to make the grid negative, positive or neutral. If neutral, the grid will not affect electron flow across the valve. If positive, it will attract electrons away from the cathode and thus amplify the electron flow through the valve.

7. If negative, the grid will repel electrons and reduce or even stop the electron flow. In this case, the valve can act as a switch or regulator.

8. Uses To get the direct current from alternative current They are component of power transrnission systems They can also be used for detection of radio signals.

9. Metal valve rectifiers Copper oxide rectifiers and
Selenium Rectifier Copper oxide rectifier Construction consist appropriate member of copper discs A- cu disc. D- aluminium cooling fin B- graphite. E- insulated rod C- lead disc Aluminium cooling fin radiates heat generated, used to lead the current in and out. Lead makes good contact with other disc.

10. Cu- n type CuO2 – p type 8 volts of current is produced in metal rectifier. If 100 volts is required 10 copper disc must be used. Selenium metal rectifier P type , tin alloy contact act as n type Rectifying unit separated by appropriate materials Produces upto 18 V.

11. Types of Rectification Rectification is the conversion of an AC to a DC This is achieved using a circuit with two diode valves or two semiconductors in it Half-wave Rectification In half-wave rectification one phase of alternating current, either the positive or negative phase of the AC wave is allowed to pass and another half is not allowed to pass. Only half of inputs of alternating current reaches for output so naturally mean output voltage decreases.

12. In half-wave rectification single diode in a single-phase supply and three in a three-phase supply are needed. As only one side of the phases either positive or negative phases are gained in output so current will be pulsed direct current. Much more filtering is needed to eliminate harmonics of the AC frequency from the output.

13. b .Full-wave Rectification In full-wave rectification conversion of the whole of the input waveform of alternating current to direct current occurs. It means it converts both polarities of the input waveform, i.e. positive and negative phases of alternating current to direct current. As in full-wave rectification, all positive and negative phases of inputs alternating current reaches for output so naturally mean output voltage will be more as compared to half-wave rectification. Two diodes and a transformer, or four diodes are needed.

14. Smoothening circuit A fter rectification direct current still varies considerably in intensity and having Large peaks and troughs. In order to depress the peaks and elevate the troughs a smoothing circuit is used which includes choke coils and capacitors The forward and back EMFs of the choke coil and the charging and discharging of the capacitor at appropriate points smooth the current.

15. Semiconductor These are materials which neither access conductor nor insulator. Semiconductors are usually metals which, because of thermal agitation or the addition of impurities, have electrons free to conduct current. (Phosphorus and silicon ) They are either n-type, with an excess of electrons, or p-type with deficit of electrons gives rise to positive holes . where a P- and n-type of semiconductor are fused together,current can only pass in the one direction I.e from n – p . semiconductor therefore acts as a valve .