8. Photo Electric Transducers: A photoelectric transducer can be categorized as photoemissive, photoconductive, or photovoltaic. In photoemissive devices, radiation falling on a cathode causes electrons to be emitted from the cathode surface. In photoconductive devices, the resistance of a material is changed when it is illuminated. Photovoltaic cells generate an output voltage proportional to radiation intensity. The incident radiation may be infrared, ultraviolet, gamma rays, or X rays as well as visible light.
Photo Electric Transducers (cont’d):- 8.1.The Photomultiplier Tube The Photomultiplier tube consists of an evacuated glass envelope containing a photo cathode, an anode and several additional electrodes caller dynodes, each at a higher voltage. Figure illustrates the principle of IN Photomultiplier. Electrons emitted by the cathode are attracted to the firs anode. Here a phenomenon known as secondary emission takes place. When electrons moving at a high velocity strike an appropriate material, the material emits a greater number of electrons than it was struck with.
Photo Electric Transducers (cont’d):- 8.1.The Photomultiplier Tube Fig (17) Principle of Photomultiplier tube
Photo Electric Transducers (cont’d):- 8.1.The Photomultiplier Tube In this device the high velocity is achieved by using a high voltage between the first anode and the cathode. The electrons emitted by the first anode are then attracted to the second anode, where the same thing takes place again. Each anode is at a higher voltage, in order to achieve the requisite electron velocity each time. Thus, secondary emission and a resulting "electron multiplication" occur at each step, with an overall increase in electron flow that may be very great.
Photo Electric Transducers (cont’d):- 8.1.The Photomultiplier Tube Amplification of the original current by as much as 105 to 109 is common. Luminous sensitivities range from 1 A per lumen or less, to over 2000 A per lumen. Typical anode current ratings are 100,uA minimum to 1 A maximum. The extreme luminous sensitivity possible with these devices is illustrated by the fact that with a sensitivity of 100 A per lumen, only 10-5 lumen is needed to produce a 1-mA output current.
Photo Electric Transducers (cont’d):- 8.1.The Photomultiplier Tube Magnetic fields affect the gain of the Photomultiplier because some electrons may be deflected from their normal path between stages and therefore never reach a dynode or, eventually, the anode. In scintillation counting applications this effect may be disturbing, and mu-metal magnetic shields are often placed around the Photomultiplier tube.
Photo Electric Transducers (cont’d):- 8. 2 Photo Electric Transducers (cont’d):- 8.2. Photoconductive Cells or Photocells Another photoelectric effect that has proved very useful is the photoconductive effect, which is used in photoconductive cells or photocells. In this type of device the electrical resistance of the material varies with the amount of light striking it.A typical form of construction is shown in Fig. (18-a).
Photo Electric Transducers (cont’d):- 8. 2 Photo Electric Transducers (cont’d):- 8.2. Photoconductive Cells or Photocells The photoconductive material, typically cadmium sulfide, cadmium selenide, or cadmium sulfoselenide, is deposited in a zigzag pattern, to obtain a desired resistance value and power rating. Fig (18) Photoconductive cell. (a) Construction. (b) Typical curves of resistance versus illumination.
Photo Electric Transducers (cont’d):- 8. 2 Photo Electric Transducers (cont’d):- 8.2. Photoconductive Cells or Photocells The material separates two metal-coated areas acting as electrodes, all on an insulating base such as ceramic. The assembly enclosed in a metal case with a glass window over the photoconductive material. Photocells of this type are made in a range of sizes, having diameters of one-eighth inch to over one inch..
Photo Electric Transducers (cont’d):- 8. 2 Photo Electric Transducers (cont’d):- 8.2. Photoconductive Cells or Photocells The small sizes are suitable where spa is critical, for example, in equipment for reading punched cards and similar applications. However, the very small units have very low power dissipation ratings. A typical control circuit utilizing a photoconductive cell is illustrated in Fig. The potentiometer is used to make adjustments to compensate for manufacturing tolerances in photocell sensitivity and relay-operating sensitivity.
Photo Electric Transducers (cont’d):- 8. 2 Photo Electric Transducers (cont’d):- 8.2. Photoconductive Cells or Photocells When the photocell has the appropriate light shining on it, its resistance will be low and the current through the relay will consequently be high enough to operate the relay. When the light is interrupted, the resistance will rise, causing the relay current to decrease enough to deenergize the relay.
Photo Electric Transducers (cont’d):- 8. 2 Photo Electric Transducers (cont’d):- 8.2. Photoconductive Cells or Photocells EXAMPLE:- The relay of Fig.(19) is to be controlled by a photoconductive cell with the characteristics shown in Fig. (20). The circuit delivers 10 mA at a 30-V setting when the cell is illuminated with about 400 IM/M2. The circuit becomes deenergized when the cell is dark. Calculate (a) The required series resistance. (b) The level of the dark current.
Fig (19) Photocell and relay control circuit. Photo Electric Transducers (cont’d):- 8.2. Photoconductive Cells or Photocells Fig (19) Photocell and relay control circuit. Fig (20) (a) Relay control by a photoconductive (PC) cell and (b) PC cell illumination characteristics.
Photo Electric Transducers (cont’d):- 8. 2 Photo Electric Transducers (cont’d):- 8.2. Photoconductive Cells or Photocells Solution:- (a) The cell's resistance at 400 IM/M2 (b) The cell's dark resistance 100 k Dark current
Photo Electric Transducers (cont’d):- 8.3. The Photovoltaic Cell The photovoltaic cell, or "solar cell," as it is sometimes called, will produce an electrical current when connected to a load. Both silicon (Si) and selenium (Se) types are known. Photovoltaic cells may be used in a number of applications. Multiple-unit silicon photovoltaic devices may be used for sensing light as a means of reading punched cards in the data processing industry. Gold-doped germanium cells with controlled spectral responses act as photovoltaic devices in the infrared region of the spectrum and may be used as infrared detectors.