Download Photo Electric Transducers (cont`d)

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. 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. 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. 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. 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. 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. 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.
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. Photoconductive Cells or Photocells
 Solution:-
(a) The cell's resistance at 400 IM/M2
I
 1 k
30 V
R1  Rcell
30V
 Rcell
I
30V

 1 k  2k
10mA
R1 
(b) The cell's dark resistance 100 k 
30 V
Dark current 
2 x 10   100 x 10 
3
3
 0.3 mA
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.