Photoconductive Cells

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Photoconductive Cells

• Butch Ellen Grace Q. Salem
• ICS 30 A

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Photoconductive Cell

• Two-terminal semiconductor device whose terminal
  resistance will vary (linearly) with the
  intensity of the incident light

VT300 SeriesPhotoconductive Cell
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Photoconductive Cell

• Also known as photoresistive device or
  photoresistor
• Developed in the later 19th century
• Oldest photoelectric device
• Photoelectric cell device that is activated by
  electromagnetic energy in the form of light waves
• A passive device, depending on an external
  current or voltage

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Photoconductive Cell

• Applications
• Familiar in supermarkets as the sensor that scans
  codes on grocery items at checkout counters
• In photography, they are designed as the light
  meters used to measure the intensity of
  illumination

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Photoconductive Cell

• Symbol

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Photoconductive Cell

• Photoconductive material mostly used
• Cadmium Sulfide (CdS)
• Cadmium Selenide (CdSe)
• CdS
• Peak spectral response 5100 A
• Response time of units 100 ms
• CdSe
• Peak spectral response 6150 A
• Response time 10 ms

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Photoconductive Cell

• Does not have a junction like the photodiode
• A thin layer of material connected between
  terminals is exposed to the incident of light
  energy.
• As the illumination of the device increases in
  its intensity, the energy state of a large no. of
  electrons in the structure will also increase,
  because of the increased availability of the
  photon packages of energy.

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Photoconductive Cell

• Results in an increasing no. of relatively free
  electrons in the structure a decrease in the
  terminal resistance.
• The resistance may change from several ohms in
  the dark to a few hundred ohms in sunlight

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Photoconductive Cell

• Another application Voltage regulator system
• Consists of photoconductive cell, bulb and
  resistor
• Maintain Vo at a fixed level even though it may
  fluctuate from its rated value

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Photoconductive Cell

• If Vi should drop in magnitude, the brightness of
  the bulb would also decrease.
• The decrease in illumination would result in an
  increase in the resistance (R? ) of the
  photoconductive cell to maintain Vo at its rate
  level as determined by the voltage-divider rule.

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Photoconductive Cell

• Voltage-divider rule