Photoresistor

from Wikipedia, the free encyclopedia
Photo resistor: round disc with a meandering photoconductor layer
CdS photoresistor, the circuit symbol below
Photoresistor circuit symbol: like the resistor symbol, but with two additional arrows to illustrate the incidence of light

A photoresistor , also called photoresistor or LDR from English Light Dependent Resistor , is a light-sensitive electrical component .

The more light falls on the photo resistor , the lower its electrical resistance becomes . The cause of this function is the internal photoelectric effect in a layer made of an amorphous semiconductor . Compared to other light sensors, photoresistors react very slowly.

construction

Cadmium selenide photoresistor

A thin layer of the photosensitive semiconductor material is applied to a ceramic base by sintering a powder on it. The electrical connections consist of two comb-like metal surfaces that are then attached and face each other. As a result, the structure of the light-sensitive layer has the shape of a meander through which the photo current flows across the course.

The entire arrangement is provided with connecting wires and coated or potted with transparent synthetic resin. Also hermetically sealed metal packages with glass windows and glass seals are common.

materials

Photoresistors often consist of a cadmium sulfide (CdS) or cadmium selenide (CdSe) layer, which has roughly the same color sensitivity curve as the human eye or photographic film .

Cadmium sulfide has the maximum sensitivity at a wavelength of 520 nm, cadmium selenide at 730 nm.

Materials such as lead sulfide (spectral sensitivity at wavelengths from 0.3 to 3.5 µm) and indium antimonide (wavelength 4.5 to 6.5 µm) are used for infrared .

function

Typical characteristic of a photo resistor according to [3]

Semiconductors that are suitable for LDRs do not use the normal internal photoelectric effect , but rather transitions to imperfections . If an impurity is ionized by the light , it acts like a doping for a few milliseconds and increases the electrical conductivity. The relatively long time it takes to neutralize the point of interference again results in a high level of sensitivity, but also the slow reaction. Because of the involvement of the impurities, the photoconductivity is not only dependent on the base material, but also on the microstructure and impurities. The semiconductor used does not have to be amorphous.

properties

A few free charge carriers that remain even after a long period of darkening cause a dark current . This can be reduced by lowering the temperature.

Photoresistors are characterized by the following parameters:

  • Dark resistance (resistance value of the photo resistor in the dark), typically 1  M Ω to 100 MΩ; is only reached after several seconds of darkness
  • Light resistance (resistance value of the photo resistor at 1000  lx ), typically 100 Ω to 2 kΩ
  • Response time (time that elapses after a lighting level of 1000 lux has been switched on until the current has reached 65% of its specified value), typical value 1 to 3 ms
  • Spectral range (material-dependent spectral sensitivity curve)

Applications

CdS and CdSe photoresistors are used, for example, in light meters and twilight switches in order to be able to measure even low illuminance levels with little technical effort - the spectral sensitivity agrees well with the light sensitivity curve of the eye. It can be used when fast response times are not required.

The curved characteristic allows, for example, to determine the illuminance over 4 orders of magnitude using the voltage on a simple voltage divider.

Since LDRs are sluggish and behave like linear ohmic resistances, optocouplers with photoresistors were used in audio compressors and voltage-controlled filters (e.g. phasers ).

RoHS compliance

The EC Directive 2011/65 / EU (RoHS 2) restricts the use of hazardous substances in electrical and electronic equipment. The use of cadmium-containing photoresistors in products is no longer permitted according to the applicable directive in the EU. Exceptions expired on December 31, 2009.

Similar components

See also

Web links

Commons : Photoresistors  - collection of images, videos, and audio files

Individual evidence

  1. ↑ a b Message from EETech Media, LLC. on photoresistors, accessed on August 1, 2020
  2. ↑ VALVO Paperback 1965
  3. ↑ a b https://cdn-learn.adafruit.com/downloads/pdf/photocells.pdf Message from Adafruit: Photocells, accessed on August 1, 2020
  4. ↑ Official Journal of the European Union: Directive 2011/65 / EU of the European Parliament and of the Council of June 8, 2011 on the restriction of the use of certain hazardous substances in electrical and electronic equipment .
  5. ↑ Official Journal of the European Union: Commission decision of June 10, 2009 amending the Annex to Directive 2002/95 / EC of the European Parliament and of the Council with regard to the exempted uses of lead, cadmium and mercury in order to adapt it to technical progress .