Photoresistor

A photoresistor (English Light Dependent Resistor LDR ) is a light-dependent resistance of an amorphous semiconductor layer.

The higher the light, the smaller is due to the internal photoelectric effect its electrical resistance. Photoresistors are surpassed only by the photomultiplier in sensitivity, however, react very slowly (range 1 ms to several seconds).

Construction

On an insulating pad (common is ceramic) is a thin layer of the photosensitive semiconductor material applied. The electrical connections consist of two subsequently applied comb-like metal surfaces, which face each other. Characterized the structure of the light-sensitive layer is in the form of a meander.

The entire assembly is provided with connecting wires and coated with a transparent resin or cast. Also, in hermetically sealed metal case with glass windows and glass feedthroughs are used.

Materials

Photoresistors often comprise a cadmium sulphide (CdS ), or cadmium selenide layer ( CdSe ), which has approximately the same color to the human eye sensitivity curve, or photo films.

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

For infrared materials such as lead sulfide used ( spectral sensitivity at wavelengths from 0.3 to 3.5 microns ) and indium antimonide (wavelength 4.5 to 6.5 microns ).

Function

In semiconductors, which are suitable for LDRs, the normal internal photoelectric effect is not used, but it used to be transitions to impurity. If an impurity is ionized by the light, the effect for a time in the range of a few milliseconds, such as doping and increases the electrical conductivity. Due to the relatively long time it takes to neutralize again the defect is obtained, a high sensitivity, but also the slow response. Because of the involvement of the impurity to the image line and not only on the base material but also the microstructure and impurities dependent. The semiconductor need not be amorphous. The usual CdS is also more likely to be a lot of crystal.

Properties

Few remaining even after a long darkening free charge carriers cause a dark current. The dark current can be reduced by lower temperature.

Photo sensors are characterized by the following parameters:

• Dark resistance ( resistance of the photoresistor in the dark), typically 1 M to 100 M; is reached only after several seconds darkness
• Light resistance ( resistance of the photoresistor at 1000 lx), typically 100 Ω to 2 kOhm
• Response time ( time that elapses after power an illuminance of 1000 lux after dark until the current has reached 65 % of its specified value ), typical value of 1-3 ms
• Spectral range ( material-dependent spectral sensitivity curve)

Applications

CdS photoresistors are used for example in light meters and cameras in twilight switches or discrete optocouplers, if any quick reaction times are required.

RoHS compliance

The EU Directive 2011/65/EU (RoHS 2) restricts the use of hazardous substances in electrical and electronic equipment. The use of photo resistors in products is due to the existing cadmium compounds no longer allowed under the current Directive in the EU. Exceptions have expired on December 31, 2009.