Photoconductive cell is a two terminal semiconductor device. Its resistance will vary (linearly) with the intensity of the incident light. It is frequently known a photo-resistive device. It is some what different from LDR.
The photoconductive materials most frequently used include cadmium sulphide (Cds) and Cadmium selenide (CdS). Both materials reasonably rather slowly to changes in light intensity. The spectral response time of CdS units is about 100 ms and 10 ms for CdS cells.
Another important unlikeness between the two materials is their temperature sensitivity. There is large change response of a cadmium sulphide in applications where the resistance of a cadmium selenide cell with changes in ambient temperature. But the resistance of cadmium sulphide remains relatively stable. The spectral response of a cadmium sulphide cell closely matches that of the human eye. That is why the cell is therefore often finds applications where human vision is a factor. For example street light control or automatic iris control for cameras.
The main elements of a photo-resistive device are ceramic substrate, a layer of photoconductive material, metallic electrodes to connect the device into a circuit and a moisture resistant enclosure.
Construction of Photoconductive cell
The circuit symbol and construction of a typical photoconductive cell are in figure.
Light sensitive material is organize in the form of a long strip, zig-zagged across a disc shaped base with protective sides. The two ends of strip are brought out to connecting pins below the base.
Characteristics of Photoconductive Cell
The illumination characteristics of a typical photoconductive cell are in figure. Initially the cell is not lit up. At that time its resistance can be more than 10 kilo-ohm. This resistance is the dark resistance. When the cell is lit up. Now the resistance may fall to few hundred ohms. Cell sensitivity is expressed in terms of cell current, input voltage and input level of illumination.
Applications of Photoconductive Cell
- The photoconductive cell used for relay control. When the cell is lit up. Then its resistance is less and the relay current is at its maximum. When the cell is dark, its high resistance reduces the current down to a level too low to energize the relay. Resistance R is to limit the relay current to desired level when the resistance of the cell is low.
Temperature variations cause substantial variations in resistance for a particular light intensity. Therefore such cell is unsuitable for analog applications. Such cells find wide use in industrial and laboratory control applications.