In certain semiconductor materials, when light falls on them, their electrical resistance varies. If constant voltage is applied to such conductor, the current varies as the amount of light falling on it varies (because resistance varies). These semiconductors are known as photoconductive cells or photoresistor or light dependent resistors (LDRs). The symbol of photovoltaic cell is shown in Fig. 1. The light energy provides sufficient energy to the electrons to become free thus causing more current to flow for some amount of applied voltage because resistance decreases.
Fig. 1: Photoconductive Cell Symbol
Construction of Photoconductive Cell
The most commonly used photoconductive semiconductor materials are cadmium sulphide (CdS), cadmium selenide (CdSe). Fig. 2 shows the details of photoconductive cell.
Fig. 2: Photoconductive Cell Construction
Characteristics of Photoconductive Cell
The electrodes are made up of gold. These electrodes are placed on the photoconductive material. This assembly is encapsulated as shown in Fig. 3. When the photoconductive cell is kept in darkness, its resistance is called dark resistance. It may be as high as 10 x 1012 Ω. When light falls on the photoconductive cell, the resistance decreases. A typical characteristic is shown in Fig. 1.
Fig. 3: Typical characteristics of Photoconductive Cell
Applications of Photoconductive Cell
A simple application of photoconductive cell for relay control is shown in Fig. 4. When light on the photoconductive cell exceeds a particular value, current through the circuit increases and relay operates. When light is less, the current is not sufficient enough to energise the relay. The phototransistors, photodiodes are used in many control circuits.
Fig. 4: Photoconductive cell for Relay operation
Drawback of Photoconductive Cell
The major drawback of photoconductive cell is that, temperature variations cause variation in resistance and in that case the resistance is not only dependent on amount of light only.