These resistors change their resistance with variations of incident light energy.
Cadmium sulphide is obtained in a highly pure state. By adding special materials, it is forms in moulds like disc. It is heated at high temperatures to remove gases. Electrodes are formed by evaporating metal in vacuumed. Leads are connected and put in plastic case with synthetic resins.
R in dark > 10 mega ohms measured after 3 minutes after darks condition.
R in light 100 to 300 ohms at 1000 lax.
Recovery > 200 kilo ohms/sec rise in resistance with in light.
Capacitance < 10pf
Permissible voltage 100 volts peak.
APPLICATIONS: In dark its resistance is high and in light it acts as a conductor. It is used in (1) proximity switches; (2) automatic brightness and contrast control in TV; (3) light failure alarms.