Increase the photocell bias voltage (Vbias) in small steps by using the rheostat. 8. Record the values of the photo current (I) on the nanoammeter as a function of the increasing photocell bias voltage, till the photo current reduces to zero.
3.1. Work Principle and Basic Characteristics of Photocell Photodetectors, also called photosensors, are sensors of light or other electromagnetic radiation which are widely used in the digital camera, optical communication, solar cells and other fields, the photocell is a basic unit of semiconductor photoelectric detector.
The sensitivity of a photocell is defined as its resistance at a specific level of illumination. Since no two photocells are exactly alike, sensitivity is stated as a typical resistance value plus an allowable tolerance. Both the value of resistance and its tolerance are specified for only one light level.
Simply stated, a photocell tends to remember its most recent storage condition (light or dark) and its instantaneous conductance is a function of its previous condition. The magnitude of the light history effect depends upon the new light level, and upon the time spent at each of these light levels. this effect is reversible.
As the light level decreases, the spread in the tolerance level increases. For increasing light levels the resistance tolerance will tighten. Likewise, for dual element photocells the matching factor, which is defined as the ratio of the resistance of between elements, will increase with decreasing light level.
This is an important characteristic of photocells because in many applications not only is the absolute value of resistance at a given light level of concern but also the value of the resistance as the light source is varied.