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Photo Conductive Cell
Posted Date:
15 Aug 2007
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Posted By: Jose Mathew
Member Level: Platinum Posted Date: 15 Aug 2007
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TOPIC: PHOTO CONDUCTIVE CELL (LOR)
INTRODUCTION
Photo resistor is also referred to as photoconductive cell. It has the property that its resistivity changes as a function of the incident illumination, called photo conductivity effect. Photo resistor is also known as light dependent resistor. (LDR)
Photoconductive cell or light dependent resistances (LDR) are devices used for detecting and measuring electromagnetic radiations. It is based upon the principle of variation of conductivity i.e., its conductivity changes according to the Wavelength and intensity of the incident radiation.
LDR is a two terminal Semi conductor device its two terminals are connected to a thin coating of the Photoconductive material, such as Cadmium sulphide (CdS), CadmiumSelenide(CdSe), Lead Selenide, Lead Telluride and Indianoid. This coating contains some impurities like antimony, Indium nitrate etc, for improving the efficiency.
Picture-1
The conductivity of the material is proportional to the concentration of charge carriers present. Light energy supplied to the semiconductor causes covalent bonds to be broken and create hole-electron pairs. These increased current carriers’ decreases the resistance of the material and hence such a device is called photo resistor or opto conductor.
In the absence of light, the resistance of the material will be more. So only a very low current flows through the photo resistor. This current is called dark current and the resistance corresponding to this current is called dark resistance. The dark resistance may be as high as few under mega ohms. When exposed to, light the resistance of the photo resistor decreases and a large current called joint current flows through it. The difference between joint current (I j) and the dark current (Id) is obviously called photocurrent ( I p)
I e, , I p = I j – Id
The main application of LDR are light sensitive alarm, height activated relays measuring lights, fast recording situation, smoke detectors, lightning controllers, automatic street lights.
AIM
To study the response of a photoconductive cell (LDR) to wavelength and intensity of incident light.
MATERIALS REQUIRED
LDR,electrical bulbs of 10W, 25W,40W,60W,100W,key,battery,micro ammeter (0- 5000µ A), various colour filters (glass paper} etc.
METHOD OF STUDY
Enclose the LDR in a cylindrical/ conical tube (usually made to suit its size by rolling paper) so that light from sideways will not fall on it.
Connection are made as shown in the diagram.
Picture -2
Draw a line centrally at right angles to the surface of LDR and place the source of light of highest wattage in front of the LDR at a suitable distance so that the micro ammeter gives almost full scale defection.
To study the response to intensity of light.
The light sources are placed one by one (decreasing order of wattage) at the fixed distance form the LDR. The ammeter readings are taken in each case. This is repeated and another sets of reading obtained by varying resistance in the box. Graphs are plotted between intensity of light and current.
To study the response to wavelength of light.
Keeping the distance and the bulb used the same, the various colour filters are held before the source. The ammeter readings are taken in each case. This is repeated and another set of readings obtained by various resistance in the resistance box. Wavelength values of the various colours are obtained from the tables. Graphs are plotted between wavelength and current.
COLLECTED DATA
To study the variation of current with intensity of light.
Least count of the micro ammeter = 10 µA
Zero error of the micro ammeter = o µ A
Graph
Graphh2
Distance of source from LDR = 10.8 cm
Table -1
To study variation of current with Wavelength of light
Least count of the micro ammeter = 10 µ A
Zero error of the micro ammeter = 0 µ A
Distance of source from LDR = 10.8 cm
Intensity of bulb used = 40 v.
Table -2
CONCLUSION
1. The variation of current with intensity of light is studied by drawing current intensity graph.
2. Variation of current with Wavelength of light is studied by drawing current wavelength graph.
3. Intensity is Greatest for yellow colour.
SUGGESTIONS
Since Yellow colour has greater Intensity. It can be used as a Fog light and can avoid accidents in snowy areas.
REFERENCE
1. NCERT text book – FOR CLASS XII
2. PHYSICS PRACTICAL- Sunil a . Pillai
- An Anamika publications for Class XI, XII
3. ROSSERY PUBLICATION- TEXT for class xii
- Xavier & Joy
4. Electronics text for diploma.
5. abc Physics.
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