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# Resistivity of Lightbulbs

Extracts from this document...

Introduction

Lower 6th Quality of Measurement:

25 January 2009

Introduction:

In this experiment I am aiming to investigate how different bulbs emit different light intensities depending on the potential difference applied to them. I will create my own sensor using a potential divider circuit using a Light Dependent Resistor (LDR) along with a lux-meter to calibrate it and then collect readings.

What is a Potential Divider Circuit?

Potential Dividers are part of many sensors. They consist of a voltage source along with some resistors in series. The potential of the voltage source[1], for example a power supply, is divided in proportion to the ratio of resistances. This means that you can choose the resistance to get the voltage you want across one of them. In the case of this experiment, the resistance across the LDR[2] varies with light intensity thus causing the potential difference across it to vary. The other resistor in series is a fixed resistor[3] which is there to compare the resistance of the LDR thus completing the ratio. The Voltmeter[4] records the potential difference across the fixed resistor thus giving the sensor a value. As the light levels increase, the resistance of the LDR falls and thus the potential difference across the LDR falls. In turn this increases the potential difference across the fixed resistor which is measured by the voltmeter. Increasing light levels increases output voltage and this is the light sensor. The Diagram below shows this:

Choosing the correct Fixed Resistor:

Middle

y regardless of whether the ambient lights were on or off. This meant that there would be no need to tape the box to the workbench.

Second Calibration:

The second method again consisted of three main circuits:

• The potential divider circuit
• The bulb and the VARIAC
• The Xlogger Lux Meter connected to the laptop

They were arranged in the following setup:

This improved method incorporates the new Xlogger lux meter which is integrated in the top of the cardboard box, right next to the LDR. This means that they will be both recording the same light intensity thus resulting in the most accurate calibration possible. The gaps in the cardboard box have been sealed and the bulb stand is also secured to the desk to prevent movement and a bias towards either sensor. To ensure a fair test, the same VARIAC was used for all experiments as actual figure of voltage through the light bulb may depend on the accuracy of the VARIAC. Of course this is not a problem so long as the same apparatus is used each time.

The above graph shows a much more regular set of results when compared with the first calibration. This is excellent as it ensures that when the real experiment is done, the light intensity figures collected by the LDR will be as close as possible to the true values. If the graph was more curved, then any values recorded at the lower light intensity (voltages)

Conclusion

As explained at the beginning a potential divider circuit works by using a variable resistor (the centre of the sensor) in conjunction with a fixed resistor along with a power supply. By measuring the potential difference across the fixed resistor, the LDR, a sensor reading can be recorded. The LDR achieves this by being a semi-conductor, that is to say that it is a material with a conduction rating between that of a conductor an an insulator. The atoms inside the LDR have bound electrons. These electrons require different amount of energy to escape from the atom and pass a current through the semiconductor. Light, which is made up of packets of photons, determines how many of these electrons are free to flow and pass a current. Therefore the higher the light intensity, the lower the resistance of the LDR and thus the lower the potential difference across the LDR.

Limitations to the method:

The method can be used to measure only a certain range of light intensities for which a) the LDR is calibrated for and b) the range that the LDR can accurately interpret. This is due to the nature of the calibration curve: there is only a specific range range of input potential differences for which the LDR can operate when used with a specific fixed resistor. This is why the experiment only measured light intensity between 80 and 140 volts because outside of that spectrum, the ratio between the 250Ω fixed resistor and the LDR is too small or too great to give an accurate reading. When presented on a graph it is at the extremities: when the gradient is close to 0 and where the gradient is close to 1.

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