Resistivity of Lightbulbs

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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:

It is very important to ensure the correct Fixed Resistor is installed in the circuit. This is due to the ratio between the fixed resistor and the variable resistor, the LDR, needing to be as wide as possible thus giving an accurate sensor reading across the input range that it will be sensing. It is therefore necessary to trial various resistors and pick the resistance which results in the highest potential difference change across the fixed resistor for the sensing spectrum. The sensing spectrum is the range of value

s the sensor needs to be able to accurately measure. In the case of this experiment different voltages across different light bulbs resulting in different light intensities. I therefore took the following readings using different resistors at either end of the sensing spectrum: at 40V (lowest light level) and at 140V (highest light level) across the light bulb:

The above table shows that the fixed resistor vaue that attained the highest range of output potential difference across the light range being tested. highlighted in red, was at 250 Ω.

First Calibration Attempt:

The first calibration attempt was fairly primitive. It consisted of three main circuits:

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  • a light bulb connected to a VARIAC to vary the light intensity
  • the potential divider circuit I had devised
  • the portable lux-meter

They were arranged as shown below:

There were many problems with the first calibration attempt and they are listed below along with the chosen correction for the second calibration attempt:

  1. The greatest problem to overcome was the quality of the lux-meter. The first calibration attempt used a portable lux meter which was not that accurate, meaning ...

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