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# Potential Divider watch

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1. I understand that to achieve the biggest change in voltage with change in input (eg.temperature) when using a potential divider (containing a sensor and a resistor), the best resistor to use is one whose value is the same as the average resistance of the sensor in the range being investigated. But why is this? What are the problems with a higher or lower resistance, hope that makes sense, if unsure what I mean please ask, thx.
2. Its because the potential difference across the fixed resistor is proportional to the ratio of the two resistances (do the maths). Hence if one is far larger than the other, a small change in either wont really change the potential difference across either.

Hope that makes sense, ask again if it doesnt
3. (Original post by martinjolly)
I understand that to achieve the biggest change in voltage with change in input (eg.temperature) when using a potential divider (containing a sensor and a resistor), the best resistor to use is one whose value is the same as the average resistance of the sensor in the range being investigated. But why is this? What are the problems with a higher or lower resistance, hope that makes sense, if unsure what I mean please ask, thx.
Try to bear with me on the explanation.

IN a potential divider, the voltage is always fixed, presuming you are using it from the mains power supply.

Therefore, the voltage across the 2 resistors must be shared. The variable sensor (i.e the thermistor, LDR etc.) will have a max resistance it will get to.

The chosen fixed resistor must be chosen carefully because it determines the sensitivity of the variable sensor. If the fixed resistor is too high, then most of the mains voltage will be dropped across that resistor, leaving only say 1V to be passed across the thermistor. This means that the thermistor only has 1V to 'play with' so the values of output are going to be small.

If the chosen fixed resistor is too small resistance, you will nto get measurable changes in ouput voltage as they will be too large from the varying resistor.

I think that's pretty much it

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