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Potentiometers acting as voltage dividers? Answered

What is the difference between wiring a potentiometer as a variable resistor vs wiring it as a voltage divider? Won't either method allow you to vary your voltage? I'm not sure I understand in what situation it would be beneficial to use one method over the other?

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randofoBest Answer (author)2017-04-19

One resistor itself doesn't change the voltage. There are a ton of websites online that explain this, but none that do so well (based on my quick search).

I have attached some pictures below.

One shows measurement of a 10K resistor in a 9V circuit. Notice it gives a 9V readout. This is because resistors in and of themselves don't change voltage.

I also have created a voltage divider with a 100K and 10K resistor. Notice how the voltage across the 100K resistor differs from the voltage across the 10K resistor (and these add up to the supply voltage).

There is a 4th picture where I measure the total voltage over both resistors. Notice that the total voltage is the same as the voltage of the power supply.

To oversimplify... What is happening is that electricity follows the easiest path to ground. When there is one resistor in the circuit, that is the easiest path by default. All of the voltage flows across it.

However, when there are two resistors, the point in the middle provides an outlet for some of the voltage. In this case, there is a potentially easier path through the middle connection between the two resistors. So, a percentage of the voltage goes towards this path, and the rest passes through the other resistor to ground. As these values change, the ratios change, and the amount of voltage present at the middle point differs.

That is why the voltage reading across two series resistors (100K + 10K = 110K of resistance) is that same as the voltage reading across one (100K) resistor and is the same.

However, the voltage present in-between any two resistors can differ from the supply voltage depending on the ratio between the two values.

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randofo (author)randofo2017-04-19

The reason you would want to use a voltage divider is when you need to provide a variable voltage in the circuit. An example of where this might prove useful is if you have an audio signal and want to adjust the volume. You can use this method to vary the voltage of the output signal to make it softer or louder.

The reason you would want to use a variable resistor is because you want to vary the amount of resistance. One common example of where this is used is an RC-timing circuit. By varying the amount of resistance between a capacitor and its supply voltage, you can change the rate at which the capacitor fills and discharges. This can then be used for creating various (adjustable) timing circuits.

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