Potential Dividers

Imagine I need a voltage of 6V but I only have a 9V cell. What can I do?

 I could set up a circuit like this. The 9V is shared between the two resistors so this circuit is called a potential divider.

The larger resistor gets more voltage. In fact the ratio of the voltages is equal to the ratio of the resistances.

i.e. 6 / 3 = 200 / 100

There are several ways we can draw this arrangement, be careful.

If you buy a potentiometer from a shop you will notice it has 3 connections. You can use the middle one and an outer one only, this will give you a variable resistor.

You can use all 3 as a mini potential divider. The sliding connection separates 2 resistors. Turning the knob changes their ratio and so also the output voltage.

The Vout may be supplying some kind of load, e.g. a motor or a bulb. Unless the output is just going to a high resistance voltmeter then the resistance of the load must also be taken into account when calculating the output voltage.

Consider this circuit;

 When it is cold the thermistor has a large resistance. The voltage across the fixed resistor is small and so is the voltmeter reading.

When it is hot the resistance of the thermistor is small. The voltage across the fixed resistor is large and so is the voltmeter reading.

We have a temperature sensor. A device that gives a voltage output that depends on temperature.

Of course we don't know if the output varies in a linear way with temperature. We would have to test it and see. The value of the fixed resistor we use would also effect the characteristics of the circuit.

Many sensor circuits are based on potential dividers, e.g. to make a light sensor we just replace the thermistor with an LDR.