Voltage Dividers

Today we looked at how we can use two resistors in series to change a signal voltage.

We know from previous lessons that the voltage in a series circuit is shared between the components.  We can use a special type of series circuit to get a specific voltage.

For example if both resistors are of the same value then the voltage will be shared equally between them:

If one resistor is twice as big as the other, twice as much voltage will be dropped over it:

Remember that the whole supply voltage must be used in the circuit, there is none left over at the end!

We can work out the proportion of the voltage dropped over the bottom resistor using the equation:

              R2      
V2 = R1 + R2      x Vcc   (V2 might also be called Vo for Voltage Out of the voltage divider)
             20     
      = 10 + 20      x 12
         2
      = 3     x 12
      = 8v

In voltage dividers we are interested in the voltage dropped over the bottom resistor.  We can use this voltage as a signal to another part of the circuit.  However it is possible to use the equation to work out the voltage dropped over the top resistor by changing the equation so that instead of R2 on the top, it is R1

The application of voltage dividers is most useful when using input transducers like a thermistor, which changes resistance based on temperature, and a LDR, which changes resistance as the light level changes.  This change in resistance will result in a change of voltage.

Thermistors
First of all we will look at thermistors.  The resistance of a thermistor changes with temperature.  They are negative temperature coefficient (NTC) which means that the resistance will do the opposite of the temperature - i.e. as the temperature increases the resistance will decrease and as the temperature decreases the resistance will increase.

Consider these circuits:

The thermistor is the bottom resistor in a voltage divider.  As the temperature decreases, the resistance of the thermistor will increase.  Therefore the share of the supply voltage dropped over the thermistor will increase (remember the more resistance there is, the more voltage is required) and so the output voltage will increase.  This makes this circuit a cold sensor.

The thermistor is now at the top of the voltage divider.  The properties of the thermistor remain the same (temperature up > resistance down) but because it is now at the top of the voltage divider, the circuit will act as a heat sensor.  As the temperature increases the resistance of the thermistor decreases.  Therefore the share of the voltage dropped over the thermistor will decrease and so the share of the voltage dropped over the fixed resistor must increase and so the output voltage will increase as the temperature increases.

There are different types of thermistor with various temperature ranges.  They are all found on this graph:

This is a "log graph" as in reality the properties of the thermistor will form a curve and not a straight line which is very difficult to read.  So instead this type of graph is used and the axis need to be interpreted.  The temperature axis acts as you would expect and the only difference is the spacing.  The resistance axis, however, is more difficult and this is the bit people get stuck with.  Reading up from the bottom the units read > 10, 20, 30 etc then 100, 200, 300 etc, then 1000, 2000, 3000 etc. 

It is also important to note that the values nearer the top are closer together than those at the bottom.  So a half way point is not half way between the two values, but closer to 1/3 of the value.  i.e. between 1k and 2k, half way would be 1.3k.

To find a value, read along the axis of the value you know (you could be given either the temperature and asked to find the resistance, or the resistance and be asked to find the temperature)  This graph shows that at 25°c the resistance of a type 4 resistor is 50kΩ.

 LDR - Light Dependent Resistor
The light dependent resistor will change resistance as the light level changes.  As the light level increases, the resistance decreases and as the light level decreases, the resistance increases.  LDRs can be used in the same way as thermistors in a voltage divider circuit.

Consider these circuits:

This is a dark sensor - as the light level decreases the resistance of the LDR increases and therefore the voltage dropped over the LDR increases and the voltage out of the voltage divider increases.

This is a light sensor - as the light level increases the resistance of the LDR decreases and therefore the voltage dropped over the LDR decreases so the voltage over the fixed resistor increases and the voltage out of the voltage divider increases.

Just like thermistors there is a graph to show the change of resistance with the change in light level.  This is another log graph so you need to be aware of the axis.  We only need to look at one type of LDR, the ORP12.  You need to be aware that in this graph, the resistance is measured in KΩ.

This graph shows how to read the graph - at a light level of 200 lux the resistance is 600Ω.

It may be necessary to adjust the sensitivity of the circuit, i.e. change the "trigger" temperature or light level.  In this example a signal voltage of 5v is required.  First of all we can use the variable resistor to achieve this voltage at a temperature of 0°c.

We can adjust the temperature which produces that 5v signal voltage to 20°c by changing the resistance of the variable resistor.