Tutorial 5 - The Potential Divider

Potential Divider

The potential divider circuit (or voltage divider) is a voltage balance, that is used to switch on an electronic circuit when the balance of voltage reaches a certain tipping point.

• Although it is simple, the potential divider is a very useful circuit.  In its simplest form it is two resistors in series with an input voltage Vs across the ends.

• An output voltage Vout is obtained from a junction between the two resistors.

We can imagine the voltage balance tipping like this as the resistance of the transducer changes.  As the resistance changes, the voltage changes.

As the voltage changes past the tipping point, the balance tips from OFF to ON.

The potential divider circuit looks like this:

You need to know this equation.  It is very useful.

This result can be thought of as the output voltage being the same fraction of the input voltage as R2 is the fraction of the total resistance.  Look at this circuit for the next example:

 Worked Example   What is Vout in this example? Make a shopping list Vs = 6.0 V R1 = 4700 ohms R2 = 2200 ohms Now put the numbers into the formula Vout =       2200        × 6.0 = 2200 × 6.0 = 1.91 volts            4700 + 2200             6900

In the potential divider circuit, we can replace a resistor with a resistive transducer.

 What is the output voltage of this potential divider?

Need for a current limiting resistor

The current flowing through the thermistor or LDR will cause a heating effect which will alter the resistance as well as the temperature change.  This is known as self heating.  The thermistor gets hot due to the increasing current through it.  This can lead to a false reading of the resistance at a given temperature and in extreme circumstances cause thermal runaway.   This happens when the resistance falls, so a bigger current flows, therefore there is a bigger heating effect.  The resistance falls further, so there is an even bigger current...  The component will glow red-hot and burn out.

 A student suggested this circuit in which a thermistor was going to be used as a thermometer.  The thermistor would detect the temperature and the ammeter would be calibrated with a suitable scale.     The thermistor has a resistance of 10 kW at 0 oC and 10 W at 100 oC.  The battery voltage is 12 V.  Discuss whether or not this would be a good circuit to use, giving your reasons, supported by suitable calculations.

To prevent thermal runaway, we place a current limiting resistor in series with the resistive transducer.

 A maximum current of 50 mA will stop the thermistor from getting hotter.  What is the value of the current limiting resistor to prevent the thermistor in Question 6 from getting too hot when the temperature is 100 oC?

If there is going to be a problem like this in a potential divider, we can use a bridge circuit to take into account any self-heating effect.

Potentiometers

Potentiometers are a kind of voltage divider.  Instead of having two resistors, they have a slider that moves from one end of a resistor to the other.  You buy potentiometers that can be used either as a variable resistor, or a potentiometer.

The variable resistor is wired in series with a component.  It takes no current from the power supply.  However it can only alter the voltage a small amount into the component.  If the component has a comparatively high resistance, the range of voltages is very limited.

With the potentiometer, we can have a full range of voltages from +Vs to 0, giving a wide range of voltage control.  A model train controller works in this way.  However it always takes a current from the supply, which will lead to a heating effect, and will run a battery down.