Electric Circuits: The Potential Divider

Electric Circuits: The Potential Divider

Understanding Potential Dividers

  • A potential divider is a simple circuit which takes advantage of the fact that the voltage drop across a resistor is proportional to its resistance.
  • A potential divider usually consists of two or more resistors in series connected across a power supply.
  • The formula Vout = (R2/R1 + R2) * Vin provides the output voltage (Vout) for a typical two-resistor potential divider circuit.

Using Potential Dividers

  • In a potential divider, the ratio of the resistors determines the fraction of the total voltage that appears across one of them.
  • If R1=R2, voltage divide into equal parts; If R1>R2, more voltage appears across R1; If R1<R2, more voltage appears across R2.
  • Input voltage (Vin) usually remains constant. The output voltage (Vout) varies depending on the resistances.
  • The current flowing through the elements of a potential divider is the same.

Potential Dividers in Practice

  • Potential dividers are used in volume controls, light-dependent resistors (LDRs), and thermistors.
  • Understanding the characteristics of a potential divider is vital in explaining how devices such as LDRs and thermistors can be used in practical circuits.
  • It’s important to recognise that in LDRs and thermistors, resistance varies with light intensity or temperature, respectively, causing a change in Vout.

Potential Dividers and the ‘Divider Rule’

  • The ‘Divider Rule’ states that the output voltage is the input voltage multiplied by the ratio of the output resistance to the total resistance.
  • This rule allows you to calculate the output voltage, given the total supply voltage and the resistances.
  • It can also be rearranged to find the unknown resistance if the supply voltage and output voltage are known.

Shortcomings of Potential Dividers

  • Power is dissipated in the components of a potential divider. This might make it inefficient for circuits requiring high power.
  • If the load resistance is not much greater than the resistance of the divider, it will draw significant current, affecting Vout.
  • A buffer stage might be required in such cases to prevent the load from interfering with the potential divider’s behaviour.