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Potential Dividers

Introduction

Potential dividers are a very common part of even the most complex circuits and it is therefore very important to understand what they do and how to do the associated calculations. Luckily potential dividers are one of the most simple circuits you can build and so understanding them shouldn't be too hard.

Basic Concepts

Potential DividerA potential divider is a pair of resistors connected in series and connected to a power supply or other source of EMF (voltage). The potential difference across ONE of the resistors is a fraction of the applied EMF. The total potential difference across the resistors is divided between the two resistors, each having its own potential difference - hence potential divider.

In essence, a potential divider is a circuit with an input and an output. You apply a voltage to the input (Vin) and get a smaller voltage at the output (Vout). The output voltage is a fraction of the input voltage.

The output voltage depends on (a) the input voltage and (b) the ratio of the two resistors.

Note: The output voltage does not depend on the actual value of the individual resistors, it depends on the ratio of the pair of resistors - they work together and the output of the potential divider depends on their relative values.


Potential Divider Equation

Potential DividerThe output voltage from a potential divider (Vout) is given by the potential divider equation:

Vout = Vin × R2 / (R1 + R2)

It is important to be able to use this equation.



Method of Ratios

Potential DividerThe potential divider equation is very useful for calculating Vout but it is less obvious when trying to calculate R1 or R2. It is usually much easier to use the ratios of the voltages to work out what resistor values to use. The ratio of the voltages is equal to the ratio of the resistances:

V1 ÷ V2 = R1 ÷ R2

Knowing Vin and Vout means you know V1 and V2 (V2 is Vout of course). Once you know V1 and V2 you can choose R1 and R2 accordingly



Link to an Interactive White Board Presentation showing the derivation of the Potential Divider Equation.

Examples

Potential DividerExample 1: Given the two resistor values and the supply voltage, find the output voltage.

The values given are non trivial so the best approach is to use the equation for Vout

Vout = 12 × 220 / (100 + 220)

Therefore:

Vout = 8.25 Volts



Potential DividerExample 2: Given Vout=5V, Vin=9V and R2=1kΩ calculate R1.

The best approach when trying to find a resistor value is to use the method of ratios.

The voltage across R2 is 5V and therefore the voltage across R1 is 4V

4 : 5 = R1 : 1000

Therefore:

R1 = 800Ω



Potential DividerExample 3: Given Vout=2V, Vin=6V and R1=12kΩ calculate R2.

Again, the best approach when trying to find a resistor value is to use the method of ratios.

The voltage across R2 is 2V and therefore the voltage across R1 is 4V

4 : 2 = 12k : R2

Therefore:

R2 = 6kΩ



Link to an Interactive White Board Presentation showing examples of Potential Divider calculations.

Effect of Changing R1 and R2

Where one of the resistors in a potential divider is variable - an LDR or Thermistor for example - it is very important to know how Vout changes when either R1 or R2 changes (and Vin remains fixed).

Potential DividerCase 1: R1 increases

R1 increases and takes a larger share of the input voltage

Therefore, the remaining voltage, Vout goes down



Potential DividerCase 2: R1 decreases

R1 decreases and takes a smaller share of the input voltage

Therefore, Vout goes up



Potential DividerCase 3: R2 increases

R2 increases and takes a larger share of the input voltage

Therefore, Vout goes up



Potential DividerCase 4: R2 decreases

R2 decreases and takes a smaller share of the input voltage

Therefore, Vout goes down



Potential Dividers with LDRs and Thermistors

Consider (and learn and understand) each of the following examples:

Potential DividerThermistor as R2:

As the temperature increases the resistance of the thermistor decreases. As the resistance decreases, Vout decreases.

Therefore, Vout goes down as the temperature increases

As the temperature decreases the resistance of the thermistor increases. As the resistance increases, Vout increases.

Therefore, Vout goes up as the temperature decreases

A similar situation occurs if an LDR is used in place of R2. If the light level increases, Vout goes down and vice versa



Potential DividerThermistor as R1

As the temperature increases the resistance of the thermistor decreases. As the resistance decreases, Vout increases.

Therefore, Vout goes up as the temperature increases

As the temperature decreases the resistance of the thermistor increases. As the resistance increases, Vout decreases.

Therefore, Vout goes down as the temperature decreases

A similar situation occurs if an LDR is used in place of R1. If the light level increases, Vout goes up and vice versa