# (6)The Potential Divider

- Created by: Olatunde
- Created on: 25-03-14 13:24

First 507 words of the document:

The Potential Divider.

What is a potential divider? Background

At its simplest, a potential divider is a circuit with a voltage source and resistors in series.

The potential of the voltage source (e.g. power supply) is divided in the ratio of the

resistances. This means that if you had a 2 ohm resistor and a 3 ohm resistor, there would

be 2/5 of the p.d. across the 2 ohm resistor and 3/5 of p.d. across the 3 ohms. Therefore

resistances can be chosen to get a required voltage across one of the resistors.

For example:

A series circuit is set up with a switch and two resistors. In the circuit R1 has R1 / R1+R2 of

total resistance. Therefore Vout = R1 / R1+R2 * Vs (voltage at switch)

If Vs = 9V and you wanted Vout to be 6V then R1 / R1+R2 would have to be 2 / 3 which gives

the ratio of R1 to R2 as 1:2. This would mean that you could have a resistance of 200 ohms

for R1 and 100 ohms for R2.

This type of circuit is mainly used for calibrating voltmeters, which have a very high

resistance. If something with a relatively low resistance is put through R1 there can be

problems. Effectively you would have two resistors in parallel, which will always have a total

resistance less than R1. That would mean that Vout would be less than the value calculated,

and will depend on what is connected to R1.

For two different Vout values i.e. V1 and V2

V1 / V2 = R1 / R2

This equation shows that the ratio of the p.d.s across each resistor is equal to the resistance

ratio of the two resistors.

Light or Temperature switch

An LDR (lightdependent resistor) has a very high resistance in the dark, but a lower

resistance in the light. An NTC thermistor has a high resistance at low temperatures, but a

much lower resistance at high temperatures (it varies oppositely to a normal resistor, only

more so). When resistance decreases Vout will increase (and vice versa).

Either of the components mentioned above can be used as one of the resistors in a potential

divider, giving an output voltage which varies with the light level or temperature. Adding a

transistor creates a switch (e.g. to turn on a light or heating system). A transistor is on when

voltage across it is high and off when voltage is low.

Potentiometers

A potentiometer has a variable resistor replacing R1 and R2 of the potential divider, but it

uses the same idea (it can sometimes be called a potential divider). The relative sizes of R1

and R2 can be adjusted. This enables Vout to be varied from 0V up to the source voltage.

This can be useful when needing to change a voltage continuously, such as the volume

control of a stereo.

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