10.5 Potential Divider circuits | 10.6 Sensing circuits

What are Potential dividers used for?

They divide the p.d to give any value you require up to the maximum supplied from the power source.

How do you make a potential divider circuit?

A circuit can be connected across one of the resistors in parallel.

The p.d supplied to this circuit (V out) can be varied to any value from zero to the maximum supplied from the power source, depending on the resistances of R1 and R2.

From Kirchoff's second law the p.d across each resistor must always add up to the p.d from the power source.

What is the ratio of resistances?

The p.d across each resistor in a potential divider depends on their resistances.

If they have the same resistance then the p.d is shared equally between them. If one has twice the resistance of the other, then this one will receive two-thirds of the total p.d

What can the ratio of resistances be expressed as?

V1 / V2 = R1/R2

Where V1 is the p.d in volts across the resistance R1 in ohms and V2 is the p.d in volts across the resistor with resistance R2.

What is the potential divider equation?

The total p.d V in and the fraction of the total resistance provided by R2 we can determine the value of V out.

V out = (R2/R1 + R2) × V in

How do you produce a varying V out?

Using a pair of fixed resistors in series in a potential divider has the effect of splitting the p.d.

To vary the Vout replace one of the fixed resistors with a variable resistor.

How does a potentiometer work?

A potentiometer can be used instead of a potential divider.

It is a variable resistor with 3 terminals and a sliding contact. Adjusting this contact varies the p.d between two of the terminals giving a variable Vout.

A and B are two ends of the variable resistors. When the contact is moved towards A, Vout increases, until at A it is equal to V in. When the contact is moved towards B, V out decreases unital at B it is zero.

The potentiometer can also be constructed so that the change in resistance is either linear or logarithmic.