The Potential Divider

Key Words

Potential Divider, Resistive Transducer

Potential Divider Circuit

Resistive transducers like LDRs or thermistors are often put into a potential divider circuit.

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 V_s across the ends.
An output voltage V_out is obtained from a junction between the two resistors.

If the output current is zero, the current flowing through R₁ also flows through R₂, because the resistors are in series. So we can use Ohm’s Law to say:

Now:

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.

Question 1

What is the output voltage of this potential divider?

ANSWER

If the light level rose, the resistance of the LDR would fall. Therefore the voltage V_out would rise. If the output were connected to a transistor, the transistor would switch on as V_out rose above 0.7 V.

Summary

Resistive transducers are passive devices.

LDR has a resistance that falls with increasing light levels;

Thermistor has resistance that falls with increased temperatures;

These are arranged in a potential divider.

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