Potential Dividers

Overview

A potential divider, or voltage divider, is a a circuit made from two resistors in series. The resistor may be fixed or variable. The potential difference across one of the resistors is measured as the "output" voltage. Potential dividers circuits are used extensively in all manner of circuits. It is very important to understand how they work and what happens to the output voltage when the value of each components changes.

Learning Objectives

• Select and apply the appropriate equation for a voltage divider
• Apply an understanding or ratios to simple potential dividers
• Understand how to use potential dividers as sensing circuits
• Use photosensitive devices (LDRs and photodiodes), thermistors and other sensors in sensing circuits
• Know that a pull-up or pull-down resistor is needed with a switch and that this is a form of potential divider

Lesson Content

Reading: Read through the first four sections that introduce the concept of potential dividers and then explain how to calculate the output voltage in two different ways.

Video (8 min): Watch the interactive whiteboard presentation that derives the potential divider equation. This is good revision of the basic current and voltage laws and of the resistor equation. Make notes about potential dividers and the potential divider equation.

Reading: Study the examples that illustrate how to calculate the output voltage of a potential divider.

Video (10 mins): Follow the examples on the video and add to your notes.

Reading: Most importantly, study and make notes about the effect of changing R1 or R2, especially when R1 or R2 are variable resistors.

Video (6 mins): The video looks at how the output voltage depends on R1 and R2 by demonstration and without calculation. Develop an intuitive understanding of how the output voltage depends on R1 and R2.

Exercises: Complete the problems (with answers) to test your understanding.

Lesson Review

Review your learning by working through the presentations or notes which summarise the website content.

Presentation: Powerpoint download. The basics of potential dividers with worked examples and questions.

Notes: PDF download. The website content formatted in to a PDF document.

Self Assessment

Complete either the questions (pdf download) OR the on-line quiz. They are the same questions.

Questions: PDF download. Questions about potential dividers requiring calculations.

Quiz: An on-line quiz about potential dividers with answers requiring some calculations.

Self Evaluation

I can:

• Use the potential divider equation to calculate V_out for a given pair of resistors and knowing V_in
• Use the method of ratios to calculate V_out for a given pair of resistors and knowing V_in
• Describe the effect on V_out of changing resistor R₁
• Describe the effect on V_out of changing resistor R₂
• Given one resistor value, use the method of ratios to determine the value of the other resistor to achieve a given value of V_out knowing V_in
• Given values for V_in and V_out, determine a suitable pair of resistor values for R₁ and R₂
• Appreciate that the potential divider theory only works when no current is drawn from the circuit
• Describe the effect of light level on the output of a potential divider containing an LDR as either R₁ or R₂
• Describe the effect of temperature on the output of a potential divider containing a thermistor as either R₁ or R₂
• Appreciate that switches used with pull up or pull down resistors are simply potential divider circuits
• Appreciate that other input transducers also need to be part of a potential divider