Potential Dividers, Semiconductor Diodes and Relays

Q: What is the primary function of a potential divider circuit?

A potential divider splits a supply voltage ( $V_{supply} $) across series resistors to convert changes in sensor resistance into changes in output voltage ( $V_{out} $).

Q: How does the resistance of an LDR and an NTC thermistor change when light and temperature increase?

For both an LDR and an NTC thermistor, resistance decreases as environmental intensity (light or temperature) increases.

Q: How does the placement of a sensor in a potential divider affect whether $V_{out} $ rises or falls?

If the sensor is at the top ( $R_1 $), $V_{out} $ rises as light/temperature increases; if at the bottom ( $R_2 $), $V_{out} $ rises as it gets darker/colder.

Q: Why is a protection (flyback) diode connected across a relay coil?

To safely suppress the high-voltage back-EMF spike produced when the coil's magnetic field collapses, protecting the driver transistor.

Technology and Design 8900 · CCEA GCSE · 8 min read · Updated: 31 Aug 2026

Unit 2 Option A: Electronic and Microelectronic Control Systems

Chapter: Potential Dividers, Semiconductor Diodes, and Relays

Welcome to this study guide! In electronic control systems, circuits need ways to sense the physical world (like light and temperature), direct the flow of electricity safely, and control powerful devices like motors or heaters using tiny signals. In this chapter, we will break down three essential electronic building blocks: Potential Dividers, Semiconductor Diodes (including LEDs), and Relays. Don't worry if circuit diagrams seem confusing at first—we will take it step by step!

---

1. Potential Dividers (Voltage Dividers)

What is a Potential Divider?

A potential divider (often called a voltage divider) is a simple circuit made of two resistors connected in series across a power supply. Its job is to split the total supply voltage ($V_{supply}$) into smaller, usable output voltages ($V_{out}$).

In sensing systems, potential dividers are crucial. Many sensors change their resistance when the environment changes (e.g., getting darker or hotter). However, electronic process devices like microcontrollers or transistors respond to voltage, not raw resistance. A potential divider solves this by converting a changing resistance into a changing voltage ($V_{out}$).

The Key Formulae

There are a few simple mathematical rules you need to know for your exam:

1. Standard Voltage Divider Formula:
To find the voltage dropped across the bottom resistor ($R_2$), which is usually our output voltage ($V_{out}$):
$V_{out} = V_{supply} \times \left( \frac{R_2}{R_1 + R_2} \right)$

2. The Voltage Ratio Rule:
The ratio of the voltages across the two resistors is equal to the ratio of their resistances:
$\frac{V_1}{V_2} = \frac{R_1}{R_2}$

3. Supply Voltage Rule:
The sum of the voltages across both resistors always equals the total supply voltage:
$V_{supply} = V_1 + V_2$

4. Ohm's Law Foundation:
Remember the fundamental rule of electronics:
$V = I \times R$

Input Sensors Used in Potential Dividers

Two main sensors appear in CCEA GCSE potential divider circuits:

1. Light-Dependent Resistor (LDR):

Made from cadmium sulphide.
Exhibits a negative light coefficient.
Bright Light: Resistance decreases (typically drops to several hundred $\Omega$ or a few $\text{k}\Omega$).
Darkness: Resistance increases (typically rises to hundreds of $\text{k}\Omega$ or even $\text{M}\Omega$).
Memory Trick: LURD → Light Up, Resistance Down!

2. Thermistor (NTC):

NTC stands for Negative Temperature Coefficient.
Higher Temperature (Heat): Resistance decreases.
Lower Temperature (Cold): Resistance increases.
Memory Trick: TURD → Temperature Up, Resistance Down!

Circuit Configurations: Where Do We Place the Sensor?

The position of the sensor in the divider circuit determines how $V_{out}$ behaves:

Case A: Sensor at the Top ($R_1$ position)
When environmental intensity increases (more light or more heat), the sensor's resistance falls. Because the top resistance drops, a larger share of the voltage falls across the bottom resistor ($R_2$). Therefore, $V_{out}$ rises as it gets brighter or hotter. This setup is ideal for automatic cooling fans or solar trackers.

Case B: Sensor at the Bottom ($R_2$ position)
When environmental intensity decreases (it gets darker or colder), the sensor's resistance rises. As its resistance increases, it takes a bigger share of the total voltage. Therefore, $V_{out}$ rises as it gets darker or colder. This setup is ideal for automatic nightlights or frost alarms.

Calibrating Sensitivity: We often place a variable resistor (potentiometer) in the opposite branch of the sensor. This lets the user adjust the circuit so it triggers at an exact light level or temperature threshold.

Key Takeaway for Potential Dividers

Quick Summary: Potential dividers share voltage across series resistors. The component with the bigger resistance takes the bigger share of the supply voltage!

---

2. Semiconductor Diodes and Light-Emitting Diodes (LEDs)

Standard Silicon Diodes

A semiconductor diode is a polarised electronic component that allows current to flow in one direction only.

Forward Bias: Current flows easily (switch is effectively closed).
Reverse Bias: Current is blocked (switch is open).
Forward Voltage Drop: A standard silicon diode requires a threshold voltage of approximately $0.6\text{ V} - 0.7\text{ V}$ across it before it will turn on and conduct electricity.
Terminals: It has two terminals: the Anode ($+$) and the Cathode ($-$). On a physical diode, the cathode end is marked with a printed stripe or band.

Light-Emitting Diodes (LEDs)

An LED is a special type of diode that gives off light when it is forward-biased.

Typical Forward Voltage Drop ($V_{LED}$): Approximately $1.8\text{ V} - 2.2\text{ V}$ (varies depending on the colour of the LED).
Typical Operating Current ($I_{LED}$): Typically between $10\text{ mA} - 20\text{ mA}$ ($0.010\text{ A} - 0.020\text{ A}$).
Protective Current-Limiting Resistor: LEDs are delicate. If you connect an LED directly across a power supply, too much current will surge through it, destroying the component. A series resistor ($R_{limit}$) must always be added to protect it.

Calculating the Current-Limiting Resistor:
To find the required resistor value, subtract the LED's forward voltage drop from the supply voltage, then divide by the required LED current:
$R_{limit} = \frac{V_{supply} - V_{LED}}{I_{LED}}$

Worked Example: If an LED requires $2.0\text{ V}$ and $20\text{ mA}$ ($0.02\text{ A}$) from a $9\text{ V}$ battery:
$R_{limit} = \frac{9\text{ V} - 2.0\text{ V}}{0.02\text{ A}} = \frac{7.0\text{ V}}{0.02\text{ A}} = 350\ \Omega$

Protection / Flyback Diodes

When inductive loads (like relay coils or electric motors) are switched off, their magnetic fields collapse almost instantly. This sudden collapse generates a dangerous, high-voltage reverse spike known as back-EMF (electromotive force), which can instantly destroy driver transistors.

To prevent this, a standard silicon diode is connected in reverse parallel directly across the coil:

During normal operation, the diode is reverse-biased and does nothing.
When the coil turns off, the induced back-EMF forward-biases the diode, providing a safe, closed loop for the collapsing current to circulate and dissipate harmlessly.

Key Takeaway for Diodes

Quick Summary: Diodes act like one-way check valves for current. LEDs need protective series resistors to limit current, and relay coils need reverse-parallel protection diodes to suppress back-EMF spikes.

---

3. Relays (Electromechanical Switches)

What is a Relay?

A relay is an electromechanical switch operated by an electromagnet. When a small electric current flows through its internal coil, it creates a magnetic field that pulls a movable contact (the armature) to mechanically open or close separate switch contacts.

Why Use a Relay? (Electrical Isolation)

The primary advantage of a relay is electrical isolation. It allows a low-voltage, low-power control circuit (like a $5\text{V}$ microcontroller or a $9\text{V}$ transistor stage) to safely turn on a completely separate high-voltage, high-current circuit (such as a $24\text{V}$ DC motor or a $230\text{V}$ AC mains heater) without any direct electrical connection between the two circuits.

Relay Terminals and Switch Types

A standard relay has distinct terminals:

Coil Terminals: Connected to the low-voltage control circuit to energise the electromagnet.
COM (Common): The moving contact connected to the power supply or load of the output circuit.
NO (Normally Open): The contact is disconnected (open) when the coil is unpowered (de-energised). When the coil turns on, the switch closes and turns the load ON.
NC (Normally Closed): The contact is connected (closed) when the coil is unpowered (de-energised). When the coil turns on, the switch opens and turns the load OFF.

Common Switch Configurations:

SPST: Single-Pole Single-Throw (simple ON/OFF control of one circuit).
SPDT: Single-Pole Double-Throw (switches one common contact between NO and NC).
DPDT: Double-Pole Double-Throw (two separate SPDT switches controlled by one single coil—useful for reversing motor direction).

Using Relays in Electronic Circuits

Small control integrated circuits (ICs) and logic gates cannot output enough current to drive a relay coil directly. Therefore, a transistor switching stage (such as an NPN bipolar junction transistor with a base resistor) is used to switch the coil current. In circuit designs, a protective diode must always be shown across the relay coil.

Key Takeaway for Relays

Quick Summary: Relays use a small coil current to magnetically flip high-power switch contacts, providing complete electrical isolation between low-power controllers and high-power loads.

---

4. Common Exam Mistakes to Avoid

Swapping $R_1$ and $R_2$ in the Divider Formula: Always ensure the resistor whose voltage you want to calculate is the one in the numerator ($R_2$ on top for $V_{out}$ across the bottom component).
Confusing NTC Behaviour: Remember that GCSE thermistors are NTC—their resistance drops when heated. They do not increase in resistance with heat!
Incorrect Diode Orientation: Make sure the protection diode across a relay coil is pointing in reverse bias relative to the normal supply voltage (cathode to the positive supply rail). Connecting it forward-biased creates a short circuit!
Forgetting $V_{LED}$ in Resistor Calculations: When calculating an LED limiting resistor, do not just do $R = \frac{V_{supply}}{I}$. You must subtract the LED drop first: $R = \frac{V_{supply} - V_{LED}}{I}$.
Incomplete Relay Explanations: If asked why a relay is used, do not just say "to turn on a motor." Always mention electrical isolation between low-power control circuits and high-power operating circuits.
Mixing up NO and NC Contacts: Double-check your resting states. Normally Open (NO) is disconnected at rest; Normally Closed (NC) is connected at rest.

Quick check

Can you answer these now?

Open each question to check the key ideas from this chapter.

What is the primary function of a potential divider circuit?

A potential divider splits a supply voltage ($V_{supply} $) across series resistors to convert changes in sensor resistance into changes in output voltage ($V_{out} $).

What is the standard formula for calculating the output voltage ($V_{out} $) across the bottom resistor ($R_2 $) in a potential divider?

$V_{out} = V_{supply} \times \left( \frac{R_2}{R_1 + R_2} \right)$

How does the resistance of an LDR and an NTC thermistor change when light and temperature increase?

For both an LDR and an NTC thermistor, resistance decreases as environmental intensity (light or temperature) increases.

How does the placement of a sensor in a potential divider affect whether $V_{out} $ rises or falls?

If the sensor is at the top ($R_1 $), $V_{out} $ rises as light/temperature increases; if at the bottom ($R_2 $), $V_{out} $ rises as it gets darker/colder.

What are the key operating characteristics of a standard silicon diode?

A silicon diode allows current in only one direction (when forward-biased) and requires a forward voltage drop of approximately $0.6\text{ V} - 0.7\text{ V}$ to conduct.

Why does an LED require a series current-limiting resistor, and how is its value calculated?

To prevent excessive current from destroying the LED; calculated using $R_{limit} = \frac{V_{supply} - V_{LED}}{I_{LED}}$.

Why is a protection (flyback) diode connected across a relay coil?

To safely suppress the high-voltage back-EMF spike produced when the coil's magnetic field collapses, protecting the driver transistor.

What is a relay and why is it used in control circuits?

A relay is an electromechanical switch operated by an electromagnet that provides electrical isolation between low-power control circuits and high-power load circuits.

What is the difference between Normally Open (NO) and Normally Closed (NC) relay contacts?

NO contacts are disconnected when the coil is unpowered and close when energised; NC contacts are connected when unpowered and open when energised.

Ready to test yourself?

Turn these notes into exam-style practice. Get unlimited AI questions on this topic with instant marking and explanations.

Practise This Topic

More Technology and Design 8900 chapters

* The content provided by thinka is generated by AI and may not always be accurate or up-to-date. Please use it as a supplementary resource and verify with official materials.

Put These Notes into Practice

Reading the notes is a great start. Now practise with unlimited AI-generated questions and get instant feedback. 100,000+ students are already improving their grades.

Start Practising Now View Pricing

Done reading? Test yourself with AI practice questions

Practise This Topic Now