Calculate the power dissipated in the series resistor of an LED on 9 V, with a 2 V forward voltage and 20 mA flowing.

The resistor dissipates 0.14 W, which is within a 0.25 W rating. P = V × I = (9 − 2) × 0.02 = 0.14 W.

Why is the next higher preferred value chosen for an LED resistor, not the next lower?

A higher resistance keeps the current just below the LED's maximum. A lower value would push the current above the maximum rating.

Compare an LED with a filament lamp as an indicator.

An LED draws far less current than a filament lamp and lasts much longer. A filament lamp wastes most of its energy as heat and has a fragile filament.

A 12 V heater is rated at 36 W. What current does it draw?

The heater draws a current of 3 A. I = P ÷ V = 36 ÷ 12 = 3 A.

CCEA A-Level Technology and Design 8900

Material Selection and Wood

Topic overview Material selection criteria Physical and mechanical properties Hardwoods, softwoods and timber stock sizes Pine, ash, beech, oak and mahogany Manufactured boards Wood finishes, veneers and laminates

Metals

Topic overview Ferrous and non-ferrous metals and alloys Metal stock forms and sizes Aluminium, copper, brass and zinc Carbon steels and stainless steel Finishes for metals

Plastics

Topic overview Thermoplastics and thermosetting plastics HDPE, LDPE and PET Polystyrene and PVC Acrylic, nylon and ABS Melamine-formaldehyde and epoxy resins

Composite and Emerging Materials

Topic overview Thermochromic and photochromic materials Phosphorescent pigments, reflective films and holograms Liquid crystal displays PEEK, PPSU and graphene

Processing Materials

Topic overview Wasting with hand tools Manual and CNC machine tools Rolling, blanking, press forming and forging Injection and blow moulding Rotational moulding and vacuum forming Sand casting, die casting and extrusion

Product Analysis and Design Communication

Topic overview Form, function, performance and aesthetics Cost, materials, manufacture and environmental issues Target audience, anthropometrics, culture and safety Design briefs, research and specifications Creativity, innovation and sustainability in design Freehand sketching and mixed media

Design for Manufacture, Quality and Safety

Topic overview CAD modelling, virtual imaging and rapid prototyping CAM and CIM Factor of safety and tolerances Testing and inspection of products Risks in workshop and manufacturing processes

Electronic Control SystemsOption A

Topic overview Control systems, feedback and electronics safety Resistors, Ohm's law and power ratings Capacitors and RC timing Potential dividers, sensors and comparators Switches, pull-up and pull-down resistors Output devices and LEDs Transistors, Darlington pairs and thyristors Logic gates, truth tables and flip-flops 555 monostable and astable timers

Microelectronic Control SystemsOption A

Topic overview Microcontroller systems, ADCs and DACs Strain gauges and bridge circuits Optical switches, encoded discs and Schmitt triggers Op-amp amplifiers and gain Zener diodes and voltage regulators Karnaugh maps and binary counters Seven-segment, LCD and dot matrix displays PIC flowcharts and programming DC motors, stepper motors and driver circuits

Mechanical and Pneumatic ComponentsOption B

Topic overview Open and closed loop mechanical and pneumatic control Safe working with compressed air and mechanisms Levers, linkages and moments Gear types and gear trains Pulleys, belts, chains and shaft fixings Cams, cranks and converting motion Compressed air supply and pneumatic valves Cylinders, shuttle valves, restrictors and reservoirs Pneumatic circuits for logic, timing and speed control

Mechanical and Pneumatic SystemsOption B

Topic overview Hydraulics, system limitations and plant safety Torque, work, power and air consumption Vacuum handling, proximity sensors and 5/3 valves Sequential and cascade pneumatic circuits Cam profiles and follower displacement diagrams Gear geometry, pulley blocks and ratchets Toggles, treadles, shafts and couplings Friction, brakes and clutches Bearings, lubrication and seals

Designing and Manufacturing ProductsOption C

Topic overview Research, questionnaires and specifications Idea generation, critiques and computer simulation Manufacturing planning and product evaluation CFRP, GRP, Kevlar and tungsten carbide Smart materials, nanotechnology and biophotovoltaics Isometric, perspective and assembly drawing Laminating, steam bending, cutting and printing processes Risk assessment and product safety law Scales of production, JIT and cell production Quality assurance, ISO 9001 and continuous improvement Design influences and intellectual property rights

Product Design in Industry and SocietyOption C

Topic overview Life-cycle environmental impact and climate change The 6Rs, waste and environmental legislation Sustainable metals, plastics and degradable polymers Product life cycles, consumer types and relaunches CAD, CAM, 3D scanning and quick-change moulding Market pull, technology push and market strategy Price, place and promotion Jigs, fixtures and ICT in quality control Design for maintenance, inclusion and scale production Rendering, typography and presentation Dyson, Baylis, Memphis and Apple

Product Development

Topic overview Choosing a product to redesign Analysing the function and fitness for purpose of existing products Materials, manufacture and sustainability of existing products Ergonomic and aesthetic analysis Redesign modifications, sketching and viability Modelling, testing and evaluating a redesign

Product and System Design and Manufacture

Topic overview Clients, end users and design briefs Researching existing solutions and writing a specification Generating and appraising design proposals Developing a concept with numerical analysis, ergonomics and aesthetics Working drawings and planning manufacture Testing and evaluating a prototype

Electronic Control Systems

Output devices and LEDs

3 things to know
AS 1 Paper 2

Overview

LED series resistors with worked values, LEDs against lamps, current calculations for heaters and solenoids, relays and latching relays, solenoid valves, buzzers and piezo sounders.

What you need to know

Answer each one before you reveal it.

Your recallNot started

Key term

What do COM, NO and NC mean on a relay's contacts?

Key term

Why must an LED always have a series resistor?

Numbers

Calculate the series resistor for an LED on 9 V, with a forward voltage of 2 V and a maximum current of 20 mA.

Questions from this subtopic

Each one with the model answer underneath.

1
Why must an LED always have a series resistor?
Model answer
An LED has very little resistance once it conducts, so the current would rise until the LED failed. The series resistor limits the current to a safe value.
2
Calculate the series resistor for an LED on 9 V, with a forward voltage of 2 V and a maximum current of 20 mA.
Model answer
The minimum resistance is 350 Ω, so the next preferred value up, 390 Ω, is chosen. R = (9 − 2) ÷ 0.02 = 350 Ω.
3
Calculate the power dissipated in the series resistor of an LED on 9 V, with a 2 V forward voltage and 20 mA flowing.
Model answer
The resistor dissipates 0.14 W, which is within a 0.25 W rating. P = V × I = (9 − 2) × 0.02 = 0.14 W.
4
Why is the next higher preferred value chosen for an LED resistor, not the next lower?
Model answer
A higher resistance keeps the current just below the LED's maximum. A lower value would push the current above the maximum rating.
5
Compare an LED with a filament lamp as an indicator.
Model answer
An LED draws far less current than a filament lamp and lasts much longer. A filament lamp wastes most of its energy as heat and has a fragile filament.
6
A 12 V heater is rated at 36 W. What current does it draw?
Model answer
The heater draws a current of 3 A. I = P ÷ V = 36 ÷ 12 = 3 A.

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Material Selection and Wood

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The output voltage is V.

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AThe total resistance is 2 kΩ.

BThe parallel pair gives (2 × 2) ÷ (2 + 2) = 1 kΩ.

CThe series resistor adds another 1 kΩ.

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