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Courses/LED & Lighting Design/Driver Circuits

lesson · 8min · Lesson 7 of 24

Driver efficiency and power factor

Course syllabusCourse overview
01LED Technology
  1. ReadHow LEDs produce light: p-n junction
  2. ReadEfficacy: lumens per watt explained
  3. ReadLED chip formats: COB, SMD, filament
  4. ReadThermal management: heatsinks and junction temperature
  5. quizLED technology quiz
02Driver Circuits
  1. ReadConstant current vs constant voltage drivers
  2. ReadDriver efficiency and power factor
  3. ReadDimming methods: PWM, 0-10V, DALI
  4. ReadDriver selection and compatibility
  5. quizDriver circuit quiz
03Emergency Lighting
  1. ReadBS 5266 categories: escape, standby, high-risk
  2. ReadSelf-contained vs central battery systems
  3. ReadMaintained vs non-maintained operation
  4. ReadTesting requirements and log books
  5. quizEmergency lighting quiz
04Lux Calculations
  1. ReadIlluminance, luminous flux, and efficacy
  2. ReadLumen method: average illuminance
  3. ReadRoom index, UF, and MF
  4. exerciseLux calculation worked example
05Lighting Control & Quality
  1. ReadColour temperature: warm, neutral, cool white
  2. ReadColour rendering index (Ra/CRI)
  3. ReadGlare: UGR and how to reduce it
  4. ReadSmart lighting systems: DALI 2 and IoT
  5. quizFinal assessment
Lesson · 8min
POWER TRIANGLEφP: real (W)Q (VAr)S (VA)PF = P / S= cos φS² = P² + Q²
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In this lesson

PurposeCore theoryWorked exampleKnowledge checkSources

In this lesson

PurposeCore theoryWorked exampleKnowledge checkSources
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ElectraCore lesson handout

Driver efficiency and power factor

Led Lighting · Lesson 7 · Intermediate

IntermediateReview: professional review pending

Purpose

Evaluate driver efficiency, power factor, harmonics, inrush and standby at the operating point.

Before you beginVoltage, current and power · Basic semiconductor and circuit concepts

Learning objectives

  • Trace electrical-to-light conversion
  • Use declared operating boundaries
  • Separate component and luminaire metrics
  • Check control and load compatibility

Efficiency describes real output/input power; power factor describes real power relative to RMS volt-amperes. A driver can be efficient yet have poor power factor or high harmonic/inrush current.

DRIVER OUTPUT AND CONTROL WINDOWDRIVER OUTPUT AND CONTROL WINDOWDRIVERLEDlight · heatinput · regulation · junction · optics · control · environment

Core theory

Measure/compare at relevant load and dim level. Driver efficiency normally falls at light load; PF can include displacement and distortion effects.

Check input current, THD/harmonics, flicker, inrush peak/duration/I²t, leakage, standby and surge immunity. Many drivers together can overload switching/protection despite low steady watts.

Use manufacturer tables to determine devices per MCB, relay/contactor and control output; do not divide breaker amperes by steady current alone.

Terms, symbols, and units
TermMeaningSymbolUnit
Power factorReal power divided by RMS apparent powerNot applicableNot applicable
THDTotal harmonic distortionNot applicableNot applicable
InrushShort high input current during energisationNot applicableNot applicable
PF = P/(Vrms Irms); η=Pout/Pin

Different ratios describing supply waveform use and conversion loss.

dimensionless
Worked exampleA driver draws 40 W real power at 230 V and 0.25 A RMS.

Assumptions: Supplied photometric/electrical values share the stated measurement boundary; Manufacturer limits remain governing.

  1. Apparent: S=230×0.25=57.5 VA.
  2. PF: PF=40/57.5=0.696.
  3. Meaning: This does not reveal efficiency without output power or inrush/THD.

Power factor is about 0.70.

Reasonableness check: The result identifies whether it describes LED package, module, driver or complete luminaire and avoids claiming field performance from one laboratory number.

Common mistakes
  • Dividing lumens by LED-chip watts while calling it luminaire efficacy
  • Connecting constant-current LEDs to a constant-voltage source without current control
  • Assuming every dimmable driver works with every dimmer

Where this appears in practice

A reliable LED installation coordinates light source, optics, thermal path, driver, supply/protection, controls, environment and maintainability.

SafetyLED outputs can create glare/photobiological hazards and drivers can contain mains voltage and stored charge after switch-off. Isolate, prove dead, observe discharge instructions and never defeat thermal interfaces or enclosure barriers.
Local code checkConfirm current luminaire/module/controlgear safety and performance standards, BS 7671, exact driver/module/luminaire data, photobiological risk, thermal/fire enclosure requirements, EMC/flicker/harmonic obligations and control-protocol certification. A CE/UKCA or DALI-2 mark does not prove two arbitrary components are electrically, thermally or functionally compatible.

Knowledge check

Does 90% efficiency imply a 0.90 power factor?

No. They measure different properties.

Answer: No. They measure different properties.

Practical exercise

Compare drivers across full/dimmed efficiency, PF, THD, inrush, standby and breaker limits.

Summary

  • Metric boundary changes the answer
  • LED current and junction temperature govern life/output
  • Driver and dimmer must match electrically and functionally

Sources and review

  • IEC 62031:2026: LED modules safety requirements: IEC; 2026; International
  • IEC 61347-2-13:2024: LED controlgear safety: IEC; 2024; International
  • DALI-2 certification overview: DALI Alliance; Current certification database/status must be checked; International

Editorial review date: 2026-08-22. Professional electrical review is pending.

Educational material for learning and preliminary checks. Verify current local requirements and exact equipment instructions. This lesson does not replace competent professional work.

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