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Courses/LED & Lighting Design/Driver Circuits

lesson · 9min · Lesson 6 of 24

Constant current vs constant voltage drivers

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 · 9min
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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

Constant current vs constant voltage drivers

Led Lighting · Lesson 6 · Intermediate

IntermediateReview: professional review pending

Purpose

Match constant-current and constant-voltage controlgear to the LED load topology and operating window.

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

A constant-current driver regulates output current while its voltage floats within a compliance window; a constant-voltage driver regulates voltage while LED modules/strips include their own current-limiting architecture.

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

Core theory

For constant current, match rated current exactly as permitted and ensure cold/hot string forward voltage remains within start, minimum and maximum output limits.

For constant voltage, match nominal voltage and keep aggregate steady and inrush power/current within driver capacity, derating and wiring voltage-drop limits.

Check SELV/isolation classification, output ripple, dimming, open/short-circuit behavior, cable length, polarity, connectors and parallel/series rules. Never mix output types by watt arithmetic.

Terms, symbols, and units
TermMeaningSymbolUnit
Compliance voltageVoltage range over which a current driver regulatesNot applicableNot applicable
Constant voltageRegulated output voltage architectureNot applicableNot applicable
HeadroomMargin between load demand and output limitNot applicableNot applicable
Worked exampleA 700 mA driver has 18–42 V compliance; a series module is 9 LEDs at 3.2 V nominal each.

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

  1. Voltage: Nominal string voltage=9×3.2=28.8 V.
  2. Window: 28.8 V lies inside 18–42 V.
  3. Continue: Check cold/hot/tolerance extrema, power, connectors and module's 700 mA rating.

Nominal voltage fits, but complete compatibility remains conditional.

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

Can a 24 V constant-voltage strip be driven directly by a 700 mA constant-current driver because wattage matches?

No. Output architecture and load regulation must match.

Answer: No. Output architecture and load regulation must match.

Practical exercise

Pass/fail candidate drivers using current, voltage extrema, power, isolation and dimming data.

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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