37. LED Selection & Failure

• A Light Emitting Diode is a PN‑junction device that emits photons when forward‑biased (electroluminescence).
• It’s optimized for light output, not rectification.

LED and connections diagram:

• Conduction: Turns ON when LED’s forward voltage (VF) is reached; current must be limited externally or by a driver.
• VF ranges (typical):
  • Red/IR ~1.6–2.0 V,
  • Green/Yellow ~2.0–2.4 V,
  • Blue/White ~2.8–3.6 V.

LED schematic symbol (fixed, RGB, bicolor)

IV curve (VF vs IF) from datasheet for red & white LEDs

1) Status indicators (power, fault, communication).

2) Backlighting (LCDs, keypads).

3) IR communication (TV remotes, proximity).

• VF (Forward Voltage): Voltage across LED at a given IF; varies with LED color (material bandgap), applied current, and temperature.

• IF (Forward Current): It determines LED brightness and heating. 
  It must always be kept below the Absolute Maximum Rating to avoid damage.
  Example: For a 20 mA LED, the recommended IF might be 10–15 mA for long life.
• IFP (Peak/Pulse Current): Short pulses allowed under specific duty cycles.
  Typical limit of duty cycle: ≤ 1%–10%, depending on LED.
• Luminous intensity (mcd) vs Luminous flux (lm):
  • Luminous intensity is the amount of light emitted in a specific direction. Unit: mcd (millicandela).
  • Luminous flux is the total quantity of visible light emitted in all directions. Unit: lumen (lm).
  • Design Rule: For indicators, use mcd (spot brightness), for lighting use lm (total illumination).

• Viewing angle (°): Beam spread; a narrower beam looks brighter head‑on.
• Wavelength (nm) / Dominant wavelength: Color accuracy for mono color LEDs.
• Correlated Color Temperature (CCT): CCT describes the apparent color of white light from an LED, expressed in Kelvin (K).
  • Low CCT → Warm, relaxing light (residential, decorative).
  • High CCT → Cool, stimulating light (offices, hospitals, outdoor, industrial).
• Color Rendering Index (CRI) – color accuracy for illumination.
• Thermal resistance (RθJA/RθJB/RθJS): Junction‑to‑ambient/board/solder; lower is better.
• Power dissipation & derating: Max heat the LED can dissipate; check derating vs temperature.
  Derating Curve : (Understand the curve carefully- although max current rating is 30mA- it declines over the Temperature rise. While operating in High ambient temperature- need to consider the curve)

• Reverse voltage (VR): Often very low; avoid reverse bias or provide a protection diode.

Example Würth Elektronik 151051RS11000 (WL-TMRC series) 5mm red LED specifications:

LEDs are polarized; current only flows from Anode (+) to Cathode (-).

1. Visual Identification

• Anode (+): Longer lead; round casing side.
• Cathode (-): Shorter lead; flat edge on the casing; internal "anvil" (larger metal part).
• SMD (Surface Mount): A printed line, dot, or notch indicates the Cathode.

2. Multimeter Testing (Diode Mode)

To verify polarity or health, touch the probes to the leads:

3. Diagnostic Results

• Healthy: Shows voltage in forward bias and OL in reverse.
• Faulty (Open): Shows OL in both directions (internal break).
• Faulty (Short): Shows 0.00 in both directions (internal fuse).

• Indicator LED’s Forword current (IF): 2–20 mA (many are visible at 1–2 mA with high‑efficiency types).
• High‑power LED’s Forword current (IF): ~350 mA (1 W), 700 mA (3 W), 1 A+ (5 W+).
• Common wavelengths: IR 850/940 nm; Red 620–630 nm; Green 520–530 nm; Blue 460–470 nm; UV 365–400 nm.
• Common packages: Through hole, 0805/1206 (indicator), 2835/3030/3535/5050/7070 (power/RGB), COB modules.

1. Understand the application: Indicator? Backlight? Illumination? IR/UV? RGB effects?
2. Pick color/wavelength or CCT/CRI:
   • Indicators: choose color for contrast; use narrow beam for bright “dot”.
   • Illumination: choose CCT (e.g., 3000 K warm) and CRI (≥80 or ≥90 for color‑critical).
3. Set brightness target:
   • Select LEDs by reviewing datasheets for high luminous intensity (millicandela/mcd) and proper viewing angles.
4. Choose electrical approach: Resistor (simple), constant‑current driver (recommended for power/RGB/precision).
5. Budget voltage/current: Verify supply headroom for max IF (Forward current) and VF (Forward Voltage). consider ambient temperature.
6. Thermal plan: Estimate power → Rθ path → heatsink/copper/MCPCB.
7. Package & assembly: THT vs SMD; reflow vs hand solder; lens/mechanical constraints.
8. Reliability: Check derating, ESD, MSL, sulfur resistance (for environments with sulfides).

• Series preferred for equal current through each LED.
• Parallel pitfalls: Current hogging due to VF mismatch → one LED overheats.
  • If you must parallel, give each LED its own resistor or use current-sharing circuitry.
• Matrix/multiplexing: Consider duty cycle; increase peak current within IFP limits to maintain average brightness; watch eye‑safety with IR/UV.

• Overcurrent/overheat: Open circuit or dark/weak LED; lumen depreciation (L70 reached early).
• ESD damage: Partial shorts/leakage; dim or intermittent behavior.
• Color shift: Phosphor aging in white LEDs; visible tint change.
• Sulfur corrosion: Blackening of silver‑plated leadframes (harsh environments).
• Moisture popcorning: Cracks after reflow if MSL not respected.
• Optics degradation: Lens yellowing/clouding with heat/UV.

Normal vs Burnt LED Image:

1. Luminous intensity vs viewing angle
   • Luminous intensity (mcd) indicates how bright an LED appears in a specific direction
   • LEDs with a narrow viewing angle:
     • Light is more focused
     • Appears brighter (higher mcd)
   • LEDs with a wide viewing angle:
     • Light spreads out more
     • Appears less bright
   • High mcd LEDs are preferred for indicators for better long-distance visibility
2. Wavelength & color
   • LED color is defined by its wavelength (in nm)
   • Infrared: ~760 nm+ (used in remotes, cameras)
   • Visible: ~450–760 nm (blue to red)
   • UV: ~365–400 nm
   • Narrow spectral output results in a purer and more saturated color
3. Color temperature (CCT) – white LEDs
   • Measured in Kelvin (K), indicates light tone
   • 2700–3000 K: Warm (yellowish, home use)
   • 3500–4100 K: Neutral (offices)
   • 5000–6500 K: Cool (industrial, hospitals)
   • High temperature or current can shift color (usually toward blue)
4. Hue (color type)
   • Represents the actual color, linked to wavelength
   • Expressed as an angle (0°–360°):
     • Red: 0°
     • Green: 120°
     • Blue: 240°
   • RGB mixing can generate any color
5. Saturation (color purity)
   • Defines how pure or washed-out a color is
   • 100% → Pure, vivid color
   • 0% → White/gray
   • Adding more colors reduces saturation
6. Intensity (brightness)
   • Represents total light output
   • Controlled using PWM (Pulse Width Modulation):
     • 0% → Off
     • 100% → Maximum brightness
   • Allows brightness control without affecting color
7. HSI vs RGB (for programming)
   • Hue: Sets color
   • Saturation: Controls color purity
   • Intensity: Controls brightness (via PWM)
   • HSI is more intuitive compared to raw RGB values