Green LED Not Lighting: Troubleshooting Steps, Voltage Checks, Resistor Values

User question

yeşil led yanmıyor

Artificial Intelligence Response

Direct answer to the question

• A green LED not lighting almost always traces to one of four things: no drive/no power, reversed polarity, an open/failed LED or series resistor, or a driver/output stage that isn’t switching.
• Start by measuring voltage directly across the LED while it is commanded ON; that single reading tells you where to look next.

Key points

• VLED ≈ 0 V → no drive or LED shorted. Trace back to the driver/switch.
• VLED ≈ supply voltage → open LED or open series resistor/trace.
• VLED ≈ 2.7–3.3 V (InGaN green) but dim/off → current too low or wrong resistor value.
• Intermittent/flicker on touch → cold solder/connector issue.

Detailed problem analysis

1) Identify the topology

• Simple indicator from DC rail: supply → resistor → LED → return (or the inverse).
• MCU-driven directly: MCU pin → resistor → LED → VCC/GND (ensure I/O can source/sink required current).
• Transistor/MOSFET driven: MCU → base/gate resistor → NPN/NMOS → LED+res → supply/ground.
• RGB/strip/channel case: each color has its own resistor/driver; a single failed color points to its path.

2) Correct forward voltage and current targets

• Modern true/bright green (InGaN) LEDs: Vf ≈ 2.7–3.3 V at 5–20 mA. Older “yellow‑green” (GaP/AlInGaP): Vf ≈ 2.0–2.4 V.
• If you swapped from red (~1.8–2.1 V) to green without recalculating R, current may be too low to light.

3) First measurements (multimeter, device powered and LED commanded ON)

• Measure VLED (anode–cathode):
  • ≈ supply (e.g., ≈5 V or ≈12 V): open LED or open series resistor/track. Ohm-check the resistor; continuity-check tracks; diode-test the LED out of circuit.
  • ≈ 0 V: the driver never pulls high/low; wrong polarity; LED shorted. Confirm polarity, then probe the driver node.
  • ≈ 2.7–3.3 V: LED is biased but probably under‑current. Verify series resistor value and the driver’s source/sink capability.
• Measure across the series resistor:
  • If VR = 0 V while “ON”: no current flows → open LED/trace/driver off.
  • If VR is correct but LED still off: LED open or reversed.

4) Resistor calculation and examples

• R = (Vs − Vf) / I. Choose I per LED’s datasheet and visual requirement.
• 5 V rail, Vf = 2.9 V, I = 10 mA → R ≈ (5 − 2.9)/0.01 = 210 Ω (use 220 Ω).
• 12 V rail, Vf = 3.0 V, I = 8 mA → R ≈ (12 − 3)/0.008 = 1125 Ω (use 1.1–1.2 kΩ).

5) Driver-stage diagnostics

• MCU direct drive:
  • Check the pin level with a DMM/logic probe. Many MCUs source ~4–8 mA and sink ~8–20 mA; exceeding this or using the wrong polarity can keep the LED off.
  • Confirm pin mode and that firmware actually asserts the LED (brief test firmware: blink at 1 Hz).
• NPN low-side:
  • ON: base ≈ 0.7 V, collector near ground; OFF: base ≈ 0 V, collector floats high via LED/resistor.
  • If base has drive but collector stays high: transistor open.
• NMOS low-side:
  • Gate must be > Vth + margin (e.g., 2.5–4.5 V for logic-level types). If gate toggles but drain doesn’t pull low, MOSFET may be open.
• High-side PNP/PMOS drivers: check for correct gate/base pull-up network and reference.

6) Physical and assembly issues

• Polarity: through-hole LED long lead = anode; flat edge/notch = cathode. SMD often marks the cathode.
• Cold solder/cracked joints around LED, resistor, connector. Reflow with flux.
• Connector pinouts (LED strips/panels): “G” channel has its own pin and transistor; a single dead color usually means open MOSFET/trace or connector pin.

7) LED health tests (power off and remove one leg if possible)

• DMM diode mode: a healthy green LED typically shows 2.5–3.0 V reading one way, OL the other.
• Coin-cell + 330–1 kΩ resistor test, or bench supply at 5 mA current limit. Never reverse-bias >5 V; many SMD LEDs have low reverse ratings.

8) Special cases

• Long LED strips: voltage drop along the strip dims green/blue first (higher Vf). Power-inject both ends; keep segment currents within spec.
• Mains-powered chargers/tools: if the green “power” indicator is dark, troubleshoot the low-voltage supply first (fuse/NTC/bridge/primary SMPS). Do not probe live primary unless qualified.

Current information and trends

• Modern high-efficacy green LEDs (InGaN, ~525–530 nm) achieve high brightness at 2–5 mA; designing for lower currents reduces heat and improves lifetime.
• Reverse-voltage robustness of many modern SMD indicators is still poor (often 5 V max); include antiparallel diodes when AC or reverse transients are possible.
• Many MCUs now default I/O pins to high‑impedance at reset; firmware sequencing may briefly turn LEDs off until initialization. Account for this in tests.

Supporting explanations and details

• Why VLED tells the story: An LED is a diode; if it is open, the node floats to the rail through the resistor (≈Vs). If shorted or not driven, it sits near 0 V. A normal forward-biased LED clamps near its Vf.
• Human perception: green is near peak luminous efficiency; you can often reduce current substantially without perceived dimming.

Ethical and legal aspects

• Safety: If the board has mains/primary SMPS sections, isolate and discharge capacitors before working. Use an isolation transformer for live measurements.
• Warranty/compliance: Opening tools/chargers can void warranties and may violate safety approvals. Observe ESD precautions when handling LEDs/MCUs.
• E-waste: Replace only failed components where practical; avoid unnecessary board scrapping.

Practical guidelines

• Step-by-step mini checklist: 1) Visual check: polarity mark, cracks, scorching, connector seating. 2) Measure VLED while “ON”. 3) Measure VR across the series resistor; compute expected current. 4) Verify resistor value with an ohmmeter; check for mis-stuffed parts (e.g., 22 kΩ instead of 220 Ω). 5) Probe driver node(s): MCU pin, base/gate, collector/drain. 6) Reflow suspect joints; clean flux. 7) Diode-test the LED out of circuit; replace if open. 8) If strip: swap controllers/channels to see if the fault follows the channel or the LED segment.
• Parts to keep on hand: 0603/0805 green LEDs, 220–2.2 kΩ resistors, logic-level NMOS (e.g., 2–4 A class), ESD wrist strap.

Possible disclaimers or additional notes

• Without schematic/model specifics, diagnosis is generic. Some devices intentionally blank LEDs via firmware or power‑save modes; confirm intended behavior.
• If the LED is part of a series string (backlights), one open device will extinguish the whole string; you’ll need constant-current testing and segment injection.

Suggestions for further research

• Check the specific LED’s datasheet (Vf–I curve, absolute max, reverse voltage).
• Review the driver IC or MCU port characteristics in its datasheet.
• For strips, review application notes on power injection and voltage drop for 12/24 V systems.

Brief summary

• Measure voltage across the green LED while it should be ON; that reading classifies the fault.
• Verify polarity, series resistor value, and driver operation; reflow or replace the LED/resistor as indicated.
• Use correct Vf (≈2.7–3.3 V for modern green) when calculating current, and respect driver current limits. If you share the device type (tool/charger, PCB indicator, or LED strip), supply voltage, and your VLED/VR measurements, I can give you a precise, component-level next step.

Disclaimer: The responses provided by artificial intelligence (language model) may be inaccurate and misleading. Elektroda is not responsible for the accuracy, reliability, or completeness of the presented information. All responses should be verified by the user.