FAQ
TL;DR: "LEDs are current devices, not voltage devices"—design for current, not forward voltage; e.g., 10 strings × 5 LEDs with ~82 Ω per string. [Elektroda, Steve Lawson, post #21662797]
Why it matters: If you try to match by voltage, brightness will vary wildly; current control fixes "how do I make my tail lights even?" for builders and restorers.
Quick Facts
- Automotive 12 V systems can rise to ~14–14.5 V while charging; size resistors or add current regulation accordingly. [Elektroda, Kent Walker, post #21662800]
- Example calc: five red LEDs (~1.8 V each) on 12 V → ~3 V across resistor; at 50 mA use ~60 Ω, 0.25 W. [Elektroda, Kent Walker, post #21662800]
- To tolerate higher car voltage, use a larger resistor (e.g., ~82 Ω suggested for ~14 V scenarios). [Elektroda, Steve Lawson, post #21662802]
- LM317 can operate as a constant-current source; see datasheet application and wire per notes. [Elektroda, Steve Lawson, post #21662803]
- Poor-quality batches can vary wildly; a Ligitek batch produced uniform results, 5 mm red, 8,000–12,000 mcd, 20 mA, 25° view. [Elektroda, Quenton Kilgore, post #21662817]
Why do LEDs with the same measured forward voltage look different in brightness?
Brightness tracks current, not nominal forward voltage. Small Vf changes cause large current changes because the I–V curve is steep. Parallel testing or selection by Vf misleads. Use current-limited strings so each string’s current is defined. “LEDs are current devices, not voltage devices.” [Elektroda, Steve Lawson, post #21662797]
How should I wire 55 red LEDs on a car for even brightness?
Use series strings with a resistor or current driver per string. For 12 V, design for five red LEDs in series and calculate the resistor from the remaining voltage. Account for charging voltage up to ~14.5 V or add a panel-level current limiter for stability. [Elektroda, Kent Walker, post #21662800]
What resistor value works if my alternator reaches ~14 V?
Increase series resistance to compensate. An ~82 Ω resistor per five-LED string helps keep current reasonable when the vehicle bus climbs toward 14 V. This reduces overcurrent and extends LED life under charging conditions. [Elektroda, Steve Lawson, post #21662802]
Can I use an LM317 as a constant-current LED driver?
Yes. Configure the LM317 as a current regulator per its application note. A single current source can feed strings directly or be multiplexed. This approach greatly improves uniformity over plain resistors. [Elektroda, Steve Lawson, post #21662803]
How do I quickly bin LEDs so strings match?
Drive each LED at the target constant current, then sort by forward voltage or luminous output.
- Set a stable constant-current source to your operating current.
- Measure Vf or brightness and group LEDs in tight bands.
- Build strings from the same band; discard outliers. [Elektroda, Steve Lawson, post #21662812]
Why did a fixed 2.0 V test produce 2–76 mA across similar LEDs?
Testing at fixed voltage exploits the diode knee. Tiny Vf differences create huge current swings. Prior parallel hookups may also stress parts and reduce brightness. Quality variations amplify the effect, so currents scatter. Avoid voltage-only tests for selection. [Elektroda, Steve Lawson, post #21662809]
What panel current should I target for 11 strings of five LEDs?
At 50 mA per string, total is 550 mA. An LM317 constant-current stage with ~1.25 V reference needs ~2.2 Ω to set ~0.55 A. Heatsink the regulator and keep the tab isolated from ground to avoid shorts. [Elektroda, Kent Walker, post #21662800]
Are 5 mm T1-3/4 “50 mA” LEDs really OK at 50 mA?
Many 5 mm LEDs are 30 mA max continuous. If you multiplex, confirm allowable pulse current, duty cycle, and frequency in the datasheet. Exceeding pulse limits leads to early dimming or failure. [Elektroda, Steve Lawson, post #21662810]
Should I trust forward voltage to match brightness across strings?
No. Forward voltage varies with temperature, aging, and lot differences. Two LEDs with the same Vf can diverge in current and brightness later. Match with constant-current testing and design each string for controlled current. [Elektroda, Steve Lawson, post #21662809]
What resistor power rating do I need per five-LED string at 12 V?
Dropping about 3 V at 50 mA dissipates 0.15 W. A 0.25 W resistor per string provides margin. If the vehicle rises to 14.5 V, dissipation increases; check power or add current regulation. [Elektroda, Kent Walker, post #21662800]
Where can I buy consistent LEDs for tail lights?
Consider reputable vendors with consistent binning. One recommendation was SuperBrightLEDs, noted for long-term consistency despite higher cost. Paying more up front can save rework and scrap. [Elektroda, Steve Lawson, post #21662816]
Any budget source that still delivered uniform parts?
A batch of Ligitek 5 mm red LEDs from Tayda Electronics produced a spot-on panel. Specs reported: 20 mA, 2.0–2.2 V, 8,000–12,000 mcd, 25° view. Consistency improved versus a prior eBay lot. [Elektroda, Quenton Kilgore, post #21662817]
How much LED variation is normal, and when is it a defect?
Small I–V variances are inherent, but extreme spread suggests operation near minimum current or poor-quality parts. Check current in each string via resistor voltage. Large current range signals selection issues, not just normal tolerance. [Elektroda, Frank Bushnell, post #21662807]
Could mis-marked polarity cause dim or dead LEDs?
Yes. Cheap batches can even have swapped lead lengths, reversing anode and cathode identification. Verify polarity before assembly to prevent backward installs and rework. This is a known failure gotcha. [Elektroda, Frank Bushnell, post #21662814]
Should I salvage a bad, uneven LED panel or rebuild?
If many LEDs are out of spec, rebuilding with a consistent lot is faster than chasing mismatches. One builder scrapped the old boards and achieved uniform results with a new batch. [Elektroda, Quenton Kilgore, post #21662818]
What’s a simple string design example I can copy today?
Use 11 strings of five 5 mm red LEDs. Add one resistor per string sized for your system voltage. For 12 V static operation, ~60 Ω, 0.25 W per string is a practical starting point. [Elektroda, Kent Walker, post #21662800]
