Why Is Only Half of My LED Sign Lit?

I've been building signs on the factory floor since 2010, and I get this call twice a month: half the letters are glowing, the other half are dark, and nine times out of ten it's not the LEDs—it's the power or the wiring between them.

Before you start yanking modules out of the face, map the failure. A clean left/right split usually points to a power supply branch or a wiring harness leg. A top/bottom split means water got in and killed a row. Alternating letters off and on means a data channel, not a power issue.

If you're ordering front-lit channel letters for a storefront, ask your factory whether the sign is wired in series or parallel. Series runs fail in blocks. Parallel runs fail in branches. That single question saves you half the troubleshooting time.

The Most Common Causes of a Half-Lit LED Sign

The usual suspects for a half-lit LED sign: a failed power supply leg, a dead LED driver channel, a broken solder joint, a data line fault on addressable signs, or water getting where it shouldn't.

Here's what nobody tells you: half-lit signs aren't random. They split along the exact lines electricians ran their wires. If the dead zone runs top to bottom on one side, that's a power leg. If it runs across a horizontal band, that's water sitting in the letter face or a dead row of modules.

I had a hair salon client whose sign lost the right half on a Tuesday. They thought lightning. It was a loose spade connector on the power supply's second output. Ten-cent fix, two-hour service call.

That's the job. Half-lit signs are usually boring, mechanical problems dressed up as catastrophic failures.

Power Supply Failure and Voltage Drop: The Usual Suspect

Look, the power supply is the cheapest part of the system and the one most likely to fail in a way that takes out exactly half your sign. A typical 24V supply has two or three output terminals. One dead terminal kills every LED module on that leg.

Before you order a new supply, check the terminal block. I can't tell you how many times I've found a screw backed out from vibration. LCL sea freight bounces your crate around 6-8 times between our dock and yours. Every touch gives a connector a chance to wiggle loose, and that's before the sign even gets mounted over a busy street.

Voltage drop is the slow killer. The SMD 2835 modules we build run 100-150 lm/W, but they're designed for a specific voltage window. Push a run too long on undersized wire and the last modules dim, flicker, then die. The first modules look fine. Half the sign looks dead.

Here's the part installers skip: if you wire a 24V supply with a long run of undersized cable, the voltage at the far end drops enough that the last modules shut down first. The first modules pull current, the last modules starve. You don't need a new supply. You need heavier wire.

We offer a 1-year warranty on power supplies and 2 years on LED modules. Cheap supplies? You're lucky to get 6 months. I've pulled power supplies out of signs that were barely a year old and found swollen capacitors. That's a factory that cut corners on parts.

LED Driver or Channel Failure: How to Isolate the Dead Zone

An LED driver isn't just a transformer. It's a current regulator, and a failed channel can mimic a bad LED module exactly. If your sign is addressable or has separate channels per color, swap the dead zone's driver output with a known-good one.

If the dead zone moves, it's the driver. If it stays, start looking at the modules and wiring. That single test isolates half the problem in about five minutes.

I've seen this go wrong: a contractor replaced 14 RGB modules because one channel wouldn't fire, and the real problem was a loose data wire at the controller. He spent $300 on parts and still had a dark section. Don't be that guy.

For back-lit halo letters, one bad module in a series run can take down an entire branch. Isolate by jumping across the suspect module. If the rest of the run lights, that module was the problem. If it doesn't, keep moving back toward the power supply.

L70 ≥50,000 hours means after 50,000 hours the LED still emits 70% of its original light. That's a lifespan rating, not a promise that nothing else fails. Drivers, solder joints, and connectors fail long before the chip itself on a decent run.

Wiring and Connection Faults That Split a Sign in Half

The ugly truth is most installers don't know how to make a waterproof splice. They use wire nuts, wrap them in electrical tape, and call it done. Water wicks up the wire and corrodes the connection from inside.

Look at the solder joints on any suspect LED module. Dull gray means cold solder or cheap lead-free alloy—that won't last 18 months. Shiny silver is proper lead-free solder. I've pulled apart failed signs where the solder looked like chalk and the module fell off the wire in my hand.

If your sign uses stainless steel letters, the body might last 10+ years, but the internal wiring won't if water gets through the conduit. Always seal entry points with neutral silicone, and don't bury AC and DC lines in the same chase without separation. Induction from AC lines can cause flicker and partial failures on DC LED runs.

One rookie mistake I still see: replacing every module when only one branch is out. Test the wiring first. You'll save $400 and a callback.

CE marking costs 20,000-50,000 RMB. Most factories will claim they have it. Ask to see the certificate with today's date. Watch them sweat. The same goes for UL 48—that's $4,000-15,000 and quarterly audits. A factory that actually paid for those certs won't hide them.

Data Signal and Control Board Failures in Addressable Signs

Addressable signs are a different animal. If you have alternating letters lit and dark, that's not voltage drop—that's a data line break. The chips aren't getting the signal to fire.

Check the controller first. Many DMX programmable controllers have per-channel fuses. One blown channel equals one dead zone. Also check the data cable terminator. A missing terminator can cause reflection and drop random pixels.

I had a nightclub job where the whole left side of a 40-foot RGB wall went dark. The owner was ready to rip it out. The fix was reseating an RJ45 connector. Five minutes. The sign had been running fine for 18 months, but vibration from the subwoofers had crept the connector loose.

Our RGB runs use SMD 5050, which is RGB capable, but if the data line is compromised, the chip sits dark regardless of power. SMD 5730 modules run 40-60 lm/chip and are great for high-brightness single colors, but they still need a clean signal path.

Environmental and Installation Factors That Cause Partial Outages

Water is the number one environmental killer. A sign mounted flat against a wall can trap water in the top row of letters. The top half dies first because that's where the water sits after a storm.

IP65 sounds impressive. It means water jets from any direction. That doesn't mean your sign survives a Florida hurricane. I've replaced signs where installers thought IP65 meant weatherproof forever. IP66 handles powerful jets for coastal jobs; IP68 is submersible for extreme weather, but even that won't save a poorly sealed installation.

For coastal jobs, don't spec 304 stainless and expect a decade. 316 stainless costs 20-30% more, but it contains 2-3% molybdenum, which buys you 15+ years in salt air. For a strip mall in Ohio, 304 is fine. For Miami Beach, pay for 316 or you're replacing letters in 3 years.

Acrylic faces don't cause half-lit issues directly, but a cracked 3mm face lets water in, which kills LEDs. A 3mm acrylic face cuts 92% light output; a 2mm face cuts 90%. Your customer won't notice the 2% but they will notice the 40% cost bump if you over-spec thickness.

DIY Diagnostics vs. Factory-Direct Repair: What to Do Next

Look, start simple. Check the power supply output terminals with a multimeter. If both legs read 24V, move to the branch. If one leg reads 0V, replace the supply or the terminal block, not the modules.

Swap a known-good module into the dead zone. If it lights, the original modules are the problem. If it doesn't, you've got a wiring or data fault. Work back toward the power supply until you find the break.

Factory-direct repair beats guesswork. When we ship a sign, it goes through an 8-12 hour aging test and a 100% QC inspection. That's how you catch infant failures before they leave the floor. If a sign dies a year later, I can tell you exactly which batch of modules was used because we keep records.

I've watched sign shops pay $800 for a front-lit letter from a domestic fabricator. Same 304 stainless, same Samsung LEDs. We ship that letter for $120. But don't pick a factory on price alone. Ask how they seal, what modules they use, and whether they run an aging test. If they can't answer, you'll be back on site with a half-lit sign in six months.

We offer a 2-year warranty on LED modules and 1 year on power supplies. Ask any factory for the brand and spec sheet of their LED modules. If they can't tell you whether they use Blue Sky (蓝景) or Rishang (日上), walk away. Cheap modules last 12-18 months. Good ones last 5+ years real lifespan. That's the difference between a callback and a referral.

Q: Can I just replace the dark half's power supply with a bigger one? No. A bigger supply won't fix a broken branch, and it can push too much current into the good half. Match the voltage and the total wattage, not whatever's sitting on the supply shelf.

Q: My sign is half-lit after heavy rain. Is it ruined? Probably not, but kill the power before you open the letters. Check for standing water, dry the raceways, and reseal with neutral silicone. If the LEDs are waterlogged, replace the affected run, not the whole sign.

Q: Why does my sign go half-lit at night and fine in the morning? Thermal expansion. A loose connector makes contact when it's cool but separates when the sun hits the letters. That's a mechanical fault, not an LED failure.

Q: Can a power surge cause half the sign to fail? Yes. A surge can blow one leg of the power supply or one LED driver channel. If the sign is on a shared circuit with compressors or refrigeration, install a surge protector. It's cheaper than replacing a driver.

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