Our showers look to be ending very soon, but it’s going to take a while to dry off. I do have a couple lights to fix out in my shop storage area... (Hopefully, the ballasts are not out and I can just replace the FL bulbs, this time with LED replacements.)
My little maintenance project did not all go well. One light ran fine with the replacement bulbs, the other ran fine for about 4 minutes and then went bzzzZZZAAATT!! - and went out. The bulb is ok (retested in 1st fixture.)
The fixtures are both rapid start types and should be compatible, but, old ballasts often can’t hack it, according to my web search. In this case, Brave Search failed, but Google AI gave a great reply:
(Sorry for the slightly run on text below, I only partially corrected that.)
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Me:
Why did a led replacement bulb put in a 48” t8/12 fixture kill the ballast? the fixture is rapid start, the bulb only says not to use in a pre-heat fixture, and it worked for about 4 minutes before the ballast (i presume) failed. the bulb still works, in another 48” rapid start fixture.
Reply per Google:
The replacement LED bulb killed the ballast because of an impedance mismatch and overheating, which commonly happens when plugging “ballast-compatible” (Type A) LED tubes into older magnetic or aging fluorescent ballasts. Because the bulb still works in a different fixture, it proves the bulb’s internal driver survived, but your first ballast could not handle the unique electrical load of the LED.
Why the Ballast Failed in 4 Minutes.
Age and Existing Degradation: Fluorescent ballasts degrade silently over time. The sudden change in electrical demand from the LED bulb often acts as the final “shock” that pushes an old, compromised ballast into total failure.
Impedance Mismatch: Type A LED tubes are designed to mimic the resistance of a standard fluorescent gas tube. However, if the ballast’s output frequency or internal circuitry doesn’t align perfectly with the LED’s driver, the ballast will experience an over-current condition, causing it to burn out almost instantly.
Thermal Tripping or Meltdown: Many older rapid start ballasts utilize magnetic coils. When paired with an incompatible electronic LED driver, these coils generate immense heat. Within 4 minutes, the ballast either melted its internal insulation or permanently tripped its internal thermal safety fuse.
How to Proceed
Do not buy another “plug-and-play” bulb or replace the ballast. The most reliable and energy-efficient solution is to completely eliminate the ballast from the equation.
Keep the current bulb for your other working fixture. Do not risk swapping it back and forth.
Convert the failed fixture to Ballast Bypass (Type B). Buy a Type B LED tube (also called a “direct-wire” tube). Cut the ballast out: Turn off the breaker, cut the wires leading to the ballast, and connect the building’s main AC power wires (Hot and Neutral) directly to the socket holders (tombstones) according to the Type B bulb’s instructions.
Bypassing the ballast removes the failure point permanently, saves more electricity, and ensures your lights turn on instantly without humming or flickering.