CFL repair

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It takes a few minutes to repair a £2 CFL lamp, so can be worth it if you use a lot. A percentage of CFL lamps can be repaired easily and quickly.

Is it worth it? It depends how fast you do it, and on the value of the resulting lamps. In short, sometimes.

CFL innards 5589-3.jpg

CFLs are readily split into 2 sections:

  • Base & Ballast
  • Tube

Lamp failures are usually a failure of the ballast or the tube, but not both, and swapping parts to make working lamps is easy and quick.

Quick method

The quickest way to determine whether a tube is dead is measure the resistance of its 2 end filaments. Both filaments should conduct; if one is open circuit, the tube's dead. If the tube's ok, assume the ballast is dead and pair up working tubes & ballasts on this assumption. This approach is imperfect, but fast, and yields a lot of working lights.

Further repairs are possible in principle, but not worthwhile.

The resulting lamps usually work fine, but expect about 1 in 6 to have issues, eg flicker or dimness.


To pair a tube with a ballast,

  • the 2 should be of the same power rating
  • the 2 parts should have run tubes of the same diameter
  • the 2 parts need to physically fit together.

Opening the lamp

Insert 2 scrapers into the crack between base and top sections of the plastic casing. Twist to separate.

There are occasionally lamps that won't open this easily. With those I put a knife blade into the crack and cut through one of the lugs. Then it comes apart more readily.

Filament arrangement

CFL tubes have 4 wires, 2 connecting to a filament at one end of the tube, the other 2 connecting to a filament at the other end of the tube. With the lamp off there's no connection between the 2 filaments. Each filament is low resistance.

Ballasts normally connect to the filaments via capacitors, so there's no need to disconnect anything to test the filaments. Just put multimeter probes straight on.


Most CFLs use soldering to connect tube to ballast. Some use wire wrap terminals or sprung contacts, both of which can be soldered.

If you can't solder, its possible to cut all wanted wires long, twist ends together and sleeve them. But its a lot slower.

Further info


CFL tubes contain tiny amounts of toxic mercury. Care is needed to not break them. If you break some, quit, its not worth it.

Obviously the bulb must be unplugged from its bulb socket when carrying out any work on it.


Mismatched parts can be paired up to some extent, if you really want.

If the 2 halves of the casing won't clip together, tape is a non-ideal option, as long as the tape used survives 10,000hrs of heat. Cellophane tape is no good.

Its possible to run a power mismatched pair where the ballast power is only slightly lower than the tube power rating. The lamp then runs at the lower power of the ballast. These behave ok indoors, but are prone to reduced light output if used outdoors in winter. 20% power mismatch doesn't have much effect on lamp life, but increasing the mismatch further quickly has a large effect, then becomes nonfunctional.

How not to

Another repair method is occasionally encountered, and it should definitely be avoided. When one tube filament burns out, its simply shorted. The tube is then slow to light, but once it has it seems to work fine at first. But the glass overheats and cracks, releasing mercury, and such lights have very short lives. Don't do this for safety reasons.

Other uses

CFL ballasts can be used to run

  • CFL tubes where ballast and tube are separate items
  • Linear fluorescent tubes of suitable wattage

Wrong base

Its just as easy to swap bases to convert lamps to a different bulbholder type.

See also