Fluorescent Tube / CFL
Discharge Lamp
A mercury-vapour arc generates UV that a phosphor coating converts to visible light — 4–6× more efficient than incandescent. The straight 'tubelight' with its choke and starter ran India's homes and offices for decades; the CFL folded the same tube around a screw cap.
Schematic symbol
Lamp
How fluorescent tube / cfl appears in a circuit diagram.
Types & variants
Key specs
Wattage
WT8 4 ft = 36/40 W; T5 = 28 W; CFLs 5–85 W. Add ballast losses (~10 W for magnetic chokes).
Length/diameter
ft / T-sizeT-number is diameter in eighths of an inch: T8 = 1 inch. Length and wattage go together — a 4 ft T8 is 36 W.
Efficacy
lm/W60–100 lm/W — vastly better than incandescent, now matched or beaten by LED.
Colour temperature
K'Cool daylight' 6500 K dominates India; warm white 2700–3000 K and 4000 K neutral also standard.
Lifespan
hours6,000–20,000 h, shortened dramatically by frequent switching — the electrodes erode at each start.
Markings
Tube ends print wattage, length code and colour, e.g. '36W/865' — 865 means CRI 80s series, 6500 K. CFLs mark wattage, voltage and equivalent incandescent wattage on the base.
Standard values
4 ft 36 W T8 and 2 ft 18 W T8 are the Indian classics; T5: 14/21/28 W; CFL: 5, 8, 11, 15, 20, 23, 45, 85 W in B22/E27.
How to choose
New installations should choose LED tubes/bulbs instead. When maintaining existing fluorescent fittings: match tube type (T8/T5), wattage and length; pair magnetic-ballast fittings with a matching starter; note T5 tubes need electronic ballasts and don't fit T8 holders. Dispose of tubes carefully — they contain mercury.
Pinout & package
Bi-pin caps (G13 for T8, G5 for T5) at each end connect to the two filament electrodes. CFLs integrate the tube and electronic ballast behind a B22/E27 cap; PL types expose 2 or 4 pins for external ballasts.
Example circuits
- Classic tubelight: mains → choke (series inductor) → tube, with glow starter across the tube to preheat and kick
- Electronic ballast driving the tube at 25–50 kHz (no flicker, no starter, instant start)
- Emergency light converting a tube to run from a battery-fed inverter
- Two 2 ft tubes in series on one 4 ft choke
Common failures
Blackened ends and failure to strike (worn electrodes), endless blinking (dead starter or dying tube), hum and overheating from a shorted choke, CFL base electronics failing (capacitor bulge) long before the tube, and low light in cold weather.
How to test
Measure filament continuity across each end's two pins (a few ohms each; open = dead electrode). Swap in a known-good starter for a blinking tube. Check choke continuity and insulation to frame. A tube that lights when rolled/rotated in its holder has worn end contacts.
Substitutes
Retrofit LED tubes replace T8s — either 'ballast bypass' (rewire holder directly to mains, remove choke/starter) or plug-in types that work with the existing magnetic ballast (replace starter with the supplied fuse-starter). LED bulbs replace CFLs one-for-one.