Ballast (Choke) & Starter
Lamp Control Gear
Discharge lamps have negative resistance — once struck, they'd draw runaway current without a ballast to limit it. The magnetic choke + glow starter pair is the classic tubelight circuit; electronic ballasts replaced both with a high-frequency inverter.
Schematic symbol
Lamp
How ballast (choke) & starter appears in a circuit diagram.
Types & variants
Key specs
Lamp wattage match
WBallast and starter are rated for a lamp wattage range — a 36/40 W choke for a 4 ft tube, S10 starter for 4–65 W single tubes.
Ballast loss
WMagnetic chokes waste ~8–12 W as heat; electronic ballasts ~2–4 W — the main reason to upgrade.
Power factor
—Bare choke circuits run ~0.5 lagging; a PF-correction capacitor across the mains, or an electronic ballast, brings it near 1.
Operating frequency
Hz / kHzMagnetic = 50 Hz (visible flicker, stroboscopic effect on machines); electronic = 25–50 kHz (flicker-free).
Markings
Choke label: lamp wattage, voltage, connection diagram. Starters mark their range (S2 = 4–22 W series pairs, S10 = 4–65 W single). Electronic ballasts list lamp count and type, e.g. '2 × 36 W T8'.
Standard values
Chokes for 18/20 W, 36/40 W tubes; S2 and S10 starters; electronic ballasts 1×18, 1×36, 2×36 W; HID ballasts 70/150/250/400 W with matching ignitors.
How to choose
1) Match exactly to lamp type, count and wattage. 2) Prefer electronic ballasts: cooler, flicker-free, instant start, and they extend tube life. 3) For HID lamps you need the correct ballast AND ignitor AND often a PF capacitor. 4) When retrofitting LED tubes, the ballast is usually removed (bypass wiring) — mixed installs cause mystery failures.
Pinout & package
Choke: two terminals in series with the phase feed. Starter: two-pin bayonet bottle plugged into the fitting's socket, wired across the tube. Electronic ballast: mains in, and two or four wires to each tube end per the printed diagram.
Example circuits
- Classic 4 ft fitting: phase → choke → tube filament → starter → other filament → neutral
- The starting kick: starter opens, choke's collapsing field spikes ~1 kV across the tube to strike the arc
- PF correction: 3.5–4 µF 440 V capacitor across the mains input of a 36 W choke fitting
- 250 W sodium streetlight: ballast + superimposed-pulse ignitor + capacitor
Common failures
Choke: shorted turns (overheating, hum, tube blackens fast) or open winding (dead fitting). Starter: worn glow bottle (endless blinking) or welded contacts (tube ends glow but never strike). Electronic ballast: blown input stage from surges, dried electrolytics (flicker, no start).
How to test
Choke continuity (tens of ohms) and megger to frame; substitute a known-good starter first for blinking tubes; on electronic ballasts, check input fuse and look for bulged capacitors. A choke that smells of varnish and hums loudly is on its way out.
Substitutes
Electronic ballast replaces choke + starter (remove the starter entirely). Electronic starters upgrade glow bottles for faster flicker-free starts. For LED retrofit, remove/bypass all control gear unless the tube is explicitly ballast-compatible.