HID Lamp (Sodium / Metal Halide / Mercury)
Discharge Lamp
High-intensity discharge lamps make light from an arc in a high-pressure gas — the orange sodium glow of older Indian streetlights and the white floods of stadiums. Being replaced by LED, but still everywhere and still in every EE syllabus.
Schematic symbol
Lamp
How hid lamp (sodium / metal halide / mercury) appears in a circuit diagram.
Types & variants
Key specs
Wattage
W70/150/250/400 W are the streetlight staples; stadium MH runs 1000–2000 W.
Efficacy
lm/WHPSV ~100–150 lm/W (great, but orange); MH 75–100 with far better colour.
CRI
RaHPSV ~25 (colours look muddy), MH 65–90 — why sports grounds use MH, highways used sodium.
Warm-up / restrike time
minutes2–5 min to full brightness; after a power dip, a hot lamp won't restrike until it cools (~5–10 min) — the famous stadium blackout delay.
Control gear
—Needs a matched ballast + ignitor (superimposed pulse) + PF capacitor; MH is sensitive to correct gear.
Markings
Lamp prints type code and wattage: SON-T 250 W, HPI-T 400 W, etc. E27/E40 screw caps or double-ended (Rx7s) for compact MH. Fitting gear tray labels the matching ballast/ignitor set.
Standard values
70, 150, 250, 400 W in E40; ignitors rated 70–400 W; PF capacitors 10–45 µF 250 V AC matched per wattage.
How to choose
New installs: choose LED street/flood lights instead. Maintaining HID installs: match lamp type AND wattage to the existing ballast (a sodium lamp on mercury gear misbehaves), replace ignitors with the same type, and replace lamps in groups — end-of-life sodium lamps cycle on and off all night.
Pinout & package
Screw cap (E27/E40) into a porcelain holder rated for the ignitor's kV pulse; gear tray in the luminaire holds ballast, ignitor and capacitor. Double-ended MH clips between two Rx7s holders in floodlights.
Example circuits
- 250 W HPSV streetlight: ballast + ignitor + 32 µF PF capacitor + photocell switch
- 400 W MH flood on a stadium mast with group re-strike delay
- Twilight switch (LDR) controlling a whole street's contactor
- Retrofit: LED corn lamp with internal driver replacing SON lamp (ballast bypassed)
Common failures
End-of-life cycling (sodium lamps strike, warm, extinguish, repeat), failed ignitor (lamp never strikes), ballast shorted turns (blackened lamp, high current), PF capacitor failure (high current draw, dim light), and arc tube rupture in aged MH lamps.
How to test
A cycling streetlight = replace the lamp first. No strike at all: check ignitor (listen for the ticking pulse), then ballast continuity and capacitor µF. Always kill power before working — ignitors pulse several kV.
Substitutes
LED retrofit corn lamps (bypass the gear) or full luminaire replacement — typically half the watts for the same lux, instant-on and no restrike delay. Within HID, lamp/ballast/ignitor must be replaced as a matched set.