Advance Transformer Co. LI551H4 Lamp Ignitor

The Advance Transformer Co. LI551H4 lamp ignitor is a round‑case ignition device for high‑pressure sodium (HPS) lighting, designed to start a single 35 to 150 W lamp in HID ballast circuits. Implemented with a compatible core‑and‑coil ballast, it delivers the energy pulse required to strike the arc and light the lamp reliably. The ignitor operates on 120–277 VAC and is specified for a maximum distance of 2 ft from the lamp, helping ensure proper ignition under typical installation conditions. In its compact package, the unit measures about 1-1/4 in diameter and 2-1/2 in long, finished in black, and is commonly cataloged as LI551H4IC or LI551‑H4 in Philips Advance ballast literature for round‑case igniters. This part is intended for 35–150 W HPS installations and is used as a replacement in fixtures using Advance/Philips HID ballast systems that require a single‑lamp ignitor within that wattage range.