PTC thermistor

Inside the Pink Glow of High-Voltage Striking Low-pressure sodium street lamps produce a lurid yellow light that matches the human eye's peak sensitivity. But striking the neon-argon gas mix inside a Philips lamp requires a high-voltage punch. When cold, the tube behaves as an open circuit, demanding a high-voltage spike to ionize the gas before the metallic sodium can vaporize. The Silicon Behind the Strike A classic Philips ignitor circuit relies on a single-sided board packed with robust analog components. The brains of the operation include a BT136 triac, a diac, and a timing network. Initially, high voltage across the unlit lamp triggers a metal oxide varistor, charging a capacitor. Once the voltage hits the threshold of the diac, it fires a pulse into the gate of the triac. The triac then shunts the line, forcing a brief current surge through the ballast. When this shunt collapses, the ballast's magnetic field dumps a massive voltage spike into the lamp, striking the arc. The Thermal Safety Valve What happens when a lamp breaks or works loose? Without a shutdown mechanism, the ignitor would hammer the ballast with continuous high-voltage spikes, destroying it. To prevent this, the circuit uses a PTC thermistor as an electronic fuse. If the lamp fails to strike within a reasonable window, the continuous current heats the thermistor, increasing its resistance and shutting down the trigger circuit. Two Components Beat a Whole Board For DIYers restoring vintage gear, the complex factory board is overkill. A ridiculously simple alternative uses just a SIDAC and a 330 nF capacitor wired in series across the lamp. The SIDAC acts as a voltage-triggered switch. When voltage peaks, it dumps energy into the ballast to strike the lamp. The trade-off is safety; this minimalist hack lacks the thermal protection of a PTC thermistor and will hammer the circuit indefinitely if the bulb fails. Modern LEDs Create Lucrative Landfill Municipalities are racing to replace vintage sodium fixtures with LED equivalents, but the environmental math is flawed. A high-quality sodium bulb can run for 18,000 hours, and replacing a dead bulb takes seconds. When a modern integrated LED fixture fails, crews must discard the entire street lighting head. This shift trades highly repairable, long-lasting hardware for costly, non-serviceable electronic waste.