Smart Photoelectric Control Senses Lighting Problems

Mill Creek, WA-A common roadside feature, High Pressure Sodium (HPS) lamps exhibit a long life span-about 5 years. They eventually fail when the arc tube darkens and lamp voltages can no longer maintain a continuous arc within the bulb.

At that point, abnormal cycling takes place, and the lamp continually flashes or attempts to start. The condition can prove especially difficult to detect because of the infrequency of the cycling. If abnormal flashing continues for more than 2 to 3 months, the lamp's starter/ballast unit eventually becomes damaged. For a utility company, this means expensive replacement of the defective lamp as well as the starter/ballast unit.

The Dual Dome Anti-Cycling HPS Street Lamp Control turns lights on at night and off in the morning, and protects against cycling. It photoelectrically senses lamp cycling using a single PROM that performs all of the logic functions in the control. "Previous attempts to disconnect cycling street lamps from their power sources required complicated circuits," says designer Frederick Blake of F.H. Blake & Associates. "Their reliability was questionable, and their cost excessive."

Mounted on the top of a conventional street light, the Dual Dome counts the lamp's attempts to start. After three on-and-off cycles during the night, it cuts current to the ballast, and a red strobe light located on top of the unit flashes on to alert maintenance crews. "The PROM combined with Triac switching simplifies the circuit and gives trouble-free life to the control," explains Blake. "The load is always switched at a current zero so that no large in-rush current is switched on or off."

Fiber-optic cable extends between the anti-cycling controller and the lamp. The cable's distal end is so positioned that a portion of the light emitted by the lamp is transmitted along the cable to the controller. At the other end of the cable, a photocell generates an electrical signal that varies as light is transmitted, or not, through the cable. Input to the anticycling circuitry, this signal enables the controller to recognize abnormal lamp cycling.

Dual Dome's circuitry and power supply are mounted on a 23/8x 2-inch-wide PC board housed in the 3-inch high, shatterproof Lexan(R) dome. Its fiber-optic cable extends from the housing; a low-heat-conduction fitting connects it to the fixture's reflector.

Under a Dept. of Energy grant, 500 field-test units are currently in production; they will soon be sent to major utility firms.

Additional details...Frederick Blake, F.H. Blake & Associates, 3103 149^th St. S.E., Mill Creek, WA 98012, (206) 337-5117.