What type of bulb do i need?

Question

I just bought a farm and there is an old yard light on a pole that does not work. It had no bulb in it when we moved in so we have no way to tell what to get to replace it. There is a dusk till dawn photocell, that we replaced and tried a metal halide bulb. That didn't work.

It is wired directly to live current from the house and that is controlled in the Excel Energy meter box so we can't shut it off to replace it. We would like to avoid having to call them or an electrician without first having tried everything we can.

Answer 1

It is likely a sodium, Na, vapor or older mercury, Hg, vapor lamp. Inside the outer glass housing, there's a second "bulb" of quartz (clear), for Hg or alumina (translucent white), for Na. If you can read writing on the lamp or fixture with binoculars, you can try to buy a replacement lamp ("bulb"); most have Edison sockets and screw in like an ordinary household lamp. However, you'll need to identify the particular lamp type -- ratings vary from 70 watts to over 1,000 watts, and bases vary.

The lamp is not connected directly to the AC mains, but through a ballast, which serves a few purposes:

• It provides a high voltage (hundreds or thousands of volts) to start an initial glow discharge, which changes to an arc as the lamp heats and the volatile metal evaporates.
• It limits the current drawn by the lamp in the preheat phase and when operating.
• It times the preheating and controls the light, often with a separate photocell head.

If the fault is a dead lamp, it is easily replaced. A 70 watt sodium vapor lamp can be found for US$10. Use reasonable safety replacing the lamp, e.g., a lamp-replacement pole, and insulating gloves. However, replacing the ballast is not recommended, if you do not see an obvious way to shut all power at the breaker 9or fuse) box.