farmerbill: Mercury Vapor Light Problem

Old 01-04-12, 02:13 PM
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farmerbill: Mercury Vapor Light Problem

I have this same fixture, and the ballast appears to have failed, but I'm still confused.

New 80w mercury bulbs will not light up, but using a multi-tester I can see that the socket is getting 120V. I tried a regular 100w household candescent bulb, and it lights up, but only at about half brightness. Are these symptoms consistent with a failed ballast? I thought if the ballast failed, there would be no voltage to the socket at all.

Note: The photo-eye works fine.

Thanks for any additional insight. I'm just trying to learn more about how these things work.
Old 01-05-12, 06:58 AM
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You may have a loose connection. The connection is good enough to register on the tester, but when a load is applied it is not good enough to carry the load.

The socket itself on a HID fixture should be higher than 120 as a ballast steps up the voltage.
Old 01-05-12, 08:00 AM
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Is the new bulb the correct wattage and kind (same as the old bulb)?

The purpose of the ballast is to limit the current flowing through the bulb.

As a mercury bulb warms up it would want to draw more and more current (amperes). A mercury light ballast is constructed so that, as the current draw goes up, the voltage goes down which in turn causes the current draw to level off.

The ballast must match the bulb in that the starting voltage has to be high enough to start the bulb glowing and, when the bulb reaches the operating temperature and current draw, the voltage has not dropped so low that the bulb goes out.

Nothing is proved if an incandescent bulb is put in a mercury light fixture. In your case the incandescent bulb immediately drew an amount of current that dropped the voltage somewhat below 120 volts.

Don't confuse mercury bulbs with metal halide bulbs with halogen bulbs (The first two are similar but usually may not be substituted for one another; the third is somewhat different, actually the third is almost the same as ordinary incandescent..).

Since you said you measured the voltage in the empty socket at 120 volts, you can try this.

Put the mercury bulb in an ordinary light socket. Since you put an incandescent bulb in the mercury fixture the socket must be the same. Turn it on for no more than ten seconds. Do you see a tiny glow in the mercury bulb? This would mean that the bulb is good and probably mean that the ballast has gone bad.

Caution: If the mercury bulb lights up and is left running in the incandescent fixture the bulb could (probably would) overheat and explode.

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