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[APD] Re: Problem with MH light

I just had a problem with one of my two Hamilton 175 Watt MH pendant lights.
When one of the lights started to flicker on as they do, I heard a fizzling
noise.  I quickly looked a the bulb and the light was flickering as it made
the fizzling sound.  I looked at the top of the pendant, and smoke was
pouring out of it.  Then the GFCI went off and shut everything down.  I
unscrewed the bulb and it looks OK (not burned).  It has a ceramic socket.
The socket looks OK.
GFCI units will normally shut off only if they see a current path that doesn't return through them. This could happen, for example, if there was water inside your light fixture in such a way that it provided a current path from the hot (black) terminal of the socket to the grounded shell of the light fixture. GFCI's will also trip from overload, although that isn't their primary function.

So I have to figure out what is broken.  It looks like there is a switch in
the bulb.  It was probably what was making the fizzling sound.  I wonder if
I need to just buy another bulb (after I inspect the wiring to the mogul
base), or do I need to take the power supplies apart (the good and bad one
for comparison) and see if the capacitor or transformer is bad.

Any advice on a plan of attack?  The bulb is about a year old (about 4000
hours on it).  It is a standard HomeDepot Phillips bulb.

Steve Pituch

The "switch" in the bulb performs a function similar to the preheat function of the old fluorescent light starters. What the switch does is allow current flow in a third electrode that is positioned near one end of the arc tube in the MH bulb, and this helps to strike the arc. After the bulb reaches some set temperature, the switch opens and the current path is now only across the entire length of the arc tube. If that switch failed you should find that the light just won't strike and will never come up to full power, and you'll need to replace the bulb to solve that problem. You should *not* see any smoke in this kind of lamp failure though since the switch is inside the sealed outer envelope of the MH bulb.

If you have the usual pendent setup with the remote ballast, and the smoke came from the light fixture, then I would guess that you probably have a bad wire in the fixture somewhere or maybe a loose terminal in the mogul socket (they are frequently assembled with aluminum or brass rivets and I see those loosen over time with the resulting loose connections causing arcing in the socket). Mogul sockets are usually about $5-10 from electrical supply houses, and make sure to get the kind with the attached leads -- they are much easier to work with and are safer since the leads are made of special high-temp wire. Wiggle all the wires and make sure they are firmly attached to the socket itself (there should be *no* movement of the wire connections), and make sure the socket holds the bulb firmly -- and especially that the bottom contact makes a firm connection (you should feel it tighten when you screw the bulb in all the way).

If you thoroughly check the fixture and find no loose connections and don't see anything wrong I would first try using a new bulb -- but watch it when you first start it. If it fails to start reasonably quickly or doesn't strike at all, then you probably have a bad ignitor (which is the AC capacitor in your ballast housing). If the new bulb never comes to full intensity but does initially strike an arc, then you probably have a bad ballast. Either way, if the problem is with the ballast or the ignitor I would recommend just getting one of the MH ballast "kits" that includes both. I purchased three 175 watt MH ballast kits made by Advance Transformer from my local electrical supply house some time back for about $30-40 or so each. I posted the part number once before and it should be in the archives, or just email me and I can get it for you. The kit includes the ballast, an ignitor, a bracket that fits the ignitor and two angle brackets with long screws to mount the ballast along with some wire connectors and instructions. Chances are if either your ballast or ignitor died than the other is probably not too far behind, and the ballast kits aren't much more than buying either part separately.


Waveform Technology
UNIX Systems Administrator

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