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I have a desk lamp which, sometimes when I start it, causes my computer monitor that is connected to the same outlet to turn off and reset, so I disassembled the lamp to see what could cause this to happen.

The lamp itself is a circular fluorescent tube of this type, and to my surprise I found that, in the armature, there are only two components whatsoever: The power switch and a glow starter, connected in the ordinary way, only there's no ballast. This surprises me because I thought the ballast was critical to fluorescent tube operation, both to regulate the current so as to not blow the tube, and for providing a pulse for striking the arc.

How can the lamp work at all without a ballast? Could the tube be somehow self-regulating? Note that I have removed the little plastic cover over the connector on the tube (as seen in the picture linked above), and there are no active components inside it; there are only leads from the connector to the tube itself. The tube has always been a bit slow to start and often needs to cycle several times; could this be how a tube "normally" starts if there's no ballast?

Also, as a bonus question, I have noted that when the lamp is turned on and I have some comparatively sensitive skin (like my face) close to the armature's bare metal parts, I can feel a slight tingling, as if the armature were faintly electrified. The armature is properly grounded, though, and all active parts are properly isolated as far as I can tell, so I've wondered how this can be. Does a fluorescent tube normally emit a strong enough EM-field that can cause induction in the armature, or something? Could this be connected to the lack of a ballast?

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  • \$\begingroup\$ How do you know the "tube" doesn't have a starter in the thicker portion where the connectors go in? Linking just to an image is pretty useless by the way. How about a datasheet? \$\endgroup\$
    – got trolled too much this week
    Commented Oct 25, 2015 at 17:45
  • \$\begingroup\$ @RespawnedFluff: As I wrote, I removed that cover and checked what was behind it, which was only leads that go directly into the glass. I can upload a picture if you'd like to see it. \$\endgroup\$
    – Dolda2000
    Commented Oct 25, 2015 at 17:47
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    \$\begingroup\$ Fluorescent desk lamps usually operate with separate ballast that connects to an outlet (which looks almost like an ac/dc adapter). \$\endgroup\$
    – venny
    Commented Oct 25, 2015 at 17:48
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    \$\begingroup\$ @venny: You are right, of course. It was just mounted at the far back of the armature where I didn't notice it. I feel a bit stupid now. :) \$\endgroup\$
    – Dolda2000
    Commented Oct 25, 2015 at 17:53

1 Answer 1

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Looking at the schematic shown here it does seem to have a built-in [corrected: suggested application] ballast. I'm guessing your teardown wasn't complete enough. Added: Well, you've found the external ballast it seems, so it was not in the tube as I initially suspected.

enter image description here

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  • \$\begingroup\$ That's really quite weird, not least because the schematic only displays two connections, while the lamp obviously has four. I can also not imagine how I could possibly miss all those things. I'm wondering if that schematic isn't some kind of "intended application", rather. Either way, the question was resolved in the comments. \$\endgroup\$
    – Dolda2000
    Commented Oct 25, 2015 at 17:58
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    \$\begingroup\$ Just for the record, here are some pictures from under that cover: 1, 2, 3. I find it very hard to imagine that not simply being the leads connecting directly to the filaments. \$\endgroup\$
    – Dolda2000
    Commented Oct 25, 2015 at 18:09
  • \$\begingroup\$ @Dolda2000: That means you were correct and that schematic is the suggested application rather than what comes with the tube. Alas the documentation on this thing is rather lacking in words. But you seem to have found the ballast. \$\endgroup\$
    – got trolled too much this week
    Commented Oct 25, 2015 at 18:11
  • \$\begingroup\$ Also I've learned a new word: "capacitative". :D \$\endgroup\$
    – got trolled too much this week
    Commented Oct 25, 2015 at 18:19

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