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All my life I was laboring under the misconception that if a bulb fits in a socket, it will also work well.

Recently, a line lamp in our bathroom stopped working, so I bought a brand new LED line lamp bulb. The new bulb entered the socket with a nice clean "click", but it does not work. I push the button and nothing happens.

The lamp has a single inscription, which seems to list three compatible light bulbs. 35W, 60W, and 120W, all operating at 250V alternating current. The bulb I bought has a specification of 18W and 230V. I assume it also uses alternating current.

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Could it be, that the light bulb is incompatible with the lamp? Wouldn't a light bulb with lower consumption just happily take less energy from the grid? Could a difference of 20V cause a light bulb not to work at all? Are the inscriptions on the lamp lower boundaries, or do they need to be satisfied exactly?

EDIT Marking on the old light tube. The last word seems to be C4b8.

Marking on the old light tube

Old tube on the left, New on the right

Old tube on the left, New right

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2 Answers 2

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Those LED replacement bulbs are not a drop-in replacement for TL tubes. You will need to do a bit of rewire to the fixture to take the starter out of the picture.

If you are uncomfortable with that then you can replace the entire fixture instead. You might want to pick a safer alternative than these which have a finger wide recess into live connectors.

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All my life I was laboring under the misconception that if a bulb fits in a socket, it will also work well.

Wow, that's not true at all. Take the common Edison E26/E27 socket. I have bulbs for that which are 120V, 75V, 32V and 12V. And I'm sure they'd all burn out instantly in any of your fixtures.

Then you have the (granted, more of a North American thing) T8 fluorescent conversions, where the efficient T8 was form-factored specifically to fit in T12 sockets (for conversions). Yet is electrically different.

Ratchet freak has the tiger by the tail. Had you bought an actual discharge light tube for this, then it would have worked. Any discharge light (fluorescent, neon, HID) does not run on mains directly. It has a ballast which re-factors the power to what will correctly drive the tube.

See, you think of electric devices as things that want a constant voltage, and then "sip" just as much current as they need, self-limiting current. That's not true either, it's an artifice that was created to make power distribution easier. Loads often want a constant current, i.e. they don't want to be self-limiting. Discharge tubes are at the top of that list, boy Howdy! And that's why they need ballasts.

LEDs running at peak performance are also members of that club, but they work differently enough that using the existing ballast is not workable. Now, we enter the wild, wild world of LED conversions, which you did when you chose to buy an LED substitute.

LEDs generally run in 2 general families:

  • Some do not require ballast change, and "make do" with what the ballast is giving out.
  • Others want you to bypass the ballast and give them hotshot mains voltage right on the FORMER tube sockets. This requires modifying the fixture.
  • And those come in two wiring schemes. (hooray!)
  • And then you have "universals" which can go either way.

So you need to (retroactively) select accordingly.




* This was a big part of Edison's problem finding a workable light bulb. Many more things could have worked in constant-current mode. However that was anathema to Edison, since it was impossible in DC power without a rotary machine. And once you have constant-current available, you might as well run discharge lighting, as Tesla did.

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  • Wonderful explanation. I had no idea the rabbit hole was so deep. Oct 30, 2021 at 9:24

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