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I have a desk lamp, that uses 11W/827 CFLs (current one is an Osram Dulux S)

The lamp uses a power supply ("Type BL1"), but it seems the transformer acts more like an AC/AC power converter, as it outputs ~230V AC (I live in Germany, we have 240V AC here.) I opened up the power supply, and the cables going off to the CFL tube, go directly from the secondary, there is no rectifier, etc. I measured the voltage that goes into the CFL at the terminals, it's the ~230V AC.

The ignition circuit must be therefore inside the (sealed) CFL casing.

I was wondering, if I could make or re-purpose a dimmer switch for this lamp. The idea is to put the dimmer after the power supply. The only dimmer switches I found, were rated for 60W-500W, which is way overkill for the 11W CFL.

What's the best approach? Could anyone suggest me a circuit for such a dimmer?

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Before you do anything else, stop and read the fine print on the side of that CFL's base. Do you find the word "dimmable" there? If you don't, then you're begging for real trouble if you try to dim it electronically. An ordinary CFL represents a capacitive load at the moment of turn-on, meaning it draws a LOT of current before it's lit and the mercury inside warmed up. If you dim that bulb by any conventional means, then 100-120 (country-dependent) times per second that CFL will go through a start-up moment, during which it draws very high current. The result: greatly shortened lifespan, excessive heat, possibly even a fire.

The best way to accomplish what you're trying to accomplish is to build a hardware shade that can be adjusted for brightness. You can come up with a clever design, I have faith in that.

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  • \$\begingroup\$ There are dimmable bulbs that fit into my lamp. If I break this one, I'll just get a new one. \$\endgroup\$ – polemon Aug 14 '14 at 3:40
  • \$\begingroup\$ Keep close track of how hot it gets - it may get BLISTERING hot, even if the dimmer is set to max brightness. Myself, I'd start with a dimmable CFL if I was determined that I needed an electronic solution instead of a physical one. Dimmable CFLs have an internal inductor that backs off their starting current. \$\endgroup\$ – TDHofstetter Aug 14 '14 at 3:43
  • \$\begingroup\$ I still need a dimmer circuit or something to re-purpose. \$\endgroup\$ – polemon Aug 14 '14 at 12:13
  • \$\begingroup\$ The "Triac dimmer circuit" shown on this page would be a good way to go for it - it's simple, and the diac/triac could be used on the secondary side of the transformer for safety's sake: circuitstoday.com/diac-applications \$\endgroup\$ – TDHofstetter Aug 14 '14 at 16:09

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