Hooked up the scope to a dimmer expecting to see how it altered the waveform, and didn't see it do much of any difference from all the way down to all the way up.

I bought an LED bulb and was told it needs a "special" CL dimmer. How do the different kind of dimmers work, and how do I manage to see what they do with my scope?

  • \$\begingroup\$ Interesting question, I've often wondered what the difference is and why a LED bulb cannot be controlled by a classic light dimmer. \$\endgroup\$ – jippie Jul 1 '12 at 9:54
  • \$\begingroup\$ @jippie This page explains why. link but given the nature of links showing up dead, I'll paste the relevant section. "Normal light dimmers are designed to only dim non-lunductive loads like light bulbs and electric heaters. Normal light dimmers are not suitable to dim inductive loads like transformers, fluorescent lamps, neon lamps, halogen lamps with transformers and electric motors. There are special dimmers available for those applications." I think the page was older than LEDs <smile> \$\endgroup\$ – rdivilbiss Jul 3 '12 at 1:35
  • \$\begingroup\$ Is it really just about inductive load? I distinctly remember seeing dimmers specially for LED's and specially for halogen. Or is the difference between LED and halogen just a marketing trick? \$\endgroup\$ – jippie Jul 4 '12 at 8:09

Wikipedia has a nice article on phase angle control which is used for most house hold light bulb dimmers. They usually don't work for halogen, LED, TL. This is a typical case where a single image says more than a thousand words:

Voltage regulated by phase control

It works by chopping a (varying) part from the sine wave using a triac. The image shows a rectified wave, but the mechanism works equally well with an unrectified one, the latter being most common in household light bulb dimmers. The dimmer is connected in series with the lamp and therefore needs to see a minimum load (in the order of 10W) to work.

This is the classical dimmer. Not sure how halogen / LED / CFL differs from this, but I know from experience (with CFL) that an unmatched dimmer can make the lamp flash at a really annoying frequency.

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  • \$\begingroup\$ Any idea why I didn't see this when I hooked up my scope. All I saw was the amplitude shrink by what I would call a negligible amount. (Standard dimmer, didn't own a CL dimmer yet) \$\endgroup\$ – boatcoder Jul 2 '12 at 2:29
  • \$\begingroup\$ Did you measure across the dimmer or across the light bulb? And are you using a conventional light bulb? \$\endgroup\$ – jippie Jul 2 '12 at 7:05
  • \$\begingroup\$ Unscrewed an incandescent bulb, and put the contacts into the socket. \$\endgroup\$ – boatcoder Jul 2 '12 at 14:36
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    \$\begingroup\$ These dimmers need a load to function, it feeds on the voltage drop across itself. Usually a minimum .. maximum rating is printed on the device. I'd expect 40W..300W or so, but you should check. \$\endgroup\$ – jippie Jul 2 '12 at 18:22

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