Why are LED light bulbs/fixtures commonly dimmed pre-rectifier? - Page 1
Forum section: electronics / projects

author: Langdon
author_rank: Frequent Contributor
timestamp: March 03, 2025, 06:05:09 pm
content: This is a good idea:AC Line -> EMI filter/PFC -> Full-wave DC Rectifier -> Filter cap(s) -> Constant current driver (best)/PWM dimmer(good)/linear dimmer(good if the room is too cold and you want a heater) -> LED fixture/bulb.However, this is common:AC line -> PWM dimmer -> Dimmable Rectifier(no clue) -> LEDs.This introduces PWM noise into the lines.The rectifier filter caps should some before the dimmer to prevent PWM noise from entering the lines.Why is this so widely used?Isn't the whole "dimmable" rectifier thing a waste of time(and, presumably, power)?Wouldn't it make a lot more sense to dim the DC power?

author: Marco
author_rank: Super Contributor
timestamp: March 03, 2025, 06:18:50 pm
content: It's not PWM, it's angle cut and it's for legacy compatibility and code compliance. Just putting rectified DC on a mains cable is generally not allowed.My main problem is that no one thought to say 20+ years ago, "for (trailing edge) LED dimmers, 90 degrees is off". Would have simplified a lot of things. When the LED is supposed to be on, it will always see the full peak voltage after rectification. Then we could have leading edge dimmer compatible dimmable LED bulbs with complicated circuitry and LED dimmer only compatible LED bulbs with trivial circuitry.

author: Langdon
author_rank: Frequent Contributor
timestamp: March 04, 2025, 01:25:57 am
content: My house has these dumb low-voltage pot lights everywhere.What about those?Why not put rectified DC on a mains cable?

author: moffy
author_rank: Super Contributor
timestamp: March 04, 2025, 02:27:56 am
content: Quote from: Langdon on March 04, 2025, 01:25:57 amBecause DC is far harder to extinguish an arc rather than AC, it needs different switches etc.

author: Langdon
author_rank: Frequent Contributor
timestamp: March 04, 2025, 02:31:45 pm
content: Very good point.That did not occur to me!Though I think pulsed DC also does not make much of an arc.Though when the dimmer is on max bightness, there would be a major arcing problems.

author: microwhat
author_rank: Contributor
timestamp: March 04, 2025, 06:38:56 pm
content: The other solution (which Langdon's house already has) is to install a driver somewhere, which usually has a 0-10V dimming control input, and then run its output (which will be DC) to the lights, which at that point are merely arrays of LEDs with no active electronics.That eliminates the phase control dimming, and the need to switch DC.I cannot think of any section of the national electrical code (USA) that forbids DC.

author: tooki
author_rank: Super Contributor
timestamp: March 04, 2025, 06:58:59 pm
content: Quote from: Langdon on March 04, 2025, 02:31:45 pmPulsed DC can also be considered as an AC square wave with a DC offset. And square waves contain tons of high-frequency energy, so they're an EMI nightmare.So in the end, an AC sine wave simplifies many aspects.

author: Langdon
author_rank: Frequent Contributor
timestamp: March 04, 2025, 07:09:41 pm
content: "The other solution (which Langdon's house already has) is to install a driver somewhere, which usually has a 0-10V dimming control input, and then run its output (which will be DC) to the lights, which at that point are merely arrays of LEDs with no active electronics."Alas, my house has not good current drivers, but low-voltage AC pot light dimmers that run dimmable LED bulbs!Every single light fixture, dimmer, and socket in my entire house was designed for halogen light bulbs in 2008, only a couple years before the advent of LEDs!Low voltage halogen, 120V halogen, all halogen!Plus some non-dimmable T5 flourescent bulbs (which are great).Random side question: I don't see any ballast connected to the T5 tube.Do thin fluorescent tubes need a ballast at all?

author: Langdon
author_rank: Frequent Contributor
timestamp: March 04, 2025, 07:17:17 pm
content: The stupid low-voltage AC LED pot lights, that my dad moronically had installed, flicker, presumably due to interference between dimmers.

author: schmitt trigger
author_rank: Super Contributor
timestamp: March 04, 2025, 07:26:30 pm
content: Quote from: Langdon on March 04, 2025, 07:09:41 pmIf the lamp you got is indeed a fluorescent tube, it definitely requires some sort of ballasting. Otherwise how would it prevent the catastrophically high currents from developing, due to the increased conductivity of the ionized gas?

author: Langdon
author_rank: Frequent Contributor
timestamp: March 04, 2025, 09:45:17 pm
content: Quote from: schmitt trigger on March 04, 2025, 07:26:30 pmPerhaps there's a ballast hidden somewhere in the fixture.Cool design!