How to design non-Triac dimmable mains led bulbs?

[treez]

As you know, all switch mode mains LED light bulbs need protection from the overvoltages that may occur if they are accidentally connected to a triac dimmer.
Given that such circuits are usually dissipative dampers, would it be acceptable, as an alternative, to simply sense the DC bus voltage with a potential divider, and if it gets too great, then an in-line fet can switch out the front end from the primary DC bus?
(ie instead of using a dissipative RC damper)

 

[andre_luis]

In fact, but increase cost...



+++

 

[treez]

a simple fet switch I don't think would cost that much?...and assures that the violent triac switching doesn't play heck with the line filter.

 

[dick_freebird]

Are you really trying to design out all forms of human
stupidity? Because that could take a while. Seems like
the usual approach is to fix the labeling and leave the
rest of the responsibility up to the purchaser.

 

[treez]

the thing is their wall dimmer isn't labelled , and they don't know if its a trailing edge dimmer or a triac dimmer. (trailing edge dimmers , of course, are harmless.)
-incidentally, any idea on what percentage of domestic households (in any country you specify) have triac dimmers fitted?......I think its around 10% max in UK..also, dimmers are exclusive to wealthy households...ie large detached houses..any ideas on the exact percentage?

 

[betwixt]

I live in a very small, remote 'hut' and I would hardly call myself wealthy but I have them. I don't think you can classify the users in that way. I would also suggest that since CFL became the normal lighting source, the number of dimmers has decreased signficantly.

Your assumtion that sharp rising edges will cause over-voltage is also somewhat dubious as it will only occur in certain circumstances. You are quite correct that it can cause ringing in line filters although the damping effect of the load will somewhat reduce that but I think you may be over-estimating it's severity. A simulation, assuming zero wiring length and instantaneous edges is not what you would see in reality. It will only occur of course when there is filter at all, many LED lamps use clusters of small LEDs which use much of the available voltage and use a capacitive/resistive dropper to take up the remainder. These are quite happy to be dimmed, as long as the dimmer circuit can handle the smaller than incandescent load current.

I see a bigger problem to be overcome, dimming without flicker. I don't mean random flickering, I mean 50/100Hz or 60/120Hz directly from the incoming AC, particularly if the waves are chopped short by the dimmer, making the off to on ratio greater. I'm assuming LEDs here which have not got an internal SMPS. LEDs have fast reaction time, dimmers expect the thermal properties of a hot filament to even out the light emission. Perhaps the solution to the problem isn't in the lamp but in the dimmer itself, maybe a controlled DC output would be a better option. It would work on incandescents, LEDs and probably dimmable CFLs and of course it shouldn't have the same EMC problems as the long output wiring would be transient free.

Brian.