On 04/12/2019 14:30, Dave Plowman (News) wrote:
In article ,
pinnerite wrote:
On Sun, 01 Dec 2019 19:46:43 +0000, rick wrote:

Been fitting new kitchen .... one of the last job was fitting 3 new
hanging lights. in and working (well sort of)

Wife had bought LED 'filament' decorative bulbs ...... they work fine
on full power or can be dimmed ... but not turned off .....
Dimmer is type you press button it dims to off .... with theses bulbs it
dims to off and then back to low.

First off thought dimmer was faulty - but put in some standard GLS bulbs
and it works fine.

After searching specs it does say non-dimmable (but not in advert or on
packaging)

I'm going to have to find dimmable LED .............. but what makes an
LED bulb dimmable or not ?

I find the dimmables do not start from zero luminance but some way up.

Dimmable requires that the LED constant current supply accepts the
waveform that the dimmer feeds it without fighting against it. Worst
case as you try to dim the LED unit it draws ever more current from a
smaller and smaller part of the mains cycle until something goes pop!

Even where you alter a plain LED via the amount of current passing through
it, can be difficult to get a smooth increase from zero. But easier to
reduce to zero smoothly. I'd guess they need some sort of start up kick.

Bare high brightness 5mm LEDs are pretty linear with current drive from
about 5uA (and upwards to 20mA) these days. You need dark adaption to
see them at such very low drive currents but they are still lit.

Power white LEDs are a little less linear due to thicker phosphor layer.
Bridging the switch of a simple LED torch with 1M makes it findable in
the pitch dark without appreciably altering the battery life.

--
Regards,
Martin Brown