Bulbs and dimmers

[rick]

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 ?

[Chris Hogg]

Are you sure the LED circuit isn't just picking up stray voltages from adjacent wiring by capacitive coupling? It's not uncommon with LED lights as they require such little power to excite them. Under such circumstances, fitting a 'snubber' is often recommended.

[Brian Gaff (Sofa 2)]

How the psu is designed. obviously if the design attempts to keep the light working no matter how mangled the waveform or how variable the voltage, its not going to dim very well! Brian

[whisky-dave]

I don't think it's down to makes. A company eithe rmakes dimmable or non-dimmable they can make both, dimmable are usually more expensive.

[Bill Wright]

If you put one tungsten bulb in it usually lets the LEDs dim properly.

Bill

[pinnerite]

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

[whisky-dave]

That's because most dimmers have a minium wattage rating that they can dim to, or words to that effect, I think mine was advertised as 5watts. if you go below that then the dimmer just switches itself to off.

[Dave Plowman (News)]

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.

[Martin Brown]

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!

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.

[whisky-dave]

Well they are called diodes and that means getting above the threshold which is noramlly 0.6V to 0.7V for a silicon and 0.3V for gernamium but as yuo had things so you get colour I guess that changes things.

if you take a basic LED they typically don't start conducting below 1V

And it;s easy if yuo start from the LED being ON and then slowlty turn it off as you'll still get a bit of persistance of vision from the eyes, which will mask the delay in turning an LED from OFF to 1V to it's conducting voltage.