Dimming 'not specifically dimmable' LEDs?

[T i m]

Yo.

A while back Mum bought a table lamp that was 'touch dimming' where repeated 'touches' to the lamp body gave you from off to 'low', 'med', full and off again. I think it suggested a 'max' 40W (ses, incandescent) mini globe but I think I tried a 30W equiv LED and it appeared to works fine.

She has now got a couple more such lamps beside a bed and so I bought a couple more LED lamps but this time I went for 40W equiv units, not considering the tops weren't 'frosted' like the first one. Now, they work ok but because you are often standing over those lights when you turn them on ... and these particular lamps go 'off / full / med / dim / off', you get a face full of fairly bright light. ;-(

Now, whilst it's no biggie, I checked back in Homebase and it seems most of the (TCP) ses LED mini globe LEDs that are dimmable are the same (clear tops) but there are frosted top lamps but they are marked as 'non dimmable'.

So, assuming the first LED lamp I tried was also non-dimmable and seems to work fine, what is it about 'dimmable' that makes them so please. Would there be any issues using non-dimmable LEDs with a dimmer like that if they actually seem to work ok or could it cause the lamp to fail prematurely or worse?

Cheers, T i m.

[tabbypurr]

immediate failure or fire due to severe overheating of the power supply resistor.

NT

[T i m]

Ok.

Well, it seems to have been fine so far so it either was a dimmable lamp in the first place, Mum has only been using it on full (not unlikely) or we have been lucky?

Well I'd *hope* it wouldn't catch fire under any circumstances (even misuse) but just 'fail to work any more' but I will be checking to see if I can confirm if it is dimmable or not (it has the TCP markings that might tell me) and not use it in that lamp if it isn't or we can't be sure.

Our bedside light failed recently (40W ses candle) and I replaced that with a dimmable LED candle.

Thanks for the feedback.

Cheers, T i m

[Brian Gaff]

I'd not expect there to be any problems, but it really does depend on several things you probably cannot know from looking at the lamp or led. For a start, the led may be a pretty crappy switch mode supply and may not take that kindly to the weird waveform being fed to it by the dimmer.,Whether it will cause a failure is hard to say. Personally I'd doubt it but at some brightness's the system may hunt up and down or just refuse to light without it being full on first. There could be inductive components in the led that could but probably won't blow up the dimmer. I'd be tempted to try it with some well known leds and see what you get. You cannot know how sophisticated or otherwise the dimmer in the lamp is of course. Brian

[Brian Gaff]

No I'd not expect any resistors in modern leds, as the current limiting is doone in the power supply, which is why some hunt up and down when given odd waveforms. The same issue can happen if you run leds off some older invertors. Mind you as has been said, if it did go, it would not I think be likely to catch fire. Brian

[T i m]

For what it's worth Brian I think Mum has had that potentially non-dimmable LED in there for some time now and there is no sign (that I have noticed) of any flickering, unexpected brightness changes or lack of response from the fitting with controlling the thing in general. As mentioned elsewhere, it *could* be a dimmable LED in there already but my question was initially 'what if it isn't'.

Agreed ... that said, I wasn't really sure how well a 'low energy' lamp would respond (lumens wise) in a fitting possibly designed for an incandescent but it works perfectly with a good range of light levels per touch on the lamp.

FWIW, I think there were fairly expensive LEDs, eight quid or but not that that would guarantee to tell us anything but it might suggest they had used better electronics than a cheaper model?

On that particular fitting / LED I'll see if I can cross reference the numbers and see if it was an actual dimmable model but if there even could be issues using a non-dimmable in a dimmer, I'll try to avoid it. I was thinking there must be a good reason why most LED lamps aren't dimmable but I was just wondering 'why'.

Cheers, T i m

[tabbypurr]

not one single point correct of course

[Syd Rumpo]

Just for info...

I took one apart - it was 4.5W golfball (30W equivalent) and supposedly not specifically dimmable, but I guess it would.

There was a series input capacitor feeding a full wave bridge with an electrolytic reservoir capacitor on the DC side, then 50 ohms to a series string of 14 white LEDs on a separate PCB. Plus other high value resistors as bleeders.

The LED PCB was well gunked onto an aluminium heatsink with thermal grease. Simple but it worked.

Cheers

[The Natural Philosopher]

And I think that is the key to why LED bulbs are pretty good. The circuitry is much simpler than the CFLs.

And, sticking what amounts to a capacitative load on the mains is going to make the Grid happy, to offset all those inductive loads in motrs etc.

But because its a capacitative load, don't except dimmers to behave as expected.

[Syd Rumpo]

Similar, except his diodes are of the dark emitting variety.

Good old US patent office, still at least it's only an application, not a grant.

Cheers

[Adam Aglionby]

Sure they got a grant on something similar, LED Patents, mumble, grumble...etc.

But it is how a lot of the 5050 based lamps are built

[Johnny B Good]

Well, it's common practice with the lower wattage LEDs (sub 10 watts, possibly even higher) to use a 'lossless' capacitive dropper in series with an inrush current limiting resistor which is designed to cope with low duty cycle switch on events as well as to typically perform as a safety fuse - it adds a small loss in efficiency but is an essential component in such a 'Lossless Dropper' circuit.

Conventional triac based dimmer switches use phase delay control over triggering the triac into conduction so that at the dimmest setting, only the tail end of each half cycle of mains is applied to the lamp (at a hundred times per second on 50Hz mains) which reduces the average voltage supplied to the lamp.

This works just fine with incandescent lamps but results in rather brutal current switching transients in devices using the 'Lossless Dropper' power supply circuit (a hundred such events per second rather than the maximum of two or three per second that a determined on/off switching freak might apply to the light switch in an attempt to break a new 'personal best' record.

The worst case condition being the notional half brightness setting where the triac turns on at the peak voltage in each half cycle to present the trailing half of the mains waveform to reduce the average voltage to half. I should imagine that most such LED lamps using the lossless dropper technique will succumb rather swiftly at this setting of the dimmer by causing the inrush current limiting/stroke safety fuse resistor to overheat and let out its magic smoke.

If it's a cheap poundland lamp and you wanted to test whether it would deal with such a dimmer, using the half brightness setting to run the test should provide the answer in the shortest time possible. You'll need to vary the setting either side of the perceived half brightness point if it doesn't fail straight away since there's rarely a direct relationship between light output and dimmer setting even with the incandescent lamp types the dimmer was designed to control (in part because the response of human vision approximates to a logarithmic one and also because of the reduced efficacy of a filament lamp at lower average power levels - non- dimmable LEDs can behave unpredictably unless you know and can analyse the circuitry used by the lamp).

[Johnny B Good]

====snip====

If enough of this type of lamp comes into common use in households nationwide, the national grid may become rather *unhappy* at such a destabilising level of leading current appearing in the grid.

It's not just cheap emergency gensets that suffer uncontrolled voltage regulation due to a 3 or 4 mF capacitor being placed across their 230v

2.8KVA output which, in the case of a 4.7mF load sent such a genset's output voltage north of the 270 volt mark.

The PSUs (Public Supply Utilties) only require that such industrial inductive motor loads be corrected to a lagging PF of 90% which suits both the PSU and its industrial customers since the cost of such PFC equipment would be doubled if they required these customers to fully correct to a unity PF.