Touch Lamp

[Tim+]

It’s just Wodney being Wodney…

Tim

[NY]

Is "bluetooth" significant here? Because we've got ordinary Philips Hue bulbs which communicate with a Hue Hub, and those do no take kindly to being dimmed with an ordinary dimmer that is designed for tungsten bulbs. Typical symptoms are failure to dim as the dimmer is turned down, followed by pronounced flickering and then total extinction: no nice controllable brightness from 100% to 0%.

Are you talking about dimming the Hue bulb with a dimmer, or are you talking about feeding the Hue with constant 240 V and using the built-in dimming capabilities of the Hue bulb itself? The former doesn't work, in my experience; the latter does work, with one proviso: there is a minimum brightness below which the bulb will not go, with the next step being totally off.

For an external dimmer (eg in a wall-mounted dimmer switch) you need one which is designed for LED bulbs. I may have got the following the wrong way round: a traditional dimmer cuts off the beginning of each half-cycle so it turns *on* part-way through the cycle at a non-zero voltage; the LED sort instead cuts off the end of the half-cycle so it turns *off* part-way through.

I have found that touch-sensitive lamps do not take kindly to non-tungsten bulbs: not only does the bulb not dim, but the thyristor/triac often fails. My father in law was an electrician and he was always having to repair people's touch-sensitive lamps - he did it to a couple of ours where we accidentally put a fluorescent or LED bulb in the socket and then started to increase the brightness. Fluorescent (CFL) bulbs may be more likely to cause damage than LED bulbs.

[zall]

Yep, you only need the one bulb.

They do their own dimming. Yes, I should have said that.

But there is when you use the app or alexa, siri or google home.

No.

Yes.

Unlikely to be a problem with a table lamp.

[NY]

Actually there *is* a minimum controllable brightness. We have some bayonet-fitting (in US and mainland Europe they'd be Edison screw fitting) "mushroom" bulbs, a lot of GU10 and some SES-fitting candle bulbs.

In all cases, you can control the brightness from 100% down to fairly dim, but then if you turn down the brightness any lower, the bulb goes out completely.

I gather this is a function LEDs: that you cannot vary the brightness right down to zero - whether the bulb is a Hue with its own dimming or a conventional LED bulb with an external dimmer that is designed for LEDs. This is unlike dimming a tungsten bulb where you can control the brightness right down to the point where, in a darkened room, you can barely see the filament glow.

[Rod Speed]

While I confirmed that with the 800 lumen white and color bulb, I have just tried it with the newer 1600 lumen white and color bulb and found that even at 1% you can still turn it off and on and see the difference. Can't see how to go below 1% in the hue app.

Can't agree given that you can certain do anything you like with a bigger dropping resistor with a discrete led used as a panel led.

[Martin Brown]

Actually it isn't really a function of the LEDs themselves so much as of the power supply design being used to drive them. The sharp cutoff is done deliberately because otherwise some LED mains lights would remain dimly lit by capacitive currents flowing in the wiring. Early ones did!

A modern bare white LED will emit light enough to be visible in a normal room with somewhere between 1 and 10uA flowing through it. Dark adapted you can use that much light as a torch. In the dark you can see the LED die emitting light down to a few nA.

It provides a way to make a DIY cheap emergency torch that you can always find in the pitch darkness (at least for one that has enough batteries in to provide the LED forward voltage). Bridge the on/off switch with 1M resistor and the glow of the LED will be enough to find it. Even enough to see by if you wait long enough.

They used to get complaints of mains LED lights not switching off and continuing to glow dimly when "off" before they tweaked them. It was particularly problematic on dual switched lights where there would be a long run of live mains next to the disconnected lead to the bulbs.

Capacitive couple was more than enough to power them. It is the sheer efficiency of LEDs at converting current into light that forced this slightly odd engineering change.

[NY]

I'm talking about conventional thyristor dimmers which delay that start of each mains half-cycle or cut the end off the half-cycle. I'm guessing that Philips Hue bulbs use the same technology within the bulb housing. With dropping resistors, I imagine you can get closer to zero.

[Rod Speed]

NY [e-mail] wrote

The 1600 lumen white and color bulbs certainly do not because they still emit visible light when set at an intensity of 1%, the lowest you can actually set them brightness wise.

In fact there is no abrupt cutoff with increasing serial resistance.

Same with brightness control by changing the current which is basically what the series resistance does.

[Dave Plowman (News]

I have lots of dimmers here. Installed maybe 40 odd years ago. Have a panel in the front room with 9 of them. Crabtree grid, and dimmer modules made by Home Automation. Different sizes too - matched to the load. None of them will dim a dimmable LED.

Have some rather later Home Automation dimmers on an MK grid. I'd guess about 30 years old. They do work just fine with suitable LEDs.