Quality Dimming at low light for LEDs

[Makeitgo]

Hi All

I’m looking to dim 2 x 13W Dimmable LED E27 bulbs (in parallel). After a bit of reading it looks like I need a trailing edge dimmer that will accommodate a total minimum load of 26W (2 x 13W) or at least be adjustable to suit 26W.

What I’m hoping to do is have them capable of full brightness at full whack, but be able to smoothly dim to about 5% of their capability when necessary. I’ve not bought any of the kit yet and was wondering if anyone had already come up with a good combo for the above.

The main criteria is for there to be good control at low light, as this for a person who has a light sensitive eye condition. It would also be good to have capable of full brightness too though, to be able to clean the room properly, although this is less important than smooth dimming (no flicker) at low light.

Any thoughts would be welcome in terms of which bulb and dimmers would be best.

Cheers.

 

[bernardgreen]

Quality and reliable dimming of LED lighting over a wide range of brightness levels is very hard to achieve using off the shelf modules.

The brightness of the lamp is set by controlling the current driven through the LED elements, This current is controlled by a current driver in the lamp. Most often this is not a smooth DC current but it is pulse width modulated which at low brightness levels cannot provide good control of brightness.

LED current drivers that are "dimmable" use the shape of the chopped AC waveform produced by the dimmer to determine the level of current to drive through the LED element(s).

It is possible to find a combination of dimmer and current driver that work together over a range of brightness levels but for smooth control from full brightness down to 5% some specialised design may be necessary.

 

[Makeitgo]

Thanks for the detail Bernard. Do you think it might be in my favour to perhaps be a little less ambitious and head for a more narrow range, i.e. at maximum brightness, not as bright as in my OP? I'm just looking for a starting point to try and suck it and see. It's unlikely that I'll be seeking a specialised design unless there is one out there already that fits the bill, given the budget I have available. I might start with Varilux V-pro for dimming unless anyone favours an alternative off the peg brand/type to see how it goes. Just wondered if anyone had stumbled on a similar set up that happened to be good at a low light setting?

 

[ericmark]

light sensitive eye condition

In which case I would use tungsten, to control the output of an LED in the main a mark/space ratio is used, i.e. the lamp is switched on/off very quickly to give the impression of dimming, with tungsten it smooths this out, so I would simply not use LED.

 

[bernardgreen]

Rather than dimming a bright lamp have you considered switching between a lamp that is bright and a second lamps that has much less output. ?

Many of the lamps in my cottage are incandescent ( tungsten ) and can be dimmed by connecting two lamps in series. Each lamps then has 120 volts which gives a soft warm light.

 

[Makeitgo]

Thanks Eric. Yes I did think about using tungsten, but thought if I used LED there might be scope for playing around with the colour more (even if the difference was between bulb types, i.e. Warm, Daylight, Coolwhite). It seems the person this is for, does seem partial to using dozens of cheap AAA powered LED torches (a quite cold light) as the light from these is apparently less distressing for them. They are living in very low light currently, which is pretty prohibitive generally. I'm just trying to give them some more indirect low level general lighting. The situation's a bit of a mare actually.
I definatley get your point about the natural smoothing attributes of tungsten though. I might have to resort to this.

Bernard, thanks, yes I might have to forget the "normal" high light level side of it for a single source solution. I love the putting stuff in series idea though, but I think it might be a tad too warm for them. I've also got to suck it and see with regard to the colour temperature here as well it seems. The plan was to have a couple up uplighters on opposite walls, so the the effect would be reflected light and not too harsh for them.

 

[Makeitgo]

OK, thought I'd report my findings just to wrap it up. I got lucky with the first bunch of stuff I bought, as I got it to play together nicely. This is the line up:
I x Varilight 2 gang 2 way V-pro dimmer - JQP252W (only using it as a one way, with one module running a single 60W GLS tungsten pendant, the other doing the 2 LED uplighters in parallel)
2 x Bell - 9W LED Dimmable GLS Lamp - ES 4000K - Code: 05619

The dimmer is programmable with 3 modes of operation:
- Mode 1 - Trailing edge
- Mode 2 - Leading edge
- Mode 3 - Trailing edge (not sure what the actual difference in characteristics is compared with [1] )
John Ward on Youtube does an excellent job of showing what's going on with the above using a scope here:

. Just became a fan of this guy, wish I had his time on my hands.

In addition to these modes, you also have other changeable settings:
- For LED lamps that require more power to start up, there is another feature that can be enabled called "DRIVE" function.
- Adjusting for minimum and maximum brightness can also be done if it's found lamps are flickering when they are either at minimum or maximum brightness.

There are about 6 things to be able to play with which is nice. However, to get the dimmer into config mode, you have to faff about switching the dimmer on and off 3 times which I found to be fairly boring. In fact, I only got to grips with it after watching the Youtube demo (

). NB, there's a lot of info in the little manual in the dimmer's box, but look online too.

I ended up setting the dimmer to Mode 3, and I think, accidentally got it into DRIVE mode "on". To be honest, I would go into config mode again to check properly, but I'm scared of having a psychedelic episode : - ) To sum up, I got the LED lamps to dim right down to almost nothing smoothly and also up to full brightness too, so this is a good combo.

Bernard, appreciated your comments. From what I saw in John Ward's video above, the difference between using a tungsten load and an off the shelf driver/LED package is vast, with regard to how the waveform across the load changes when you switch from one to the other. Looks like trying to design dimmers and lamps independently to work together is a bit of a mare as you've suggested. Interestingly, although the dimming does the range that I've hoped for, it's a bit urgent at the low end as if there's an anti-log pot being used, but that's my only criticism.

Cheers all.