Swap out the phase angle dimmer in an Ikea lamp to a linear one - Page 1
Forum section: electronics / beginners

author: prosper
author_rank: Regular Contributor
timestamp: January 04, 2023, 07:14:58 pm
content: I have a couple of Ikea bedside lamps with integrated dimmers. They suck. I assume they're a phase-angle control dimmer, and they do a horrible job of dimming the LED. The range of dimming goes from about 60% brightness to 100% brightness, and they flicker and drift all over the place.The bulbs themselves have a simple string of LED's in series, a bridge, and a linear driver chip in them - they're definitely not switchmode drivers. So, I had the idea to replace the SCR circuit with a variable linear current source - I mean, the LED's only draw 30mA at 120VAC, so, it's not like I need to cope with a ton of heat or anything.I drew up the following circuit. Does this seem decent? It uses a capacitve dropper, which means that nothing is isolated from mains, but I think that shouldn't be a problem - the existing dimmers aren't isolated either.

author: Gyro
author_rank: Super Contributor
timestamp: January 04, 2023, 07:57:54 pm
content: From a quick look. You won't find a 220nF capacitor in Y2, it should be X2. Also a capacitive dropper won't work with a half-wave rectifier (nothing to discharge the capacitor), it needs to go on the AC side of a bridge rectifier.It's not clear where all the GND connections return to. It can't be protective ground (earth) as you are not permitted to actively pass current through the ground conductor as part of normal operation.Ah, ok, it's the negative side of the bridge rectifier. Why aren't you using a (resistive) dropper from the +ve side? The transistor base current shouldn't be that high.

author: prosper
author_rank: Regular Contributor
timestamp: January 04, 2023, 08:21:36 pm
content: good call, I hadn't even considered a resistive divider. That'd be much simpler. Thanks!

author: james_s
author_rank: Super Contributor
timestamp: January 04, 2023, 08:40:52 pm
content: Have you tried different bulbs? Some of them work much better than others on triac dimmers. You could also remove the dimmer entirely and use a smart bulb that has built in dimming, those work very well and you can get various wireless remotes to control them directly without needing any other infrastructure.

author: prosper
author_rank: Regular Contributor
timestamp: January 04, 2023, 09:47:44 pm
content: It's one of those bayonet-style lamps. There's not really a ton of options for different bulbs, unfortunately.

author: IanB
author_rank: Super Contributor
timestamp: January 04, 2023, 10:01:35 pm
content: Quote from: prosper on January 04, 2023, 09:47:44 pmAre you in North America? I didn't know it was even possible to obtain bayonet cap bulbs for 120 V service? I have never seen one.As far as dimming is concerned, it appears to be important to have a good quality bulb (e.g. Philips), and also best if the dimmer is a trailing edge design rather than the older and more traditional leading edge design. Rather than changing to a linear dimmer, you might just try swapping it out for a trailing edge dimmer?However, in my experience I have found Philips bulbs to dim perfectly well on old dimmers. On a newer dimmer such as Lutron the Philips bulbs will go down to a barely perceptible glow and remain stable.

author: prosper
author_rank: Regular Contributor
timestamp: January 04, 2023, 10:09:40 pm
content: It's GU10 - like this:https://www.ikea.com/ca/en/p/solhetta-led-bulb-gu10-380-lumen-dimmable-40498781/They're reasonable common in Canada at least. The dimmer's built into the fixture. I could look at building a trailing-edge dimmer, but linear seemed simpler. And it's not like it's a high-power lamp, so, efficiency isn't so concerning

author: prosper
author_rank: Regular Contributor
timestamp: January 04, 2023, 10:14:34 pm
content: I re-drew it with a resistive dropper instead of the capacitive. The dropper consumes about half a watt, and can provide ~3mA of base drive. With a β of >> 10, I think it should work OK. C1 should hopefully reduce the flicker a bit. Oh, and R3=100Ω, not 180.

author: prosper
author_rank: Regular Contributor
timestamp: January 05, 2023, 06:19:11 pm
content: I prototyped portions of the circuit, and tested at lower voltages with a dummy load. Seems to work as expected. Tweaked a few values here and there, but I think I'm ready to build it up and test it out at mains voltage, using the actual lamp, to validate my assumptions about the lamp driver.

author: prosper
author_rank: Regular Contributor
timestamp: January 09, 2023, 04:20:54 pm
content: Update: well, it doesn't work. The driver in the bulbs doesn't like being current controlled, I guess. Bare LED strings work fine, they light up and dim nicely. But the bulbs I was wanting to use this with - they don't light up at all. Guess I'll start working on a ZVS trailing-edge dimmer.

author: tunk
author_rank: Super Contributor
timestamp: January 09, 2023, 04:48:27 pm
content: Quote from: prosper on January 04, 2023, 07:14:58 pmWhat's the linear driver chip?Many of the linear chips use a sense resistor to set thecurrent, and may not be designed to work with dimmers.

author: edavid
author_rank: Super Contributor
timestamp: January 09, 2023, 04:56:22 pm
content: Quote from: prosper on January 09, 2023, 04:20:54 pmIs it possible that the bulbs don't work on DC? That should be easy to test.

author: prosper
author_rank: Regular Contributor
timestamp: January 09, 2023, 05:05:40 pm
content: It does work on DC, I tested that. And it works at reduced brightness when half-wave rectified.I don't know the details of the driver circuit, I'd need to smash one open to get a real good look. I can see a bridge, a SOIC8, a moderately sized capacitor (maybe 10uF?), a SOT23, and a few passives. No inductors that I can see. It is 'dimmable' to a degree, so that implies some sort of smarts to the controller.

author: Zero999
author_rank: Super Contributor
timestamp: January 09, 2023, 05:15:29 pm
content: Quote from: prosper on January 09, 2023, 05:05:40 pmHave you tried PWM?

author: prosper
author_rank: Regular Contributor
timestamp: January 09, 2023, 05:17:35 pm
content: I could, though is PWM at line voltage just asking for trouble noise-wise?

author: IanB
author_rank: Super Contributor
timestamp: January 09, 2023, 05:24:24 pm
content: I could mention that I tested various LED bulbs of different kinds and different brands with a modified sine wave inverter, and found that none of them worked properly. They absolutely seemed to be expecting a proper sine wave AC supply.The problem with the modified sine wave was that the bulbs consumed too much current and ran the LEDs much too hard. In some cases at more than 200% of the design power. So the bulbs "lit up" and seemed OK, but if I measured the supplied current and power things were badly out of spec.Consequently I would not trust any bulb with DC or with PWM without doing some careful measurements first.

author: prosper
author_rank: Regular Contributor
timestamp: January 09, 2023, 06:21:59 pm
content: I played around with a few other configurations, including slow PWM (120Hz) and trailing edge phase angle control from my function generator, as well as variable voltages AC and DC voltages. (in retrospect, I probably should have done this first...)In all cases, the end result is the same: the LED is only capable of dimming down to about 50% brightness, then it cuts out.I suspect I'll have better luck if I find a 'non-dimmable' bulb that uses a simple linear driver instead of a 'smart' scr-aware dimmable driver.

author: IanB
author_rank: Super Contributor
timestamp: January 09, 2023, 06:42:41 pm
content: Quote from: prosper on January 09, 2023, 06:21:59 pmSurely this then is a problem with the bulb, and a different bulb could produce different results?If I give you an example, I have dimmable LED bulbs from Philips that go down to 10% on a Lutron (trailing edge) phase angle dimmer and remain quite stable. I can dim a bulb down to the point where the glow is barely visible. I have similar results with bulbs from GE or Feit Electric.The Philips bulbs also dimmed quite effectively on an older leading edge phase angle dimmer, so the trailing edge dimmer was not strictly necessary.QuoteBefore trying a non-dimmable bulb, why not try a different brand of dimmable bulb with your phase angle tests?

author: prosper
author_rank: Regular Contributor
timestamp: January 09, 2023, 06:54:49 pm
content: Yes, I'll do exactly that. Thanks!For what it's worth, the best results I got were with a pseudo-PWM control, with this circuit:

author: Zero999
author_rank: Super Contributor
timestamp: January 09, 2023, 08:01:22 pm
content: Quote from: prosper on January 09, 2023, 06:54:49 pmThat's not PWM at all. It's a current sink and a very poor one at that. The current will be highly dependant on the hFEand VBEof the transistors.

author: prosper
author_rank: Regular Contributor
timestamp: January 09, 2023, 08:42:51 pm
content: That would be true, if the LED wasn't internally limited to about 30mA. As it is, the transistor starts to turn on fairly sharply (due to its high gain) as the voltage comes up, and hits the current limit of the LED pretty rapidly at most settings of the potentiometer. I'm using it as a (low quality) comparator, effectively.I certainly wouldn't recommend it as any sort of solution, though. But as a quick and dirty proof of concept - it worked all right-ish.

author: Zero999
author_rank: Super Contributor
timestamp: January 09, 2023, 10:18:31 pm
content: Quote from: prosper on January 09, 2023, 08:42:51 pmIt's still a current source. If the LED is turning on and off quickly then it's because it's interfering with the internal current regulator, causing it to oscillate.

author: james_s
author_rank: Super Contributor
timestamp: January 09, 2023, 10:47:33 pm
content: There are LED drivers that have built in dimming support, the PT4115 is one that I've used a few times that is a low voltage buck regulator, others are available that are off-line drivers for series arrays. You could gut a bulb and wire the LED array directly to the terminals and then install a driver in place of the dimmer.

author: IanB
author_rank: Super Contributor
timestamp: January 10, 2023, 02:04:53 am
content: I thought I would try to illustrate the range of dimming available with a Lutron dimmer and a current generation Philips LED bulb. It almost exceeds the dynamic range of the camera and was quite difficult to capture. The two attached pictures show full brightness and the lowest reasonable dimmer setting, both with the same exposure. This level of dimming is far below any usable light output and you can only see the glow in a dark room, but it is stable with no flicker.