Removing flickering of LED lighting

Question

I am trying to install recessed, dimmable LED lighting in the living room. We're talking about an end solution of maybe 15-20 spot lights. My home has 120V/60Hz.

Currently I am experimenting with a single spot light, a 4 x 1W warm white LED LemonBest 120V AC. Besides the main lamp the device has a small box which is labelled 'LED driver'.

I installed a Leviton slide dimmer in the wall and the spot light into its hole in the ceiling. There is functionality. The light can be turned on and dimmed.

However, the light that is produced has a flicker in it. It is especially noticeable when the light is dimmed. This flicker is not an option. I am trying to get rid of the flicker.

Since this is the 2nd dimmer I am trying I gave up on a solution a la 'just get the right dimmer'. This problem needs to be solved differently.

Measuring the output of the 'LED driver' results in 25V down to 15V DC depending on the setting of the dimmer. Now, I guess that the 'LED driver' contains a rectifier and a resistor. I further guess that the flicker is a result of voltage as a function of time which probably goes like in the image (the resistor is not shown). The LED has probably a minimum voltage that is needed for operation and on a frequency of 120Hz this minimum voltage is not provided. Thus, the flicker.

My question is: Is there an inexpensive, but still professional way to achieve a non-flickering LED light? E.g. would a capacitor added to the wiring at each lamp smoothen the voltage? Or is the only solution to add a 25V DC source to the wall and 'dimm' the DC current (and remove the 'LED driver' at each lamp)?

EDIT:

The LED lamps are designated as 'dimmable' and the dimmer slider I use is designated as for the use of LEDs. As I said, the lights are dimmable and for most use cases it's fine. But in a very ambient setting (low dimmer setting) and when uses as the only light source in the room, one notices the high frequency flicker. (If I fan my finger in front of a book page I see a stroboscope effect). This is annoying and I'd like to solve this before installing ~20 of these lights.

I opened the 'LED driver' and took 2 photos. See below. Not sure what the brown-ish thing is. A capacitor maybe?

EDIT:

And this is the lamp:

And the other side (with the LED driver connected):

For completeness the specs of the lamp (source Amazon):

Specifications:

Power: 4W(4x1W LEDs)
Input Voltage:AC 100-145V
Dimmable: Yes
Material: High Strength Aluminum
Color Temperature:Warm White(2800-3500K)
Luminous Flux:480-650LM
Beam Angle:45 degree
Life Span:50000 hours
LED Working Temperature:-25°C - +65°C
Size:D3.39*H1.65inch
Cut-out:2.68inch (68mm) 

EDIT:

I looked up what a PWM is and it basically puts the signal periodically to zero on a high frequency simulating a 'dimming' effect. The signal would look like:

Thus for the dimmable LED lights to have the best result one needs to go for the highest frequency (or fast-acting) PWM. If the frequency is high one should not notice the flicker. But what would be high? As I see it, right now I guess my flicker is at 120 Hz and it is annoying. (Honestly I think it's at 60Hz but technically I can't explain 60Hz, so I say 120Hz, comes from the two zeros per sinus wave). Guessing I need a PWM at at least 240 Hz. Does this make sense?

Answer 1

I sort of have an answer but it may not be one that you like.

I have the same problem in a mobile audio recording truck that I am working on. The person with whom I am working has tried every combination of every single LED fixture and wall-mount dimmer that was available locally as well as several units purchased online.

All of these have some degree of flicker at the low end of lighting.

My solution is sort of drastic. I'm going to rip the electronics out of every LED fixture in the control room and install a very-fast-acting PWM constant-current driver inside each fixture. Then I'm going to use one of the standard PWM LED dimmers that we build and simply supply 12V PWM to each fixture.

I have the freedom to do this - I was worried that flicker might be a problem (past experience) and made sure when wiring the control room that I would be able to isolate those particular lighting runs from the 120 Vac supply and instead use DC.

My particular case is fairly unique in that I have several hundred Amp Hours worth of 12V battery in the truck's battery compartment from which to draw power. You would have to find somewhere to install your DC power supply, whatever voltage you want to use.

I have two standard LED PWM frequencies that I use: 977 Hz (1024 us tick rate) & 4 KHz. Both work well and I have never experienced a flicker problem with either. I'll probably wind up using the 977 Hz version just to give the constant-current LED drivers a fighting chance of working well.

Answer 2

Ordinary dimmers cut the power for part of the sine wave:

(Source: Wikipedia.)

The LED driver is designed for a full sine wave. You need a dimmer and driver that are compatible with each other — perhaps even sold as a set.

I doubt the driver is as simple as that. LEDs work best with a constant power supply, which implies no flickering at all. The LED semiconductor should see constant voltage and current.

EDIT: Ah, the driver isn't entirely built-into the bulb. Are you sure its output is DC? LEDs work at constant voltage, so the only possibility at the LED chip itself is PWM. You need to figure out what the individual components are actually doing to figure out the problem.

My guess, if everything you have is designed to be compatible, is that it's producing DC at full brightness and 60Hz PWM when dimmed. You need a higher PWM frequency for good quality, which implies replacing/redesigning everything between the mains AC and the LED chip.

Whatever you do, do not connect the LEDs to an adjustable, ideal DC source. The effect will be a binary on/off, and increasing the voltage past "on" will tend to burn them out.

Answer 3

As others have kinda confirmed, dimming LEDs is non-trivial. Unlike incandescent bulbs where brightness is proportional to applied voltage, AND that there's a huge thermal lag in that burning filament that allows 50/60Hz 'chopper' triac-based dimmers to work without/much perceived flicker, LEDs have a significant minimum voltage before any light comes out, and then only a proportionally small increase in voltage to increase the brightness.

In fact, you set an LED's brightness by either changing the current through it, not the voltage applied to it, and by PWMing that current to achieving dimmability.

So in practise, to dim LEDs down really low, you really do need high frequency PWM dimmers, which drive a constant-current through the LEDs when the PWM is on, and then the width of the W in PWM sets the brightness, and human persistence-of-vision does the rest.

Unfortunately the domestic LED lighting industry is nascent, and either clueless or unscrupulous retailers and designers are selling inappropriate dimmers as being suitable for LEDs, or selling LED fixtures that they say can be dimmed, but in reality can't very much, certainly not down to 'mood lighting' levels. For full or near-full dimmability, you need the combination of an LED dimmer that truly does do PWM dimming (and we're talking kHz) coupled to LEDs that too can be dimmed via PWM.

Answer 4

Naked LEDs have a very fast response time – they are often used for data transmission – compared with the very slow incandescent or medium fluorescent tube response time. This means that there is no ‘luminance inertia’ to smooth out variations in supply current and ANY variations in current will instantly change the light output. Nearly all dimmers work on some sort of PWM system wall phase control dimmers are just PWM linked to the supply waveform. Analogue dimmers supplying DC are possible, but waste a huge amount of energy – I remember those ancient rheostat dimmers in my school theatre producing so much heat we nearly melted! Incandescents work very well at 50/60Hz PWM because of the (slow) rate at which the filament temperature responds to supply. High frequency fluorescent drivers, which convert to DC, then run PWM at around 40KHz, produce an almost completely flat light output because of the phosphor persistence, but have been relatively expensive and too large to integrate into conventional ‘bulb’ shapes. There are advances being made in LED drivers that are very similar to the fluorescent HF ballasts and run the LED PWM at around 1KHz which subjectively, because of the human eye’s persistence make the LED appear to be flicker-free. However, many older LED designs still DO switch on and off very quickly and therefore still flicker at the PWM frequency and, for example, are not recommended for illumination where rotating machinery is in use because of the danger of the stroboscopic effects. Advances in phosphor technology in new LED designs are at last introducing sufficient persistence to fill the PWM troughs and matching LED response to the PWM should result in almost flicker-free dimmable LED lamps, but they are unlikely to be the cheapest ‘e-Bay specials’. LEDs have a further complication that they operate at (very near) constant voltage; below this voltage they produce little or no light, and attempting to run them at higher voltages will usually just burn them out. So the only way to obtain a constant light output from an LED is to run it using DC at a constant current which massively reduces system efficiency especially where dimming is required. So hopefully the advances in LED design coupled with better PWM drivers will fill the gap (pun intended), but there is still a way to go!. AJC.