Some questions about dimming LEDs - Page 1
Forum section: electronics / beginners

author: Blitzschnitzel
author_rank: Regular Contributor
timestamp: September 12, 2023, 03:18:33 pm
content: Hi, I need to pack a larger area with LEDs and they shouldn’t be too bright. But they also need to be white with 6500K in colour temperature and have CRI above 90%. Unfortunately, I can’t find that as a low power LED, so I need to dim them.I get that I can reduce the voltage a bit but the LEDs would shut off long before it dims to the lower brightness percentages. When I look at datasheets, the forward current to luminosity graph starts at 0,0 and goes up almost linearly. Does that mean if I get an adjustable constant current power supply, I can freely adjust the brightness without a shut off point?I have also seen regular constant voltage power supplies with built in pwm but not constant current power supplies with pwm. Is that because that wouldn't be necessary for dimming LEDs or does constant current and pwm not work well together?

author: RJSV
author_rank: Super Contributor
timestamp: September 13, 2023, 02:00:29 am
content: Constant current is a kind of opposite, to PWM. Pulses are non-constant.Confusing, as you started with Voltage, up against a 'turn off' point, but then revert to a constant current source.That constant current source is going also end up set at some voltage...what that (varying) source situation does to the absolute COLOR temperature is variable, by exact device type.Some LEDs just simply go way dim, as voltage goes down, to a point, but that varies by device, and not sure what shift of color output can happen.

author: Infraviolet
author_rank: Super Contributor
timestamp: September 13, 2023, 03:13:53 am
content: It might apply differently to big high powered LEDs, but usually you can get to an arbitrarily low brightness by having the LED only on for a small fraction of some duty cycle period, and making that ull cycle period far too short for the eye (or, given your requirements I suspect, a high quality video camera) to perceive. LEDs, normal ones atleast, respond very fast to changes in supply, so when it is on for that brief moment it would electrically behave like it was on for a longer period. Most LEDs can switch at MHz and more without difficulty, so creating a PWM situation at 10s of KHz should be easy, and in the long term your eye or a camera operates over 10s of KHz might as well be constant illumination. PWMing an LED isn't like PWMing an inductive load, the PWM doesn't actually give a low voltage here the way it can when PWMing a DC motor, it just gives a low energy output when averaged over the longer term.

author: arcfault
author_rank: Contributor
timestamp: September 13, 2023, 04:36:35 am
content: Given that simply lowering the voltage doesn't work in this case, I think PWM might work. It'd also probably be noisy and cause issues, especially in HDR photos.My low tech and silly solution would be to put something between the light and the room to make the light dimmer, more diffused, and hopefully keep the same color. Maybe some acrylic or some 3D printable? Might be the easiest solution, though certainly not an ideal one.

author: Haenk
author_rank: Super Contributor
timestamp: September 13, 2023, 05:19:14 am
content: There is milk glass - this has been in use for a long time, to get uniform backlighting. Should be availabe in large panels. Also available as plastic sheets.Just remember to get rid of the heat, a large number of LEDs will produce a lot of heat. Less an issue with pulsing...

author: RoGeorge
author_rank: Super Contributor
timestamp: September 13, 2023, 05:37:07 am
content: Quote from: Blitzschnitzel on September 12, 2023, 03:18:33 pmYes, LEDs must be powered with constantcurrent, so you'll need acurrent source, not a voltage source. A current source adjusted for a lower than the nominal current of the LED will give you less light. At very low currents, the spectrum of the light or the CRI might change, too, or at least that's how it seems to me by simply looking with naked eye (without any instrument to measure the light).PWM means pulses, can be either current PWM or voltage PWM.In conclusion, to lower the lights, either use a constant current that provides less than nominal current, or use a PWM current source. PWM might flicker at very low intensity, while low current might change the spectrum. You should try and see which one matters more for your application.If you must keep the CRI at all costs, you may also use PDM (pulse density modulation) instead of PWM (pulse width modulation). PDM might flicker less than PWM when the LEDs are dimmed very low.

author: pqass
author_rank: Super Contributor
timestamp: September 13, 2023, 05:59:44 am
content: Quote from: Blitzschnitzel on September 12, 2023, 03:18:33 pmUnlike resistors LEDs are non-linear. Very little current flows until the forward voltage is reached. Above that, the current increases exponentially with increasing voltage. So a small voltage change of a couple of volts (above Vf) will have a large effect on brightness. See attached typical IV curves. However, if you apply too much voltage (causing more current to flow than max. continuous If) you will destroy the LED. Therefore, it is best practice to limit the greatest current possible via resistor or active current limiter (eg. transistors, regulator IC).QuoteYes. Just don't apply more than max. continuous If.QuoteI can't say I've ever seen a CV power supply with PWM output capability.However, a power supply with selectable max. output current whose output can be PWM'd should work to adjust LED brightness.The max. CC knob protects the LED from destruction while the PWM adjusts its brightness.Q:Why do both (CC and PWM)?A:You can just use the CC supply but then it would be continously dissipating power which may be a lot if driving high-power LEDs. If you add PWM to modulate brightness, then the off-time dissipates no power. Also, choice of PWM frequency may cause flicker while recording.See attached for a rudimentary CC LED driver with PWM.

author: EPAIII
author_rank: Super Contributor
timestamp: September 13, 2023, 06:16:16 am
content: You need to pack a larger area with LEDs.Just what kind of "larger" area are you talking about? What is large in one context is tiny in another.Are you talking about a larger room? A stadium? A factory?Or are you talking about something like a square foot or square meter? Or perhaps even less? A "larger area" under a microscope can be less than a square cm.And when you say "LEDs" that also can mean a number of things. LEDs come in types and sizes from tiny chips that are around 1 mm square, are intended to be mounted on PCBs (Printed Circuit Boards), and that produce only 1 mcd up to LED lamps that can illuminate parking lots or stadiums or large factories or warehouses."LEDs" can be just a LED or they can be the assembled products that include the circuitry for regulating the current from mains power (115 or 230 VAC) that you use in your home's lighting fixtures. In today's world, there are many, many "LEDs" and each of them would need a different type of control to vary it's brightness. Some even do not have a good way to do that.So, please share with us exactly what you are talking about.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: September 14, 2023, 10:17:33 pm
content: Quote from: EPAIII on September 13, 2023, 06:16:16 amI'm guessing it's the ceiling of a football stadium.But overall, as many already wrote here. the luminosity of a LED is pretty linear with the current through the LED, and usually this holds true all the way down to micro Amperes. But at the very low end CRI will suffer, and there may be a very low current below which there is no light at all anymore.At the other end, with very high currents, the efficiency of the LED goes down. The LED will still produce more light, but not as much as you would expect from the current. Depending on the type of led, this is often also quite noticeable in a quickly rising temperature. For example, I have bought a bunch of LED panels which have about 50 LED's on an aluminiumumyum *1). substrate, and according to the chinese manufacturer these panels should be good for 10W. However, up to about 6W these panels stay almost at room temperature, they are moderately warm at 8W and get quite hot at 10W. So just from the sudden temperature rise it's clear that efficiency goes down significantly. And also, life expectancy of LED's goes down significantly at elevated temperatures.*1). Stupid name. Just call it Element 13.

author: themadhippy
author_rank: Super Contributor
timestamp: September 14, 2023, 10:40:12 pm
content: Quotethats more diffusion than dimming,for dimming whilst maintaining the colour temperature nd filters were the weapon of choice

author: RJSV
author_rank: Super Contributor
timestamp: September 14, 2023, 11:25:17 pm
content: I've worked a lot with what's termed as a 'Light Diffuser panel', an open grid of square holes, and sold in 2 by 4.(feet) rectangles. Mounted directly below the usual florescent fixture, and allowing for blowing conditioned air into a space.In California the got is TAP Plastics store, San Leandro, CA.Maybe also called a 'register'.'Maker' type folks, can find these register panels to be an interesting source material, for some futuristic looking components. A simple needle nose pliers can, selectively, be used to eliminate a wall here, and there, by piecemeal breaking off.

author: RJSV
author_rank: Super Contributor
timestamp: September 15, 2023, 01:53:10 am
content: Ah, the term I couldn't recall, is 'EGG-CRATE light diffuser...that's the official label, for the light fixture.

author: Blitzschnitzel
author_rank: Regular Contributor
timestamp: September 15, 2023, 05:04:22 pm
content: Thank you all for the answers that helps me a lot. So, if I understand correctly, I can dim LEDS with a constant power supply freely from 0 to 100%. Although, in practice the adjustable power supplies have predefined steps. At the same time reducing the current can change the LEDs colour, so I might want to use a fixed constant current power supply at optimal current for the LED and reduce the brightness with PWM.Quote from: RoGeorge on September 13, 2023, 05:37:07 amI find this very confusing. I thought PWM is just mosfet cutting the connection for a while. So PWM would cut both voltage and current to 0 in a set frequency.The plan is to have a light that comes as close to sunlight as possible, including very parallel light rays. Kind of like a white laser that emits light from a 40x40cm area. I had tested it with an aluminium PCB and added narrow beam lenses. Everything, including the thermals is great. Only the shadows are split tenfold. For the next step I want to place the LEDs way denser but run them dimmer and keep the overall lumen output the same. So, adding diffusers is not an option.

author: sparkydog
author_rank: Regular Contributor
timestamp: September 15, 2023, 05:51:20 pm
content: Quote from: Blitzschnitzel on September 15, 2023, 05:04:22 pmHave you looked into using asinglelight source with proper lenses? Or maybe a parabolic reflector? The technology to make beams of light with minimal spread has existed for a long time...

author: Jr460
author_rank: Regular Contributor
timestamp: September 15, 2023, 06:47:06 pm
content: I'm confused...First off stop thinking of things in terms of LEDs. I'm sue whatever problem you have has been done with old fashioned hot lights. Once you figure out how to modify the light, then how you dim the source is just a slight add on.If I wanted something that covers a large area, I'd use a standard photo soft box. They are made for flash heads, which start out close to a point source, and smoothly spreads it out over a good size, for example 48" x 48".Wanting 6500K and a good CRI, I get, yea that would daylight, or in the area of flash. But, if you want the effects of sunlight, then a large area for the source is not what you want. From a photo standpoint, the sun is more light a point source and gives hard well defined shadows. If I want smooth look, then I'd use a larger source like a soft box and get it as close to the subject as I can, and then crank down the flash power to get the light levels you want.

author: Zero999
author_rank: Super Contributor
timestamp: September 16, 2023, 02:17:23 pm
content: Quote from: pqass on September 13, 2023, 05:59:44 amThe resistor needs to go in series with the PWM driver signal.

author: pqass
author_rank: Super Contributor
timestamp: September 16, 2023, 04:53:20 pm
content: Quote from: Zero999 on September 16, 2023, 02:17:23 pmYes, but the PWM output should swing high enough to be able to fully turn-on the MOSFET (although Q2 won't let that happen).For a typical (non logic-level) MOSFET like the IRF540, that would be 10V.The original circuit would work if the PWM output was an open collector.See attached for a more practical/complete version of the CC LED driver with PWM.Simulator link hereChoice of Rs (shunt) will limit the maximum current through the LED string.It's best to string LEDs in series (vs. paralleling them); enough so that the total Vf *quantity is just under Vdd.The LED supply (Vdd) can be different from the 555 supply but doesn't have to be.The advantage with a higher Vdd is being able to support more LEDs per CC driver.