Using PWM to increase LED life - Page 1
Forum section: electronics / beginners

author: sairfan1
author_rank: Frequent Contributor
timestamp: November 13, 2024, 07:32:01 pm
content: Not sure if I'm correct, I'm going to use 1W and 3W LEDs to use as light source on table, i came to know if we use PWM it may increase LED life,Otherwise I was going to use with static DC current, I wanted to know is it really worth to use a PWM circuit to control LEDs?Picture of LED type is attached for reference

author: Siwastaja
author_rank: Super Contributor
timestamp: November 13, 2024, 07:34:41 pm
content: PWMing compared to 100% full blast all the time -> life time increase because obvious reduction in power dissipationPWMing compared to achieving same average brightness by lower DC current -> life time decrease because light output per current is non-linear, efficiency is worse at high currentIf you just want to have light and do not need to control it dynamically, PWMing makes no sense.

author: Konkedout
author_rank: Frequent Contributor
timestamp: November 13, 2024, 11:22:03 pm
content: Efficiency may fall off at very low current also. PWMing will dim the visual brightness proportionately. But if you just want to get long life, simply reduce the static current. I think that anything under 20-30% of max current will give you long life unless the environment is hot.I have what I call a "dim light" in our pass through mud room (laundry room). It uses four 1W LEDs, each running at about 5%. It has been running continuously for several years.

author: thm_w
author_rank: Super Contributor
timestamp: November 13, 2024, 11:44:05 pm
content: More important is to measure the temperature of the LED and ensure it doesn't exceed recommended levels. Keeping in mind the case temperature is less than the die temperature inside.If you want it to last long, keep it under 85C.Whether its run at 1W or 3W doesn't really matter if its cool.

author: neillnz
author_rank: Contributor
timestamp: November 14, 2024, 03:56:24 am
content: You can expect to get better life by operating the LEDs at around 85% of the current rating, eg. 300mA for a LED rated at 350mA.As others have said you must of course also provide adequate heat dissipation.I'm not sure about any difference between 350mA @ 85% duty cycle and 300mA DC. There will be a small change in the forward Voltage and hence power. Personally I would use 300mA DC as it's less complicated.Note that our eyes have a log like and not a linear response and as such you will hardly notice the difference between 85% and 100%.

author: MrAl
author_rank: Super Contributor
timestamp: November 14, 2024, 05:13:30 am
content: Quote from: sairfan1 on November 13, 2024, 07:32:01 pmHi,It depends what level you are going to use the LED at normally. If you use PWM and still go for 100 percent 'on' time, there is no change in life expectancy. If you want to PWM at 50 percent, then you can expect a life of about twice that as at 100 percent, which is a fairly linear relationship because PWM runs the LED at full current just for only part of the time.If you instead use constant DC current at 1/2 rated current, then the life should be more because the life goes up a lot more with reduced DC current, but only goes up linearly with PWM.So it depends a lot on what level you will normally run the LED at. At 100 percent no change, at anything lower than maybe 80 percent you would get longer life with a steady DC current, and around 50 percent or so you would see a marked increase in life with a steady DC current.

author: Siwastaja
author_rank: Super Contributor
timestamp: November 14, 2024, 07:13:59 am
content: Quote from: neillnz on November 14, 2024, 03:56:24 amAlso photons out per coulomb in decreases at higher current, so efficiency drop is caused by the combination of these two effects: less light per mA, more V needed, quadratic effect on power.But yeah it's not a large effect, it depends on how manufacturer rates their LEDs, some are almost perfectly linear in light produced per current up to the rating and only start tapering off after rating is exceeded. Some others benefit significantly from derating.There is a curve in every datasheet which shows the latter (current vs light) phenomenon. The Vf rise is in a separate curve so you need to combine the two to see the total efficiency drop.

author: MrAl
author_rank: Super Contributor
timestamp: November 14, 2024, 10:51:02 am
content: Hello again,I used to have the formula for the life expectancy of an LED based on a steady DC current but it is lost to time.We can get an idea how this changes though by assuming an inverse square relationship, which is not unreasonable as an estimate.For example, if we run a 20ma LED at 10ma, the life expectancy would go up N times:N=(20/10)^2which of course means N=4 (not N=2 because of the square).That's probably a good enough estimate in the absence of any real data published by the manufacturer.Note there is also a time factor in there. If we run the LED for 12 hours out of every 24 hours, then that is only 1/2 the time. That means we have twice the life expectancy when we consider the entire lifetime. That leads to another factor T:T=12/24which of course is the actual run time over the total run time (per day in this case). So we have the total factor F:F=N*TThis also means that since when we use PWM we bang the LED with the full current, we do not get any benefit from N, so N=1. That means we are left only with:F=TThat means if we run that 20ma at an average 10ma PWM, the life expectancy for 12 hours out of 24 hours is:F=12/24which is only a factor of 2.With constant current of 10ma and run for 12/24 hours, we have the advantage of both ratios so we get a much longer life expectancy:F=4*2=8Of course 12 hours out of 24 hours is still PWM, but usually we use smaller time periods like maybe 1ms 'on' and 1ms 'off', which means 1ms/2ms=2 again. This still leads us to a factor F of 8 which is a considerable increase in life span.Most significant here though is of course the exponential increase with reduced DC steady current, which in these examples was 4 times. That's a lot. If we can get away with using two LEDs instead of one and running them both at 1/2 steady DC current, we can get 4 times the life and about the same light level output as one LED run at full current (actually slightly higher light output with two LEDs run at 1/2 current). Pretty nice.

author: tggzzz
author_rank: Super Contributor
timestamp: November 14, 2024, 11:00:18 am
content: Quote from: sairfan1 on November 13, 2024, 07:32:01 pmYou aren't using LEDs for illumination, you are using phosphors.You need to understand the efficiency characteristics of the phosphors (and amount of phosphor) in your specific LED, LED efficiency, and the human eye response. That's not a lunchtime projectGet your LED+phosphor, use a signal generator plus transistor/resistor, and suck it and see (literally!). That is a lunchtime project.

author: Siwastaja
author_rank: Super Contributor
timestamp: November 14, 2024, 12:52:41 pm
content: Quote from: tggzzz on November 14, 2024, 11:00:18 amOr, you know, we have these things calleddatasheetswhere manufacturers list the properties of their products based on their testing and/or physical modelling, so that end users do not need to understand semiconductor and phosphor physics.Light output (in handy unit called lumen, which takes human eye response for different wavelengths into account built-in) as a function of current, as well as Vf as function of current, are available on nearly every LED (for illumination) datasheet. By multiplying these two plots you can simply calculate efficiency at different currents without deeper knowledge.

author: tggzzz
author_rank: Super Contributor
timestamp: November 14, 2024, 02:28:20 pm
content: Quote from: Siwastaja on November 14, 2024, 12:52:41 pmSimple it isn'tI refer you to TAoE3 X-Chapters...

author: Siwastaja
author_rank: Super Contributor
timestamp: November 14, 2024, 02:57:12 pm
content: Quote from: tggzzz on November 14, 2024, 02:28:20 pmYes. That is exactly why it's useful that white leds for illumination purposes are rated in Lumens by the manufacturer. Thus you can compare visual light output e.g. as a function of current, given datasheet data.

author: sairfan1
author_rank: Frequent Contributor
timestamp: November 14, 2024, 03:19:32 pm
content: I understand that for good LED life i need to take care of heat desipation and keep Amps a little under specs.How can i reduce amps, like with a general voltage regulator we can limit the volts

author: Siwastaja
author_rank: Super Contributor
timestamp: November 14, 2024, 03:35:58 pm
content: Quote from: sairfan1 on November 14, 2024, 03:19:32 pmYou use a current source (constant current supply). If this is difficult or impractical, you use voltage supply and a series resistor. In latter case the accuracy of current regulation is better the more voltage you afford wasting over the series resistor.

author: Zero999
author_rank: Super Contributor
timestamp: November 14, 2024, 06:45:22 pm
content: Is this ultimately being powered off the mains? If so, just buy an off the self constant current LED driver. Job done.

author: Faranight
author_rank: Unknown
timestamp: November 15, 2024, 06:15:35 am
content: Yeah, adjustable constant current LED driver is what you want (google "Meanwell"). It took me a while to realize and understand this concept myself. When I first got access to eBay and Aliexpress, I was initially so happy that I could buy cheap chinese LED strips to build custom lights out of them and power them by constant-voltage PSU's only to later realize how inefficient they are because they have so many series resistors on-board. Once I finally got my hands on some raw 3W LED's, a heatsink, and a constant-current power supply, I couldn't be happier.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: November 16, 2024, 01:20:47 am
content: Quote from: Faranight on November 15, 2024, 06:15:35 amYes, that's also my understanding. LED life expectancy get shortened significantly by temperature rise.On top of that, LED efficiency goes down with higher LED current. I.e. DC current will have higher efficiency and lower heating then a pulsed curren with the same RMS value.Running your 3W LED's at 2W will be swell or put two of those LEDs in the armature together.I think I like the aluminum panels with lots of COB LED's on them. But I don't have enough data to say anything about their longevity. The 10W panels I have stay quite cool upto around 6W.

author: sairfan1
author_rank: Frequent Contributor
timestamp: November 18, 2024, 10:15:48 pm
content: I have 12 volt 3 Amp adapter, I'm using 4 3W LEDs in series to run on this adaptor, in total there will be 4 LED modules in parallel each module will have 4 3W LEDs

author: Zero999
author_rank: Super Contributor
timestamp: November 19, 2024, 08:29:39 am
content: Quote from: sairfan1 on November 18, 2024, 10:15:48 pmThat's not the correct way to do this. LEDs require a constant current supply, whilst your mains adaptor has a constant voltage output.Your post is confusing.How many LEDs in total?What's the forward votlage? Normally it's around 3V.What forward current? You said 3W LEDs, so presumably it'll be 1A, but you also talk about running them at a lower power so you might want to reduce it.

author: sairfan1
author_rank: Frequent Contributor
timestamp: November 25, 2024, 06:29:01 pm
content: LED specifications are 3.2-3.6V 700mA (3W) 4500K Natural light (attached image from aliexpress listing where from i bought)Please refer to schematic that i draw for this post, there are in total 8 LEDs on one module,4 LEDs in serial Where 3.2 x 4 = 12.8v needed by one row, each module has two rows that means total 8 LEDs but in serial only 4 LEDs in each row (refer to schematic)Please refer to attached image of LED module where soldered LEDs are shown, Total 8 LEDs, 4 in each row soldered in serialHere is how am I gona use it with power adapter.1) Two modules (8 LEDs each module) 8x2 =16 LEDsPower source: 12v 3Amp adapter2) Four modules (8 LEDs each module) 8x4 = 32 LEDsPower source: 12v 3Amp adapterI know how to limit the voltage but I have to idea how can i limit the current.

author: mariush
author_rank: Super Contributor
timestamp: November 25, 2024, 07:00:13 pm
content: You need a current limited power supply, or you need to limit the current through various methods .Easiest is adding a resistor in series with the leds... you calculate the resistor like this :Input voltage - ( number of leds in series x forward voltage of single led ) = current x resistor valueSo for example with your 12v power supply, assuming 3 leds in series and a forward voltage of 3.4v and 650mA (for safety, margins etc )12v - ( 3 x 3.4v ) = 0.65A x R=> R = (12-10.2)/0.65 = 2.77 ohm - use 2.7 ohm or parallel 2 x 5.6 ohm resistorsThe power dissipated in resistor is P = Current x Current x R = 0.65 x 0.65 x 2.77 = 1.17 watts, so you'll need to use a resistor rated for 2-3 watts.The easiest for you would be to buy those to-220 / to-247 resistors and screw them to the same heatsink you're gonna screw those leds to.Those leds HAVE TO BE HEATSINKED ... a big flat piece of copper or aluminum sheet at minimum.Here's a bunch of them :https://www.digikey.com/short/9759vqqt- select your desired value from the filtersDon't parallel them like in the pictures, use one current limiting resistor or a driver on each series of 3 leds if you insist on using a 12v power supply.You can use proper led drivers and set the current to your desired value using a resistor.Example of a relatively cheap linear driver that can do at least 600ma :STCS1 (up to 1.5A) :https://www.digikey.com/en/products/detail/stmicroelectronics/STCS1PHR/1578365orhttps://www.digikey.com/en/products/detail/stmicroelectronics/STCS1APHR/1852420STCS2 (up to 2A) :https://www.digikey.com/en/products/detail/stmicroelectronics/STCS2SPR/1820076orhttps://www.digikey.com/en/products/detail/stmicroelectronics/STCS2ASPR/1852422

author: mariush
author_rank: Super Contributor
timestamp: November 25, 2024, 07:32:48 pm
content: You can also determine the forward voltage of each led at various currents using an adjustable power supply and a multimeter.For example, place a 10 ohm resistor in series with your led. When there's going to be 100mA of current going through the led and resistor, you're going to have a voltage drop across the resistor of V = I x R = 0.1A x 10 ohm = 1vGet an adjustable power supply - it can be an adjustable dc-dc converter from eBay, for example:https://www.ebay.com/itm/176096878133Or you can make your own using a linear regulator, a potentiometer and some resistors, for example LM317, and it could be powered from 5v or 4 AA batteriesSet the initial output voltage very low, like at 2v-2.5v, connect your led + resistor to the output, and put the multimeter on DC voltage range and the probes on the resistor leads.Slowly increase the output voltage and look at the multimeter until it shows 1v across the 10 ohm resistor - that tells you 100 mA goes through the circuit. Now you can move the probes to the output voltage of the dc-dc converter and you substract 1v and that's the forward voltage of the led at 100mA.You can go slowly go up to 200mA (you'll see 2v across the 10 ohm resistor) or 300mA (you'll see 3v) ... heatsink the leds if you go higher and make sure the resistor is rated for the current if you go above 300mA - the 10 ohm resistor will dissipate 0.9 watts at 300mASo this way you could bin your leds into various voltage levels ... ex use 3 leds with 3.25v forward voltage at 300mA and pick suitable resistor to limit current , and make another series of 3 leds with the ones that need 3.4v at 300mA , and adjust the resistor value to limit the current.

author: cosmicray
author_rank: Frequent Contributor
timestamp: November 25, 2024, 08:54:05 pm
content: Quote from: sairfan1 on November 14, 2024, 03:19:32 pmControlling the current per LED, is basically deciding how much current you want to run (and this may need to be experimentally determined), then calculate how many parallel circuits, and set your current regulator to I_total (parallel_circuits x current_per_circuit). As an example, the board I've been working with has 100 LEDs on it. RatedDC forward currentis 60 mA each. All are in parallel. My I_total is 109 mA, or ~1.1 mA per LED. The net total is a considerable amount of light, but very little heat. The theoretical wattage is 100 x 1w each, or 100w, but I'm only using ~2.2w total. To accomplish this I use a constant current circuit (LM317).

author: Zero999
author_rank: Super Contributor
timestamp: November 26, 2024, 08:25:42 am
content: Quote from: sairfan1 on November 25, 2024, 06:29:01 pmNo, don't use a 12V mains adaptor.Use an LED driver. Here are some suggestions. I would opt for the 350mA or 500mA model and connect all the LEDs in series.https://www.digikey.com/en/products/detail/mean-well-usa-inc/LPC-20-350/7704817https://www.digikey.com/en/products/detail/erp-power-llc/ESS020W-0500-32/7227761https://www.digikey.com/en/products/detail/mean-well-usa-inc/APC-25-500/7702575