LED Lighting (350 mA). Is there any standard for # of LEDs is a string? - Page 1
Forum section: electronics / general

author: Benta
author_rank: Super Contributor
timestamp: August 08, 2022, 08:02:18 pm
content: The title says it best.My idea is reusing laptop PSUs (normally 19 V, 90 W = 4.7 A) for lighting. The landfills are full of them, and I think it's a recycling project worth considering.But to keep it simple, 19 V would only supply 4-LED strings.Is this a normal length? Is there even some kind of standard?Thanks.

author: tom66
author_rank: Super Contributor
timestamp: August 08, 2022, 10:14:32 pm
content: Not as far as I'm aware. Most LED power supplies are constant current, with a maximum output voltage under SELV limits (~100VDC is as high as I've seen them.)If you want to drive LEDs with a laptop supply, you can, but be aware that it will require a power resistor to balance the current between each string. That resistor might well drop 5V @ 350mA = 1.75W. So not insignificant power to dissipate.

author: Benta
author_rank: Super Contributor
timestamp: August 08, 2022, 10:40:02 pm
content: Thank for your reply.No, not resistor-drop, but active regulation and splitting. I'm fully aware of how to drive LEDs.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 09, 2022, 01:57:59 am
content: Good LEDs are down around 3.0V; should be able to run at least five, preferably six at exactly 19V. Even at 3.6V, five should be easy. If they're over some distance, one less would be acceptable of course. Four may be about ideal if using a CC buck converter to drive them.Yeah, not aware of a standard either, but on the other hand, laptop supplies are surprisingly well agreed on something around 19V. Sounds like you've got your standard!Tim

author: Zero999
author_rank: Super Contributor
timestamp: August 09, 2022, 12:49:48 pm
content: Quote from: T3sl4co1l on August 09, 2022, 01:57:59 amI've previously run six old LEDs off a 19V constant voltage power supply, but they had forward voltage of more than 3V at the rated current, so were been under-run.Six modern LEDs, run at 80% of the rating, with the standard op-amp+reference+transistor+current sense resistor, is feasible off 19V.You don't need anything fancy. The crappy old LM358, a generic NPN transistor, such as the BC337 and a reference, such as the TL431, with a potential divider to make the sense voltage in the hundreds of mV.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 09, 2022, 04:09:24 pm
content: A four-transistor circuit will also do:But this needs about as many components / as much cost, and you may find the simplicity of an off-the-shelf LED regulator is worth the purchase.Ed: to expand on this a little further, it'll take about 5 components (discounting the reference: pull-up, zener/431, divider) to use the op-amp, about 10 here -- to build two channels, while making use of dual components, except the power stuff. And not much power is really needed, so a dual output transistor would be fine here too. Say '4401 BJT, or preferably low-Vce(sat) type, or NMOS as shown.I suspect the cap can be optimized out by tuning tail current (increasing R2, R3) to achieve compensation. And D1 doesn't really matter, as long as Vgs(max) < VCC, or a BJT is used.Hopefully, the same is true of the opamp case; perhaps shopping around for suitably slow, or enough phase margin, op-amps would be worthwhile here, eliminating compensation components.Heh, since supply is regulated, well, no big deal using that for reference, just a bare divider will do in this case.Foldback would be nice, to limit power dissipation in case of shorted LEDs; should be easy enough to add two resistors and call that done. May need to use SOT-89 or larger transistors to ensure adequate dissipation at worst case (say, half nominal load current at a couple volts?). Could also use a thermistor for added thermal protection, with the downside that it can't be supplied from a resistor pack (dual/quad).Tim

author: jmelson
author_rank: Super Contributor
timestamp: August 09, 2022, 04:43:02 pm
content: Quote from: Benta on August 08, 2022, 08:02:18 pmFor ~ 90 W, you'd need a boost regulator, there are tons available. I have built up strings of 20 lighting LEDs running at 300-350 mA.They run at about 65-68 V for the string. So, that's only about 20 W/ string. You could put 4 strings in parallel with individual boost regulators or just a small resistor to balance current between strings. Watch out, though, a bank of 80 1 W LEDs puts out a LOT of light. We have 3 strings of 20 and they light up our whole large kitchen quite well.Jon

author: tooki
author_rank: Super Contributor
timestamp: August 09, 2022, 05:29:00 pm
content: Quote from: Benta on August 08, 2022, 08:02:18 pmHow do you arrive at this number?!?LEDs made for lighting come in various voltages, and some of the most powerful ones do not have the highest voltages. For example, the Samsung LH181A LEDs I used in a recent project are rated from 2.82V at 350mA to at 1.5A at just 3.15V. So you could almost run 7 of them with 19V, and given that many “19V” laptop chargers are actually a bit higher (19.2-19.5V), 7 would be no problem.19V/4 is 4.75V per LED, which is far too high for almost every visible-light LED. You have to get up to UV-B LEDs to require such a high voltage. (Yes, some very high-power blue LEDs use voltages in this neighborhood, but it’s rare.)

author: bdunham7
author_rank: Super Contributor
timestamp: August 09, 2022, 05:53:42 pm
content: Quote from: Benta on August 08, 2022, 08:02:18 pmI recently made a bench light running 9 in series from a 30 V supply and in the process of making the PSU I learned a few practical things. First, the 'normal' way of doing LEDs in a standalone situation seems to be to use as long a string as practical. Second, for the white-ish LEDs that I tested, the voltage at theratedcurrent my vary a bit, perhaps 3.0 to 3.3V, but the knee of diminishing luminosity/current is in the 3.0V range. I didn't test that many so perhaps this isn't a general rule, but I didn't find a good reason to go much over 3.0V when making my lights. So you should be able to run 5 lights using an LM317 or similarly crude current regulator, six if you do a bit better.If you are reusing a laptop brick and are willing to chop it open, it might not be that hard to tap into the voltage feedback circuit and convert it to a voltage-limited constant current supply.

author: nctnico
author_rank: Super Contributor
timestamp: August 09, 2022, 06:17:37 pm
content: Quote from: Benta on August 08, 2022, 08:02:18 pmAFAIK not. From what I've seen many LED lamps made from a string of LEDs require 50V to 80V to drive. A long string reduces wiring costs and lowers current (=less fire hazzard). You'd need a constant current boost converter.

author: Benta
author_rank: Super Contributor
timestamp: August 09, 2022, 06:50:23 pm
content: OK. no standard for power LED strings. I can live with that, but it's a bit of a nuisance.I'm going completely KISS here, so no step-up converters or other things. This has to be as simple and cheap as possible.Also, when I say "LED string", I don't mean metre-long things with 100s of LEDs. I'm talking about white power LEDs for lighting.At this time, it seems the electrical optimum is 4 LEDs per string at 19 V supply. The laptop supply could then drive up to 13 strings (12 is probably a better number) that can be mounted optically separately or in groups.Each string will have its own simple controller.The simplest I've come up with is this using Osram OSLON SSL 150 whites, and it works like a dream, spot-on at 350 mA:

author: Zero999
author_rank: Super Contributor
timestamp: August 10, 2022, 10:39:18 am
content: How many strings are you planning to have per luminaire?Consider having a lower reference voltage, then you can have an extra LED or two and cut down power dissipation in the transistor.

author: mariush
author_rank: Super Contributor
timestamp: August 10, 2022, 11:01:31 am
content: You could use a basic linear driver like BCR421 which can do up to 350mA (absolute max rating somewhere around 500mA) using an external resistor (defaults to 10mA without one) :https://www.digikey.com/en/products/detail/diodes-incorporated/BCR421UFDQ-7/793077132 cents if you get 100, 20 cents if you buy 1000 ... datasheet here :https://www.diodes.com/assets/Datasheets/BCR420UFDQ-BCR421UFDQ.pdfNote that the enable pin can only handle up to 18v but it can also be used as pwm. The IC has a voltage drop of around 1v ... if you allow for 1.5v drop you're looking at 18v - 1.5v = 16.5v / 3.2v = 5 leds in series.If you go with a step-down led driver, you could have groups of 3 leds in parallel, so your step-down led driver would support a wider range of laptop adapters ex 12v ...18.5v or even only 16.5v .. 18.5v and output 8.5...10v (3 leds x 2.8v...3.3v in series)for example AL8843 :https://www.digikey.com/en/products/detail/diodes-incorporated/AL8843QSP-13/10668306it can do up to 3A , so you could safely do 8 groups of leds in parallel as 8x 0.35 = 2.8A ... so then have 3-5 leds in series, then 8 such series in parallel, for a total of 24-40 leds controlled by one driver.

author: nctnico
author_rank: Super Contributor
timestamp: August 10, 2022, 12:34:06 pm
content: LEDs strings in parallel don't work well. You'll see differences in brightness. I've seen current mismatches up to 25%. For the best results, you need a current source per string (even if it is a simple dropping resistor).

author: mariush
author_rank: Super Contributor
timestamp: August 10, 2022, 01:18:28 pm
content: You can bin the leds so that the forward voltage of every series ends up very close to the forward voltage of other series that are in parallel. Super easy to make a test fixture of some sort to measure the forward voltage and brightness at various current levels.They do this parallel thing with led strips for monitor backlights and in other places and nobody complains about differences in brightness.

author: tom66
author_rank: Super Contributor
timestamp: August 10, 2022, 02:14:24 pm
content: I don't know of many monitors that don't have independent current balancing for each parallel string, but even then the most common approach is all LEDs in series. Yes that does mean if one fails, the whole panel goes dead but hey, e-waste is not their problem.

author: Benta
author_rank: Super Contributor
timestamp: August 10, 2022, 02:59:43 pm
content: Quote from: Zero999 on August 10, 2022, 10:39:18 amNot decided yet. I'll need to experiment a bit with the subjective optical power, which also has to with materials, optics and enclosures.I though about using a lower reference voltage, but the simplicity and incredibly good current stability of the TLV431 solution led me to that choice.Also, the output of the laptop power supply is marked as pulsating DC, so I'll need a bit of headroom to be sure of correct operation.Thanks for Your inputs All.

author: Benta
author_rank: Super Contributor
timestamp: August 10, 2022, 07:57:13 pm
content: Quote from: mariush on August 10, 2022, 11:01:31 amNice little device, but not very precise. With the LEDs being 1.50 Euro a pop (=6 Euro), I'd rather be certain of my LED current.But Thanks.

author: Zero999
author_rank: Super Contributor
timestamp: August 10, 2022, 09:25:42 pm
content: Quote from: Benta on August 10, 2022, 02:59:43 pmI was thinking use the TLV431 as a reference for an op-amp. If you used a quad, such as the LM324, it would actually work out cheaper than four TLV431s.QuoteThat's odd. Does the label/manual say what frequency it's pulsating at? What does the waveform look like on an oscilloscope? I think it's likely you're mistaken. The symbol for steady DC is a solid line over a dashed line.⎓Oh and just to be pedantic, there's no such thing as pulsating DC. It's really AC with a DC bias, but I know what you meant.

author: Benta
author_rank: Super Contributor
timestamp: August 10, 2022, 10:18:32 pm
content: Quote from: Zero999 on August 10, 2022, 09:25:42 pmTo your first point: that would defeat the purpose here. The idea is to have the controllers as POL regulators, one regulator per string of four LEDs. This allows me to place the strings in any physical configuration. Consider it as a "19 V, 350 mA, 4-Led module".Imagine a "splitting box" with twelwe 19 V outputs powered from one laptop supply. The LED strings will do the rest themselves.The voltage headroom on my design also allows me more freedom in cabling (losses).Concerning the "pulsating DC": of course you're right, bad choice of words on my side, sorry. I interpreted it as "mediocrely regulated DC".-----------------------Something that might interest you (you seem to like playing with circuits).The TLV431/Darlington power BJT is dangerous, which I only found out after having serious problems with my first circuit. An emitter follower (which this circuit basically is) can oscillate with significant amplitude at >1 MHz frequencies. In a discrete circuit it's usually not a problem, but add in a TLV431 which has a lot of gain, and it gets very hairy.The TLV431/BD679 combination proved impossible to tame, which made me go back to the BD437 plus a relatively large base resistor. This is stable and works reliably. R1 as 1 kohm and R2 as 5.6 kohm works even better.You live and learnPS: (added later). Do you remember the "backlighting" thread where temperature stability was brought into the discussion?Well, look at this:

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 10, 2022, 10:31:42 pm
content: Try a resistor from Rshunt to REF, and a C or R+C from REF to A.Tim

author: Benta
author_rank: Super Contributor
timestamp: August 10, 2022, 10:46:51 pm
content: Quote from: T3sl4co1l on August 10, 2022, 10:31:42 pmSorry, but throwing capacitors at the problem does not help (I already tried all that in all permutations).I suggest you read the following (scroll down to "Advanced Amplifier Topics and Design Examples"):https://www.sciencedirect.com/topics/engineering/emitter-follower

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 10, 2022, 11:20:29 pm
content: No not the emitter follower, the TLV431.Follower stability is just, add base resistance.Tim

author: Benta
author_rank: Super Contributor
timestamp: August 10, 2022, 11:26:14 pm
content: Seems you're in stenographic/shorthand mode today. Not really intelligible.Perhaps tomorrow.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 11, 2022, 02:33:57 am
content: I mean if you want to discuss, you're welcome to, but...Quote from: Benta on August 10, 2022, 10:18:32 pmQuote from: T3sl4co1l on August 10, 2022, 10:31:42 pmYou already discovered the base resistor, that's fine.You seemed to indicate reducing TLV431 gain might be helpful......That's the way to do it.It's not clear if your "impossible" search included such. So it's worth mentioning.Perhaps a diagram is missing; I take words for granted much too often.This for example stabilizes a '431 into a C load:You already have the R3, more or less. R2 is missing. C1 can be R+C for additional phase margin.Some damping (R+C) at the output (C-E or C-GND) can also help, when the load is squirrelly. LEDs wouldn't usually be long enough I think, but a high fT transistor might be able to anyway.I could further describe how to solve for such values, but I'm guessing that isn't necessary here.Not sure how much more detail to add. Emitter ferrite bead?Tim

author: Zero999
author_rank: Super Contributor
timestamp: August 11, 2022, 07:34:42 am
content: Quote from: Benta on August 10, 2022, 10:18:32 pmIf you're set on strings for four LEDs, then why use the TLV431? The LM317 would do and still have plenty of headroom for cable losses.I'd say 19V is a bit on the high side, for strings of four + a linear regulator. Have you considered hacking the power supplies? It's highly likely changing a resistor will get the voltage down to between 14V and 15V, which still gives enough headroom for a linear regulator.

author: mariush
author_rank: Super Contributor
timestamp: August 11, 2022, 08:13:29 am
content: Quote from: Benta on August 10, 2022, 07:57:13 pmThe built in sense resistor of around 95 ohm (for the 10mA current limit) will be less precise, and will allow for +/- 10% as the datasheet says, so 9..11 mA.You parallel an external resistor with that built in resistor, for example on Page 11 , Figure 7... it shows 2.1 ohm used for 350mA current.So you could choose a good quality low ppm resistor to get let's say 320-340mA of current, then even with all the variation of the internal 95 ohm resistor and external resistor due to temperature change, the current in the series won't go over 350mA of current.Basically... I'd look at a 2.2 ohm or a 2.4 ohm resistor ... for examplehttps://www.digikey.com/en/products/detail/koa-speer-electronics-inc/SR733ATTE2R40F/9845654Or you could even have two resistor footprints by the driver chip, and for example parallel two 4.7 ohm resistors (common value, mass produced, pennies, and less heat per resistor)example :https://www.digikey.com/en/products/detail/vishay-dale/CRCW08054R70FKEAHP/2226710Keep in mind that the TL431 will also drift with temperature, as will resistors and capacitors in same circuit, and if you have these on same PCB with the LEDs the pcb will probably be heated up to 60+ degrees CelsiusSo account for that when you think other solution will be more precise at keeping current at 350mA.

author: nctnico
author_rank: Super Contributor
timestamp: August 11, 2022, 11:03:22 am
content: Quote from: mariush on August 10, 2022, 01:18:28 pmThat is way too much hassle. Typically you buy the LEDs from the manufacturer binned for color & brightness but you still have to expect differences large enough that make parallelling several strings problematic.QuoteThey likely have a way to deal with proper current sharing otherwise you will see difference in brightness in the panel. Been there, done that.

author: Zero999
author_rank: Super Contributor
timestamp: August 11, 2022, 07:59:34 pm
content: A few transistors can be used to make a switching regulator with current limiting, which can be used to drive a string of LEDs. The zener diode can be removed, or you might want to keep it and increase it to a higher voltage, such as 15V,https://www.romanblack.com/smps/a04.htm

author: Benta
author_rank: Super Contributor
timestamp: August 11, 2022, 08:41:40 pm
content: Quote from: mariush on August 11, 2022, 08:13:29 amI think you missed my replly #19 and the LED current plot.I'm really not worried about TLV431 temperature drift. Even if if was 10 times worse, it's still good.

author: Kjelt
author_rank: Super Contributor
timestamp: August 11, 2022, 09:09:32 pm
content: Quote from: Benta on August 08, 2022, 08:02:18 pmAFAIK only difference is between LV (upto I believe 50VDC but could be a bit higher) and HV. HV you need more isolation larger distance between traces etc.And there is a difference in driver being mains isolated or not. Latter needs extra safety.Quote from: nctnico on August 10, 2022, 12:34:06 pmI disagree, I have seen many ledlight pcbs from A brands and they use 3 to 4 parallel strings even without resistor. I can take pictures for you, from pro lighting fixtures for parking garages costing €150 a piece to kitchen lighting. The trick is to use A brand leds from the same bin. For our hobby use buy a reel and use those they are also from the same bin.The delta in forward Voltage drop is less than 0,1%. But when one led dies or is damaged then you have to replace the pcb in total ofcourse (average 25000 hours).

author: Benta
author_rank: Super Contributor
timestamp: August 11, 2022, 09:22:40 pm
content: Quote from: Kjelt on August 11, 2022, 09:09:32 pmSource/documentation?

author: nctnico
author_rank: Super Contributor
timestamp: August 11, 2022, 09:48:01 pm
content: Quote from: Kjelt on August 11, 2022, 09:09:32 pmThat may be, but my experience is different (also with monitor backlights). When looking at white LED datasheets from Osram, they specify a variation in forward voltage of about 1% within a bin. It is possible that the lamps you have seen, have the PCB traces act as a dropping resistor just in case. LEDs from the same batch may have a much closer match but I doubt a manufacturer is going to bin LEDs within a 0.1% match.

author: Benta
author_rank: Super Contributor
timestamp: August 12, 2022, 12:17:00 am
content: Thanks to a lot of helpful inputs and feedback here, I think I've now arrived at my final circuit. Some adjustments/tuning were indicated.For a 19 V input, I've settled on a 5-LED string. This leads to a power dissipation in the current regulator circuit of around 1.5...2 W, which is acceptable to me.LED current is set at 340...350 mA (pretty standard for power LEDs).Neither voltage nor temperature stability are an issue, I attach plots of the LED current for anyone interested.Cost is around 60 cents for the the controller plus 7.50 Euro for the LEDs. This is not a cheap China product, but uses OSRAM quality LEDs.Feel free to copy/modify the design as you like. (KiCAD design file (.zip) is also attached).Cheers.

author: mariush
author_rank: Super Contributor
timestamp: August 12, 2022, 06:21:42 am
content: I don't know where you get that 1.5 euro per led from ... I can find Osram Oslon 150 degree leds at under $0.5 in quantity (600+)You have the CM series with CRI 90+ , the EM one with CRI 80+ and the PM with CRI 70+The better CM ones are 1$ each for 10 on Digikey, 0.7$ for 100 :https://www.digikey.com/en/products/detail/ams-osram-usa-inc/GW-CSHPM1-CM-KSKU-XX58-1-350-R18/6599016Same price for the EM ones (CRI 80+) :https://www.digikey.com/en/products/detail/ams-osram-usa-inc/GW-CSHPM1-EM-LPLR-XX58-1-350-R18/6599021Sure add VAT and shipping but we're still at much less than 1.5 euro per led.Anyway, unless those osram are better binned or your need those lenses on the leds ... these Samsung leds seem to be just as good for much lower price, at 15 cents if you buy 100 :https://www.digikey.com/en/products/detail/samsung-semiconductor-inc/SCS9WT93HPL2WLS03F/6569400Still CRI 90, but it's 5.9v -ish forward voltage, 150mA current ...

author: Kjelt
author_rank: Super Contributor
timestamp: August 12, 2022, 07:17:16 am
content: Quote from: Benta on August 11, 2022, 09:22:40 pmTestreports of reels of leds.With the 3.xx V types you will have max ten mV's differences on leds from the same reel ( ofcourse when in series with the same current).If you get different reels , different production dates your mileage may vary.

author: Kjelt
author_rank: Super Contributor
timestamp: August 12, 2022, 07:27:03 am
content: Quote from: nctnico on August 11, 2022, 09:48:01 pmYou always keep a safe margin with your specs or you get many returns.So lets say it is 0,05% in reality that is ten mV's difference in Vfd.If it is evenly distributed you have no problem. If you are really worried measure each led and put it in different new bins with tighter spec. But as I said even A brands don't bother.QuoteThat could be but the traces are pretty wide. I will do a measurement on them again, measuring each led for Vfd and the voltage between the leds (trace resistance).

author: Kjelt
author_rank: Super Contributor
timestamp: August 12, 2022, 07:46:58 am
content: I can't find this back in the datasheets. Most manufacturers do Lm and CRI binning.Only a few manufacturers do Vfd bins it seems.

author: nctnico
author_rank: Super Contributor
timestamp: August 12, 2022, 10:58:08 am
content: With very large batches you can do your own verification and gamble it but if the parts start to vary wider for some reason, you have no claim at all.A fellow engineer once told me their company needed 1% capacitors for a product. Because the 1% capacitors where expensive, they bought 5% capacitors and tested them theirselves. The out of spec capacitors where used elsewhere so nothing got lost. After some time they started to receive batches of capacitors where all capacitors where outside 1%. Turns out the manufacturer had started checking the capacitors themselves to sell as 1% tolerance.Closer to home: I have been bitten as well by components that got worse. At some point a batch of a products started to show strange behaviour. Turned out the decoupling capacitors where not as good as they used to be. Same part number and still in spec but simply made worse.

author: Zero999
author_rank: Super Contributor
timestamp: August 12, 2022, 05:29:34 pm
content: Here's what I was talking about earlier. It requires just under 1V of headroom.It depends on your requirements. Does it actually matter if the current falls a bit below the set point? You'll probably not notice if it's 300mA, rather than 350mA.I suggest you measure the voltage of the power supply and the forward voltage of the LED strings at different currents.

author: deadlylover
author_rank: Frequent Contributor
timestamp: August 12, 2022, 06:46:19 pm
content: Which LEDs are you using? Were they the OSLON square on eBay binned below the BBL? (like this 3500k) Those are an excellent choice.I've always wanted to try them but I have been a Nichia fan lately. (it helps that Cutter Electronics in Australia is kinda local to me, so I just use their strips for DIY projects)A while back I upgraded my room/desk lighting using the Nichia Optisolis 757 series. Museum grade lighting because I'm a bit of a colour pencil hobbyist too.

author: Benta
author_rank: Super Contributor
timestamp: August 12, 2022, 07:58:04 pm
content: Quote from: Zero999 on August 12, 2022, 05:29:34 pmOK, so right now I have three resistors,* a BD437 and a TLV431. (*) Ok one is actually three to hit the value and spread power.And now I need the same PLUS an LM358 (plus an additional dropper resistor for the reference). Not really a simplification to my mind.

author: Zero999
author_rank: Super Contributor
timestamp: August 12, 2022, 08:13:00 pm
content: Quote from: Benta on August 12, 2022, 07:58:04 pmIt wasn't supposed to be simpler, but more efficient. Now the transistor can be cheaper and doesn't even need a heatsink. A cheaper TL431 can also be used if the values of R2 & R2 are changed to 160k & 12k.It depends on how many strings of LEDs you want per luminaire. If you want to use two or more strings, then my circuit requires fewer components. For another string all that's needed is an extra transistor and current sense resistor, then there's the LM324, if you need three or four strings.

author: Benta
author_rank: Super Contributor
timestamp: August 12, 2022, 09:37:52 pm
content: Quote from: Zero999 on August 12, 2022, 08:13:00 pmIn a perfect world, yes. But (just as an example) the OSLON voltage binning (K2...M2) spans 2.75...3.2 V. So the BC337 will either run nice and cool or be in flames, depending on LED bin availability.Second, and I've said this before: the idea is to have the 4/5/6-LED strings running autonomously at 19 V for free placement.

author: Zero999
author_rank: Super Contributor
timestamp: August 13, 2022, 09:31:13 am
content: Quote from: Benta on August 12, 2022, 09:37:52 pmYou have many LEDs in series, so the variation in forward voltage drops will cancel each other out. You won't have six 2.75V LEDs, or 3.2V LEDs, but an even distribution of forward voltages, giving a total forward voltage of about 3V.As I said before, you should get hold of the real components and measure the voltages at different currents. It's probably also a good idea to breadboard it before finalising the circuit.I strongly recommend running the LEDs at less than their ratings. It doesn't reduce the brightness much, but greatly increases the efficiency and lifetime.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 13, 2022, 03:56:54 pm
content: I don't mind running at ratings, personally; as long as they're good parts, that's what really matters. That, and thermal management. Which, it sounds like they're the pricey kind, so they ought to perform well (high lm/W).Last pricey ones I got were, let's see... Cree XTEAW-00-0000-00000HGE3. Far smaller than the old gull-wing-cylinder style they replaced, but their efficiency is so much greater, they still run cooler. It's... a little insane. And quite wonderful.So, if it's already designed to run an acceptable temperature with older chips, or, like, resistors (i.e., assuming 100% applied power goes to heat -- in reality it's more like 50% now!), they'll run quite cool. If it's new design, and you're accounting for dissipated power only, or don't have much opportunity to dissipate heat in the first place (compact, enclosed?), yeah, derating may be desirable.Tim

author: Zero999
author_rank: Super Contributor
timestamp: August 13, 2022, 06:02:56 pm
content: Quote from: T3sl4co1l on August 13, 2022, 03:56:54 pmI'm just going from my experience of commercial LED bulbs, which generally run the LEDs at their ratings and invariably don't last as long as they should. Then there's the Dubai lamp which is more efficient and lasts for much longer too.If budget isn't an issue, the LT3080 can be used as a current sink with a low drop-out voltage.

author: nctnico
author_rank: Super Contributor
timestamp: August 13, 2022, 06:30:59 pm
content: Quote from: T3sl4co1l on August 13, 2022, 03:56:54 pmThermal management is easy to underestimate. Even 60mW LEDs need like 4x4mm pads in order to have sufficient heatsinking in real life applications. Otherwise they will (not can, but will!) fail. A LED is a semiconductor so their operational temperature is rather low compared to passives like a resistor. It is quite similar to power transistors. Once you've got them mounted onto a heatsink and apply derating for elevated environment temperatures, you often realise you can only dissipate 25% of what the transistor is rated for.

author: Benta
author_rank: Super Contributor
timestamp: August 13, 2022, 07:00:22 pm
content: Quote from: Zero999 on August 13, 2022, 09:31:13 amI have an engineering rule that I always stick to: never do a basic design based on probabilities, typical values or wishful thinking.You can always downgrade afterwards based on experience.And thinking that a reel of parts exhibits a nice Gaussian distribution is wishful thinking. You never know if a really big customer has asked the manufacturer to select parts with a certain VF. This happens much more often than you think (don't ask me how I know, I've been in the semiconductor industry for 40+ years). The rest is sold on the open market, but the Gaussian distribution is gone, although all parts confirm to specs.

author: Benta
author_rank: Super Contributor
timestamp: August 13, 2022, 07:04:22 pm
content: Quote from: nctnico on August 13, 2022, 06:30:59 pmVery true, but in this case quite easy: total PDis 19 x 0.35 A = 6.65 W. Not insurmountable. The idea is to mount the lights/LEDs/regulator on an aluminium baseplate anyway. Shape TBD.

author: Zero999
author_rank: Super Contributor
timestamp: August 13, 2022, 07:08:55 pm
content: Quote from: Benta on August 13, 2022, 07:00:22 pmThat's exactly why I keep saying do some actual measurements, before finalising the design.

author: Benta
author_rank: Super Contributor
timestamp: August 13, 2022, 07:20:07 pm
content: And why I'm saying do worst-case/best-case or minimum/maximum spec analysis first. Measurements will not cure the "wishful thinking" part. Calculations will.

author: nctnico
author_rank: Super Contributor
timestamp: August 13, 2022, 08:45:14 pm
content: Quote from: Benta on August 13, 2022, 07:04:22 pmTo be sure: I'm writing about 60 milli-Watt = 0.06W!

author: Benta
author_rank: Super Contributor
timestamp: August 13, 2022, 09:03:43 pm
content: Quote from: nctnico on August 13, 2022, 08:45:14 pmI understand fully what you're writing.But I've designed 150 W Class-AB amps and 20 W Class-A amps. Thermal management of 6.7 W is easy.You keep on harping on this. Why? A standard 0.3 W resistor won't even get a little warm at 0.06 W.

author: Zero999
author_rank: Super Contributor
timestamp: August 13, 2022, 09:09:35 pm
content: Quote from: Benta on August 13, 2022, 07:20:07 pmCalculations putting the worst case into everything will often result in over-engineering, which might be acceptable in safety critical applications, but otherwise counterproductive.

author: nctnico
author_rank: Super Contributor
timestamp: August 13, 2022, 09:17:09 pm
content: Quote from: Benta on August 13, 2022, 09:03:43 pmBecause some LEDs will fail without proper heatsinking even at power levels that look like you don't need to care about at all. You can't compare a resistor with a ceramic substrate with a LED that is mostly plastic. All in all: it is not 'harping on', it is sharing practical experience. But my remark about thermal design was aimed at T3sl4co1l. I got the impression that you missed the m for milli.

author: Benta
author_rank: Super Contributor
timestamp: August 13, 2022, 09:41:46 pm
content: Quote from: Zero999 on August 13, 2022, 09:09:35 pmSomehow, this is evolving a bit into a Pub discussion, which I like... but we should rather meet there! won't rub your nose in British car manufacturing that used your approach vs. "German over engineering". It's history.You take a Pint, I'll take a Mass.Cheers.

author: Zero999
author_rank: Super Contributor
timestamp: August 13, 2022, 10:07:06 pm
content: Quote from: Benta on August 13, 2022, 09:41:46 pmGerman over-engineering didn't win the war.How many of these are you going to make? Your original post says you're using second hand power supplies. I doubt it needs to meet stringent quality assurance. It's definitely feasible to just measure the forward voltages.

author: Kjelt
author_rank: Super Contributor
timestamp: August 13, 2022, 11:19:25 pm
content: Quote from: Benta on August 13, 2022, 09:41:46 pmUhum the most reliable cars remain the japanese cars, some german cars being very unreliable. Just google it

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 14, 2022, 12:41:41 am
content: Quote from: nctnico on August 13, 2022, 09:17:09 pmIndeed, a recent project I had to use 0402 white and green LEDs for indication, and had no room for even much trace width to keep them cool. They ended up multiplexed at fairly modest intensity though (partly for matching intensity with the much dimmer red LEDs, partly didn't need full intensity anyway) so it's probably fine. 4-layer board as well, but that really isn't going to help as much as you'd want when the component width is barely more than the laminate thickness (well, something like that anyways; would be neat to get some measurements to see by how much!).Tim