Correctly wiring and powering 10+ 3W LEDs? - Page 1
Forum section: electronics / beginners

author: ghawkins
author_rank: Newbie
timestamp: July 14, 2021, 05:41:02 pm
content: I've seen no end of projects on the web where people wire up large numbers of high Wattage LEDs (i.e. 3W or more) and invariably someone in the project's comments section points out that the design has many serious issues.So I want to know what is the _right_ way to wire up e.g. twenty 3W LEDs?What are the common issues people point out with the designs that I've seen:1. Wiring - they wire up the complete sequence of e.g. twenty LEDs in parallel - see more discussion below.2. Current - the power source is a constantvoltagesource when what LEDs need is a constantcurrentsource.3. Power source - they use a source that's completely incapable of continuous delivery of the required total Wattage.4. Heat dissipation - they make no effort to dissipate all the heat being generated by the LEDs.I see a lot of contradictory information out there, in particular about points 1 and 2.Some people seem to feel that running long sequences of LEDs in parallel is perfectly fine while others insist you should split your LEDs into strings of e.g. 5 (which you run inseries) and then run the strings inparallel(so for 20 LEDs, you'd end up with 4 strings being run in parallel).Other people then point out that running LEDs in series requires a constant current source, i.e. a standard 12V 5A power supply is out, as a constant voltage source will "will allow an LED that goes into thermal runaway to pull as much current as it can, resulting in rapid failure."So is the best solution a constant current LED driver and strings of LEDs run in series and these strings then run in parallel?If strings are the way to go then, as I understand it, I have to total up the voltages of the LEDs in the string.Looking at the datasheet for e.g. this product from Sparkfun -3W white LED- I see the voltage listed as 3.2 to 3.8V, i.e. a range. Which value should I choose when totaling up?Say (for the moment), I take 3.8V as my value then for a string of five LEDs, the voltage is 5 x 3.8V = 19V.OK - but the LED drivers, I've seen, are either 12V or 24V. So how do I deal with this discrepancy, do I add enough LEDs to the string to get just above the 24V or do I have to adjust down the voltage somehow?Sorry - I'm confused by the idea of a constant current source so I'm unsure about what approach to take.Then, as I understand it, the constant current should be the forward current of an individual LED (e.g. 750mA) multiplied by the number of strings, e.g. 4 giving 3A.So it seems I would want a 24V 3A LED driver. LED drivers seem to be sold according to their Wattage, so I want a 72W driver.I can't get a driver that's exactly 72W - so do I have to go lower, e.g. 60W, and provide less current than the setup could consume for maximum brightness? Avoiding providing too much current seems the whole point of these drivers to I would hardly choose a slightly higher Wattage, e.g. 80W.Sorry for such a long question - I wanted to show, I had thought about things and tried to work things thru rather than just asking a one-liner "how do I do this?!?".

author: BeBuLamar
author_rank: Super Contributor
timestamp: July 14, 2021, 06:43:14 pm
content: If you want 5 LED in series you can use the 24V driver. The extra voltage will have to dissipate as heat if the driver is linear and not switching. I would use 3 drivers and wire them in parallel rather than 1 driver that deliver 9 amps.

author: ledtester
author_rank: Super Contributor
timestamp: July 14, 2021, 06:58:09 pm
content: I'm not sure it's such a great idea to drive multiple strings in parallel with a CC driver. If one string fails its current will be forced onto the other strings. Have a look at the end of this article:https://www.ledsupply.com/blog/wiring-leds-correctly-series-parallel-circuits-explained/QuoteCan you provide links to these drivers?As for a driver capable of driving a single string of all your LEDs, have a look at the second page of this product sheet for the MeanWell ELG-150-C series:https://www.mouser.com/datasheet/2/260/ELG_150_C_SPEC-1109884.pdfThe ELG-150-C1400 has enough power and will allow you to run between 14 and 28 of your 3.8V LEDs. And the -A version has an external pot which allows you to control the current between 700 and 1400 mA or get the -B /-AB version if you want to control current electronically.

author: ghawkins
author_rank: Newbie
timestamp: July 14, 2021, 08:14:31 pm
content: ledtester- I must admit, I'd been looking at drivers liketheseon AliExpress. It seems I've misunderstood things and these are only suitable for LED strips designed to take either 12V or 24V. I don't know if these are actually any different (in any meaningful way) to a standard 12V or 24V power brick.I had also looked at MeanWell drivers but it's only now, looking at the datasheet that you linked to, that I think I understand how this is supposed to work. So I should look for a device that can provide the current I'm interested in e.g. 700mA and can deliver the wattage needed to drive the number of LEDs that I have and that's it.So you wouldn't bother splitting things into strings or anything complex like I mentioned, you'd just run all 20 LEDs in series?If it's as simple as that I wonder why so many projects (e.g. this the two listed below and endless more on YouTube and elsewhere) run their LEDs in parallel. I guess (like me) they're confused and want to use a plain old voltage source, that they have knocking around, rather than getting a LED driver.BeBuLamar- thanks for your reply. My impression from ledtester's reply is that it's simpler just to run all the LEDs in series if I use a suitable constant current source. Or do you think there are still good reasons to consider using multiple 24V source?Cost might be one? The MeanWell driver that ledtester mentions is quite expensive. But I'm assuming you still mean a current limiting 24V source, i.e. one that provides ~750mA?When you say "I would use 3 drivers and wire them in parallel" do you mean wire them independently, i.e. power three completely independent sequences of LEDs in series with three independent LED drivers? Or do you mean something else - if you mean wire them in parallel in the normal circuit layout sense, I'm not sure I understand why this would be a good idea?

author: ghawkins
author_rank: Newbie
timestamp: July 14, 2021, 08:18:13 pm
content: Sorry - I completely messed up the URLs in the last message. Corrected now.

author: mariush
author_rank: Super Contributor
timestamp: July 14, 2021, 09:04:07 pm
content: The Sparksfun page says :Forward Voltage: 3.2-3.8VForward Current: 750mAViewing angle: 125±5 DegreesLuminous Intensity: 160-240LMTemperature Color: 6000-7000KNow if you do the math, 3.8v x 750 mA = 2850 mW or 2.850 watts, not 3 watts.You decide on a power supply ... you could have 12v, 18v (a laptop adapter), you could have 24v ... the higher the voltage the lower the current you're gonna need, and the lower the losses in the wires.You can connect leds in series and that group of leds will require a higher voltage to light up, but the current remains the same.So for example:* 3 of these leds in series = 3 x 3.8v = 11.4v* 4 of these leds in series = 4 x 3.8v = 15.2v* 5 of these leds in series = 5 x 3.8v = 19.0v* 6 of these leds in series = 6 x 3.8v = 22.8vNow you need to decide on a driver... and you can either use linear driver or a more advanced driver.A linear driver can be placed in series with a bunch of leds and simply acts like a resistor or a diode, adjusting the voltage drop across it so that the current never goes above your set limit.Typically, such linear led driver requires the input voltage to be at least around 1v above the sum of forward voltages of the leds.A more advanced led driver can be similar to a switching regulator, using an inductor and a diode and pwm to send a bunch of pulses of electricity to get a specific voltage on the output and the desired amount of current.In your particular case, as you have 20 leds, a simple basic approach would be to make groups of 4 or 5 leds and have 5 or 4 groups in parallel connected to the power supply (18-19v or 24v)If all you need is limiting the current and you don't need fancy brightness adjustment and so on, then you could use basic linear drivers.An example of such driver is DLD101-7 :https://www.digikey.com/en/products/detail/diodes-incorporated/DLD101-7/2217122See application circuit at page 7 in datasheet:https://www.diodes.com/assets/Datasheets/ds32007.pdfIt supports up to 50v input voltage so you could even power your strip with a 48v power supply.So with a 18-19v laptop adapter power supply, you'd have 5 such driver chips, one for each group of 4 leds. The leds would need around 15-16v, so there's a couple volts to be dissipated in the chip.With a 24v power supply, you could have 24 leds , and make 4 groups of 6 leds in series.Other such linear driver, maybe in a more beginner friendly footprint but note maximum 700mA current:https://www.digikey.com/en/products/detail/taiwan-semiconductor-corporation/TS19601CP5-ROG/7359748

author: ledtester
author_rank: Super Contributor
timestamp: July 14, 2021, 09:25:31 pm
content: Quote from: ghawkins on July 14, 2021, 08:14:31 pmYes, those supplies are meant for the pre-manufactured LED strips. The 12V strips have a resistor for every group of 3 and the 24V strips one for every group of 6 LEDs - or something like that. They are probably just constant voltage supplies.In the first video:https://youtu.be/xsd5b9p19nMis an ok way to do things because:1. he's adjusting the voltage so the LEDs don't get too hot2. the "voltage booster" module is inherently current limited3. the battery pack itself also has current limitsIndeed, those XL6009 boost converters generally have very poor performance. This video shows that one struggles to boost 12V to 24V at even just 1A:https://youtu.be/IFiupEP_vB8The second video looks like it was just a demonstration to see how much light they could get out of the 15 LEDs. I wouldn't use that as a permanent set up.