High power UV LED array - Page 1
Forum section: electronics / projects

author: adam440
author_rank: Newbie
timestamp: August 06, 2018, 02:09:27 pm
content: Hello,I’m designing SLA 3D printer and I need to design UV LED array for LCD screen backlight. Preliminary design is 5x3 (5 parallel lines with three LEDs in each). All LEDs will be soldered on PCB.The catch is that every LED will have 1W and my inner engineer cries out for active regulation, even though I understand, that passive regulation (via resistors) would be much simpler. I’m just high school student with very little knowledge about electronics, so could you point me to some solutions?I’m going to solder the PCB myself and I’m not working on the project full-time, so I would definitely prefer something simpler (maybe regulation via MOSFETs?). I don’t need complicated dimming of individual LEDs via IC in BGA package or something like that.LED parameters:voltage: 3.2-3.6Vcurrent: 300-350 mAfurther info:https://www.aliexpress.com/item/1W-3W-5W-Warm-White-Royal-Blue-Orange-UV-Violet-RGB-High-Power-LED-Chip-Light/32804097246.htmlI’m sorry for any grammar mistakes, I’m not a native speaker.Best regards,Adam Schuppler

author: Zero999
author_rank: Super Contributor
timestamp: August 06, 2018, 02:53:43 pm
content: There are plenty of constant current regulator ICs available.And why do you want UV for an LCD back light? Presumably there will be a phosphor on the screen, to convert it to a visible wavelength.It's possible the phosphor won't absorb enough of the UV so make it safe, so a filter may be required over the screen to protect the user's eyes from the UV radiation.

author: senso
author_rank: Frequent Contributor
timestamp: August 06, 2018, 03:07:44 pm
content: I think he wants to use an lcd instead of the more usual DLP projector used in SLA printers, no phosphor needed, the UV is what cures the resin.

author: Zero999
author_rank: Super Contributor
timestamp: August 06, 2018, 04:01:26 pm
content: Quote from: senso on August 06, 2018, 03:07:44 pmThat's what I initially thought, until he mentioned using UV LEDs for the LCD screen back-lightQuote from: adam440 on August 06, 2018, 02:09:27 pmUnless a phosphor is used, an LCD back-light is completely the wrong application for UV LEDs.

author: adam440
author_rank: Newbie
timestamp: August 07, 2018, 10:38:28 am
content: It's new but already proven design. Look at these examples:http://www.anycubic3d.com/products/show/1022.htmlhttps://plus.google.com/communities/109609533907401814291I'm going to use 410 nm LEDs. I've already tested the display with single 30W 410nm LED, and it's working well. But for better homogenity of the light, simpler cooling and more compact size, LED array is more appropriate option. I've already asked on other places and they all seem to point me to constant current LED drivers (sold on eBay and Aliexpress). However if one of the parallel lines will be shorted by faulty LED, the rest of the LED parallel lines will have more current (the same current won't be divided across 5 lines, but 4). This could damage the LEDs. Slightly better solution would be to use resistors, but they dissipate a lot of heat (2.2 ohm resistor would dissipate 1.1W of heat). And the best solution would be to use something like LM334 on each parallel line. Am I right?

author: Siwastaja
author_rank: Super Contributor
timestamp: August 07, 2018, 03:03:51 pm
content: Put as many leds in series as you can, they share current perfectly.Use off-the-shelf LED driver IC which works in boost mode.Boost rations up to about 4-5x are sensible. Voltages over around 100V are not safe. Voltages between about 50-100V start reducing the selection of ICs to choose from.So, if the number of LEDs is high enough so that you'd end up with voltages impractically high when all in series, divide them into a few chains. For each chain, replicate the driver, i.e., use a completely dedicated IC + inductor + capacitors.But for 15 LEDs times 3.6V = 54V, a single chain should be well achievable, starting from 12V input, but if you have a bit more available (like a 19V laptop brick), go for it.Ebay modules may or may not work. The selection is poor to choose from. You may need to roll your own PCB. OTOH, deadbugging a switcher on a piece of unetched copper clad is simple and quick and works for leaded ICs so you can bend the legs.A lot of constant current switch mode driver ICs, really. Most only require an inductor, possibly a switching diode, input capacitors, and possibly output capacitors.https://www.digikey.com/products/en/integrated-circuits-ics/pmic-led-drivers/745?k=&pkeyword=&pv494=5&pv494=93&pv494=270&pv494=646&pv494=673&pv494=697&pv494=650&pv494=24&pv494=663&pv494=702&pv494=703&pv494=56&pv494=649&pv494=696&pv494=806&pv494=695&pv494=750&pv494=664&pv494=678&pv494=809&pv494=764&pv494=804&pv494=653&FV=11280006%2C1cd4000a%2C1cd4000b%2C1cd4000e%2C1cd4000f%2C1cd40010%2C1cd40012%2C1cd40013%2C1cd40002%2C1cd40014%2C1cd40018%2C1cd40019%2C1cd4001a%2C1cd4001e%2C1cd40021%2C1cd40024%2C1cd4002a%2C1cd40032%2C1cd4003b%2C1cd4003d%2C1cd40007%2C1cd4004d%2C1cd40008%2C1cd40009%2Cffe002e9&quantity=0&ColumnSort=1000011&page=1&stock=1&nstock=1&pageSize=25Filters:In stock, normally stockingBoost typeInput max voltage at least 16VOutput current at least 350mA

author: Zero999
author_rank: Super Contributor
timestamp: August 08, 2018, 12:56:17 pm
content: Quote from: adam440 on August 07, 2018, 10:38:28 amThose appear to be using violet for curing the resin, not for the LCD back-light.QuoteThen that's not UV, but visible violet, which is a bit safer than UV, but still not good for the eyes.https://en.wikipedia.org/wiki/Visible_spectrumhttps://en.wikipedia.org/wiki/Violet_(color)QuoteWhy not use purple LEDs? Which are blue LEDs, with a red phosphor. They will be safer, much more efficient and give off less heat, for the same apparent brightness as violet LEDs. Another alternative is a mixture of blue and red LEDs, with blue being double the radiant power of red, but mixing the colours can be challenging.

author: DJohn
author_rank: Regular Contributor
timestamp: August 09, 2018, 12:44:38 pm
content: Quote from: Hero999 on August 08, 2018, 12:56:17 pmYou seem to be assuming that the LCD is being used as a human-visible display. There may or may not be one of those in an SLA printer, but the one in consideration here is not intended to be seen.You have a tank of light-sensitive resin sitting on top of a source of light of the wavelength it is sensitive to. In between you put an LCD, to control which bits of resin get exposed. The model gets printed an entire slice at a time. An LCD with a UV backlight is exactly what you want.

author: Zero999
author_rank: Super Contributor
timestamp: August 09, 2018, 01:15:51 pm
content: Quote from: DJohn on August 09, 2018, 12:44:38 pmThank you for explaining that to me. It was not obvious from the original post or the links to the machines.Then it's not an LCD, which is a Liquid Crystal Display, because it's not performing the display function, but acting as a printhead. Of course you don't want UV, i.e. <380nm, but visible violet, since many plastics are pretty opaque to UV and can also be damaged by it, hence why 405nm is a popular wavelength. The websites calling it UV, is marketing bunk.

author: adam440
author_rank: Newbie
timestamp: August 10, 2018, 12:06:54 pm
content: Thanks for replies.I'm sorry for late response. I'm probably going to design custom PCB and I'm deciding between two ICs, LM317 and LM334. I could use LM317 in constant current mode, but I'm afraid I'd have to use heatsink. Or I could use LM334 in following way (figure 3):http://www.bristolwatch.com/ccs/Which one is better?@Hero999: I'm sorry for confusion. In 3D printing community a lot of people calls it UV, but now I see it's misleading. Anyway, 405nm wavelength is used mainly because it still goes through the liquid crystals (through the blue filter), but also cures the resin. For maximum efficiency you'd want something like 380nm or so (every resin is bit different), but commercialy available LCDs aren't transparent for such wavelenghts. LCDs also detoriate rather quickly when subjected to high intensity sub 400nm light (at least from anecdotal evidence). The printed parts then need to be post-cured after printing in UV light oven (also called curing station).

author: Zero999
author_rank: Super Contributor
timestamp: August 10, 2018, 01:12:09 pm
content: Quote from: adam440 on August 10, 2018, 12:06:54 pmThe LM317 is the better of the two circuits. The LM338 design is bad practice because it relies on the Hfe of the transistor which varies with the temperature and is a poorly controlled parameter: it varies widely even across parts with the same number and from the same manufacturer.Both are linear power supplies which aren't very efficient. The LM317 will need about 4V of voltage headroom to work properly.Why not use a switched mode power supply? There are plenty of off the shelf switched mode LED drivers available.What power supply voltage is available? Is the mains supply available? If so, there are plenty of ready made LED drivers which can drive 15x3.6V LEDs in series at 350mA, at high efficiency. They safely convert mains voltages to a constant current of 350mA and limit the open circuit voltage to under 60V for safety reasons.Here's an example, which is available from RS Components, Farnell and plenty of other distributors.https://www.sunpower-uk.com/wp-content/uploads/sites/42/2018/03/PCC-18TD-Series.pdfAnd another one:https://www.saxbylighting.com/products/product-detail/46896-led-driver-constant-currentIf it needs to run off DC, then you're best bet is a buck-boost converter, such as the ones linked below. Unfortunately their maximum output voltage is 40V, so you'll need two.https://www.mouser.com/ds/2/468/RBD-12-958434.pdfhttps://www.jameco.com/Jameco/Products/ProdDS/2219621.pdfThere might be off the shelf low voltage DC LED drivers which will power all 15 LED in series at 350mA, but I didn't search for that long. I've seen some buck converter modules, but they need 60VDC to get the output voltage you need.QuoteThat makes sense. So ideally you need UV to cure the resin, but it's incompatible with liquid crystals, so violet needs to be used instead.