Can I get away with not using a heat sink/attaching to a pcb? - Page 1
Forum section: electronics / projects

author: Dr.Rob
author_rank: Newbie
timestamp: June 07, 2017, 07:35:37 am
content: Morning all,First time poster so I hope this alright/ in the right place.I'm looking at using some UV-LEDs for a project and the cost of a SMT LED is considerably cheaper than the a through hole of similar wavelength (£20 compared to £116).I have found two that look interesting (a) LG LEUVA66B00HF00 (b)VLMU1610-365-135Datasheets for each are here:(a)http://docs-europe.electrocomponents.com/webdocs/13f5/0900766b813f56fa.pdf(b)http://docs-europe.electrocomponents.com/webdocs/156f/0900766b8156f4eb.pdfKey facts from datasheet(a) Vf Typ. = 6.5 V, If = 20 mA, Rth j-s @ 20 mA = 37 degC/W(b) Vf Typ. = 3.5 V, If = 20 mA, RthJC = 53 K/WSo back to title, as this is a prototype can I can get away with soldering wires onto the pads to prove proof of concept with a 275 ohm and 75 ohm resistor respectively. Initially they will be ran in air, eventually moving to a box.CheersRob

author: Berni
author_rank: Super Contributor
timestamp: June 07, 2017, 07:46:22 am
content: Yep its perfectly fine to solder a wire to it and light it up. However you will most definitely not be able to run them at full power.To reliably work at full power these need a PCB with tones of vias around to take the heat away.

author: Fungus
author_rank: Super Contributor
timestamp: June 07, 2017, 08:46:46 am
content: Quote from: Dr.Rob on June 07, 2017, 07:35:37 amSoldering to a PCB is probably OK, so long as you have plenty of copper fill to move heat around (plus VIAs going to the back of the board and more copper fill there).The problem is putting them in a box. Anything enclosed in a box can overheat. You may need a fan to move the air around.(you don't say how many LEDs you're going to put in it, how tightly they'll be packed, etc. so it's impossible to give more than general advice)

author: Dr.Rob
author_rank: Newbie
timestamp: June 07, 2017, 08:58:24 am
content: Quote from: Fungus on June 07, 2017, 08:46:46 amI had hoped not to use a PCB at all at this stage, I think that a single LED of either type would be enough for this application.CheersRob

author: kalel
author_rank: Frequent Contributor
timestamp: June 07, 2017, 09:33:12 am
content: Quote from: Dr.Rob on June 07, 2017, 08:58:24 amI have done some quick tests (just with hand, not any temperature meters) of a supposedly 0.5w (150 mA 3.3v) warm white SMD LED from China on open air in room (this one costs only a few cents so it's easier to experiment with)It didn't seem to heat up at all noticeably at 30 mA at ~3v, also not much until around 40 mA (at around 3.1v), anything over that and you could feel the heat quickly.So, 20 to 30 mA might be fine on a similar spec LED, but then again I don't know if it will heat up after several hours, or when in an enclosure.I guess you could test it yourself by running it on a current or voltage regulated supply (I don't have a proper current regulated supply so I can only alter the voltage and monitor the current with a cheapometer), and slowly increase the voltage/current and measure the temperature yourself, either with a probe or just using your hand.You might not be able to do this easily in an enclosure, in which case a probe would be better.If it doesn't get hot, it shouldn't become damaged easily. Once you have decided on a voltage/current, you can leave it running for a few hours or more (depending on how long you intend to use it normally) and monitor the temperature. If it's stable and not too hot for the expected standard usage time, you should be fine.If not (gets too hot), of course you should turn it off before it gets damaged and you could consider buying some piece of aluminum heatsink that can fit inside your case.

author: dmills
author_rank: Super Contributor
timestamp: June 07, 2017, 10:22:18 am
content: You are aware that those two diodes are at very different wavelengths?Regards, Dan.

author: Dr.Rob
author_rank: Newbie
timestamp: June 07, 2017, 10:34:02 am
content: Yeah, that's a good thing. (b) was a lot cheaper (£1.20) so less issues if I destroy it. I didn't look at the dimension though (1.6 mm x 1.6 mm) when I ordered it.That is 7-0.2 cable.Rob

author: MagicSmoker
author_rank: Super Contributor
timestamp: June 07, 2017, 11:59:27 am
content: Quote from: dmills on June 07, 2017, 10:22:18 amQuote from: Dr.Rob on June 07, 2017, 10:34:02 amYou seem to have missed dmills point: LED (a) emits at 278nm, which is at the long end of the UVC range, while LED (b) emits at ~365nm, which falls in the middle of the UVA range.For many applications requiring UV irradiation - e.g., curing UV epoxy, disinfecting surfaces, exposing photoresist, etc. - you won't get equally good results with UVA and UVC; one or the other will be preferred or even necessary.

author: Dr.Rob
author_rank: Newbie
timestamp: June 07, 2017, 12:10:33 pm
content: Don't worry, the application requires a variety of wavelengths so the fact that these two are so different is a good thing.CheersRobert

author: Fungus
author_rank: Super Contributor
timestamp: June 07, 2017, 01:56:02 pm
content: Quote from: Dr.Rob on June 07, 2017, 08:58:24 amIn that case, no worries!Use thick wire and that will be more than enough to absorb the tiny amount of heat you'll get from a 20mA LED.

author: tooki
author_rank: Super Contributor
timestamp: June 07, 2017, 02:41:40 pm
content: Quote from: Dr.Rob on June 07, 2017, 08:58:24 amHeat aside, using a PCB (even a small bit of perfboard) is far superior mechanically.