Salvaged LEDs for TV , what power ? - Page 1
Forum section: electronics / projects

author: KD0CAC John
author_rank: Frequent Contributor
timestamp: April 01, 2015, 02:52:01 pm
content: I saw 3 TVs by the dumpster , so thought ok with 3 of them I'l bite .One was a LED TV with LEDs mounted on aluminum strip , I just assumed that they would run on 12vdc , hooked up to my bench supply , set the volts on 12 and from 0 started to turn the amp pot .No light , doggie dew .These could be handy , but do not light , so maybe I am assuming wrong , what voltage / power ?ThanksJohn

author: Isad
author_rank: Frequent Contributor
timestamp: April 01, 2015, 02:56:19 pm
content: Quote from: KD0CAC John on April 01, 2015, 02:52:01 pmU will be surprised that those thing actualy runon more then 50V if i am correct.I foudn a led strip inan old tablet they wouldnt run on 12v but 50v.

author: xrunner
author_rank: Super Contributor
timestamp: April 01, 2015, 02:57:12 pm
content: Well I don't know specifically about the strip you have, but if they are all in series 12V won't cut the mustard. Depends on how many there are ...

author: georges80
author_rank: Frequent Contributor
timestamp: April 01, 2015, 03:12:34 pm
content: Assuming they are white LEDs, their Vf will be in the 3 to 3.5V range EACH. Likely all in series, so count the number of LEDs and multiply by the Vf.The amount of power (actually constant current x Vf) will depend on the type of LEDs and how good the heatsink/thermal path is.cheers,george.

author: DanielS
author_rank: Frequent Contributor
timestamp: April 01, 2015, 03:17:39 pm
content: Blue and White LEDs do not light much below 3V, so if you have 10+ LEDs in series on those strips, you can expect to have little to no light until you hit 30V or more. This is not surprising at all since driving a single series LED strip with moderately high voltage is a much simpler way of achieving fairly uniform brightness than driving them in a series-parallel mix with shunt resistors.Make sure you have the polarity right (you can verify this by setting your PSU to 3.5V with 5mA limit and test the first LED on the strip) then leave the current limit to 5mA and start cranking the voltage up until the strip lights up.

author: KD0CAC John
author_rank: Frequent Contributor
timestamp: April 01, 2015, 10:18:07 pm
content: Oh well , will not use as I thought .Once I took a closer look and use my magnifier , it says 90v

author: amyk
author_rank: Super Contributor
timestamp: April 01, 2015, 10:22:59 pm
content: If you're referring to "94V-0", that'sa flammability rating, not the voltage... remember that LEDs are constant-current devices so it's not unusual to require a high voltage but low current. Don't try driving these long series chains with constant voltage supply/current-limiting resistors since the total Vf varies much more as they warm up.

author: georges80
author_rank: Frequent Contributor
timestamp: April 01, 2015, 11:31:55 pm
content: Again, HOW many LEDs are on the strip??As stated 94V-0 has nothing to do with the operating voltage.cheers,george.

author: KD0CAC John
author_rank: Frequent Contributor
timestamp: April 02, 2015, 01:56:19 am
content: 2 sections one has 9 LEDs , and the other has 10 , then when mounted , there are 2 sections plugged together , so a total of 19 per row and 6 rows in the TV .

author: georges80
author_rank: Frequent Contributor
timestamp: April 02, 2015, 02:09:54 am
content: White LEDs are around 3 - 3.5V Vf EACH, so if you know how many are actually in series per string you'll have a good idea of the total operating voltage of each string.cheers,george.

author: DanielS
author_rank: Frequent Contributor
timestamp: April 02, 2015, 03:53:27 am
content: Quote from: KD0CAC John on April 02, 2015, 01:56:19 amAt just over 3V per LED, that would mean you need about 60V per strip, assuming the two sections are in series.

author: KD0CAC John
author_rank: Frequent Contributor
timestamp: April 02, 2015, 01:49:48 pm
content: That brings up another lack of info on my end , I have some LED tape that has about 3 LEDs per inch , about 4 ft. and I am running that with a wall wart that outputs 12v @500ma .Part of the reason that I figured I should be able to run these strip out of the TV , so it would seem some thing missing if you say that each LED needs 3-3.5 v ?

author: DanielS
author_rank: Frequent Contributor
timestamp: April 02, 2015, 02:02:22 pm
content: Quote from: KD0CAC John on April 02, 2015, 01:49:48 pmIf you are confused about the concept of series connections, you may want to review fundamental electronics.If you have 19 LEDs connected in series and each one has a forward voltage of just over 3V, you need 19 x 3V or just about 60V before getting any significant light out of them since there won't be much current going through them until then.Your 12V LED strip likely has has a pattern of three LEDs in series (3 x 3.xV = about 10V) and a current-limiting resistor repeating every inch. That's a series-parallel array.The electrical connections are not arranged the same way.

author: georges80
author_rank: Frequent Contributor
timestamp: April 02, 2015, 03:24:54 pm
content: Quote from: KD0CAC John on April 02, 2015, 01:49:48 pmAt has been mentioned a few times. It's not just the total # of LEDs, it is HOW are they wired on the strip/board etc...That's why series/parallel and words like string have been mentioned. Until you determine how the LEDs are wired you can't calculate the total string voltage.You'll notice your 12V strips are design to be 'cut' apart every 'segment', so for white LEDs there will be 3 series LEDs plus a current limiting resistor PER segment. They route the strip to carry 0V and 12V along the strip and distribute it to each 'segment'. Essentially you ave a series/parallel wire strip (3 LEDs in series + resistor PER segment and then each segment in parallel).cheers,george.

author: KD0CAC John
author_rank: Frequent Contributor
timestamp: April 02, 2015, 03:51:34 pm
content: OK finally getting something going , I know about series / parallel , but did not bother to look , most of the time I have been at coffee shop when I posting here , so not able to look & test .So just now at home , and looking for indications of how theses are connected [ to keep storage handling easy , I have been using just one half of strip , there length ] , so now looking at the surface to see traces under white paint , and then start thinking that I may need to connect the 2nd half of strip to complete the circuit , hook up to variable power supply .And the findings are the 1st noticeable light happens at about 42.2v with current set at min.And brightness stops at 49.4v , current set at min.Then max brightness at 59.9v , current at 200ma .Then with voltage at 59.9 , they start to dim at 100ma .Glad I chose to get a 0-60v , 0-2a power supply , some time ago , thinking may end up playing with tele-com gear - 48v .

author: KD0CAC John
author_rank: Frequent Contributor
timestamp: April 02, 2015, 03:53:53 pm
content: Just to clarify , no response the 1st time connecting to PS , with just half a strip .Results came when connecting the 2nd segment - completing the circuit .