What Should Be the Voltages Reading on the Led Strip for Samsung 55inch Smart Tv ?

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Question Samsung 48" Ability Supply and LED Problems.

• Thread starter HMark
• Start date Oct 25, 2021

Oct 25, 2021

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• #one

Samsung 48" PS and LED problems
Hi: I'g new to the forum merely not to electronics. I'm a retired RF engineer with over 50 years in broadband telecom and video delivery. I took in a 48" Samsung Television set (recent model) to see if I could repair it for a friend. No picture or sound. The red power LED light blinked 6 times when attempting to power up manually. According to Samsung, this indicated a switch model power supply problem. Upon checking the voltages on the SMPS board, all appeared to be normal except for the voltage fed to the LED backlighting. Given the number of LED's in series the voltage should be around 85vdc merely was well over 200vdc! The LED cord was open (tested using a variable DC power supply), then I 'causeless' the PS board went bad and burned open up ane or several LED's. I ordered new LED's plus a replacement SMPS board from shopjimmy.com. I've replaced the LED'due south (3 of the viii strips had at to the lowest degree i open up LED) and have the new PS installed, but am hesitating to connect the LED cord, as the voltage on the new board besides measures over several hundred volts dc, and am afraid information technology might be lacking and I'll burn out the new LEDs. Meanwhile, I still go an error lawmaking and the TV however won't plough on. It now blinks v times vs the original 6, but both codes indicate a PS problem. My questions are: (ane) should the Idiot box turn on with the LED'south disconnected or tin can it sense they are not working? - and (2) is the high voltage for the LED strip feed normal (is this a current limiting circuit that will reduce the voltage and electric current to the proper values once I connect them)? Whatever suggestions or thoughts are appreciated.

Oct 25, 2021

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I'm posting an update to my own mail service. I'k now reasonably sure that the switch mode power supply (SMPS), when working properly, provides current limiting to power the LED strips. Just to be sure, all the same, I've calculated that the entire string (at minimum LED firing voltage) should require 85.2 volts and draw .026 amperes. Using Ohms law, I can work from in that location to determine at minimum LED firing voltage the LED string has 3.3 yard ohms (equivalent) resistance and would dissipate 2.2 watts. I've ordered a wire-wound three.three k ohm resistor with a five watt rating. I can then substitute the resistor for the LED cord and burn down everything upward while monitoring the voltage and current at the resistor. If everything measures as it should, I'll cross my fingers and plug in the LED string. Also, the SMPS does indeed monitor the LED circuit voltage and electric current, and will shutdown the SMPS if parameters are not within a proper range, so that I recall explains why the Television receiver nevertheless won't plough on. If I had more than experience with these modern TV's I wouldn't be and so cautious. I'll post one time more when I'chiliad finished with this thing, one way or the other.

Sep 6, 2020

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• #three

When a backlight LED opens upwardly, the voltage does rise some. You nigh likely need to supersede the LED backlight strips. Never just replace one, every bit this is a big delicate task, and the whole TV needs to come apart to fifty-fifty
test the LED strips. It'due south non a job you desire to practice again shortly afterwards replacing one strip, plus they lose intensity over time, and might exist a little unevenly lit if all aren't done. I am reasonably certain you will find an open. I accept recently had a lot of bad backlight strips as the trouble, and so know first-hand. You must be SUPER cautious not to interruption the very thin LCD screen.
Talk to me if yous want before getting too far in.

Oct 25, 2021

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• #iv

Hi, thanks for your answer and suggestions. I haven't repaired a television since the mid 1970'due south and they have obviously changed a slap-up deal since then. I'm attempting to repair a 48" Samsung for a friend, recently widowed, who can't afford a new one. I do some repair work now that I'g retired, but I stick mostly with RF test equipment; spectrum analyzers in particular.

Let me effort to summarize what I've done thus far.

1. Initial troubleshooting with a LED tester indicated 1 or several of the LED strings (4 rows of 8 LED's) were open up.
2. The voltage on the SMPS (switch mode power supply) feeding the LED string was at approx. 230 VDC, much college than the 130 max VDC stamped on the lath.
3. I take removed the old LEDs; and take installed and tested new LEDs. My LED tester shows the new entire series string just turning on at 85 VDC at effectually 85 ma.
4. I ordered a replacement ability supply board as I suspected the erstwhile SMPS might exist bad; yet the replacement lath shows the same approx. voltage (approx. 230 VDC) as the original WITH THE NEW LED Cord Nevertheless Non CONNECTED.
5. Testing and test of the onetime LED's showed multiple LED'southward burned out, including one location where 4 bulbs in a row had arrondi over, indicating some sort of voltage surge?
6. I'm at the point where the next logical step would exist to connect the new LED string to meet if the Boob tube is functional, but have this dubiousness that I'm missing something and I'll burn down up the new LED's. That would mean another $50 on LED'southward, not to mention disassembling the LCD assembly once again.

My question is: "is there some manner to simulate the LED cord 'load' on the SMPS to make it think they are continued, and then that I tin confirm the supply is working properly earlier actually plugging the new LED string in?" Or am I only being too cautious?

Thanks in accelerate for any advice you can offer.

Oct 25, 2021

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• #v

I had a typo in my response only now. The new LED string just fires (minimum brightness) at 85 VDC, merely at 26 milliamperes, not 85 ma.

Sep 6, 2020

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• #6

I,one thousand glad yous antiseptic as it changes my response completely. FYI, I am an Electrical Engineer, so you need not spell out SMPS for me, nosotros tin can talk techie all day.
I was thinking that because these power supplies are more current than voltage limiting that that may be the reason for the abnormally high voltage, but that just didn't seem right from my feel.
Yous must accept a bad LED power supply. For a exam load, I would use large carbon resistors, as the wire-wounds that would be handier might add too much inductance, leading you astray on the problem. And the LED'southward are being strobed, another reason to go with a non inductive load.
For ability (Yep you know this) each led is at most 3V * 50mA (Lets be conservative on the power draw for the SMPS) So .15W * your number of LED'south) . As apparently the number of LED'south vary from Tv to Television set. Y'all know you voltage, 85Volts? At present make your load to fit. So go out your O-Telescopic, as I'1000 betting you don't have a clean voltage source. Caps are by far the highest failure part in a SMPS, so look at those electrolytics first. Next the driver for the switcher(due south). They are usually used in 2 or more banks for the sake of meliorate moving picture clarity. And so you may need to determine the number of loads you demand. The concluding few TV'southward I've replaced LED'due south on, had ii banks. The principal board determines how the LED'southward strobe, frequently with a 5V strobe signal. It is even so a single power supply that drives the banks. Y'all may want to consider buying a new power supply board, as they unremarkably are pretty low toll, or if you lot don't discover a relatively easy problem to repair.

Squeamish of y'all to practise this for a friend BTW.

Sep 6, 2020

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• #7

If you do replace SMPS caps, make certain they are Depression ESR caps, or they will last five seconds to 5 days, I recommend Panasonic FM series caps, which you lot tin can get from Digikey. ( in the .0XX of an ohm range)
For a P.S. board replacement, Shop jimmy has earned a dandy reputation, and volition help yous make sure you go the right version board. Their prices are skillful as well.
Beats taking your chances on EBay in my opinion.

Additional thought, find a vivid prototype, and intermission information technology, and you see switching dissonance on top of the voltage under load, the problem is probably the big LED output filter caps. Compare to the voltage to the input of the switcher circuit, if the output caps are bad, it will still be pretty noisy, but meliorate % -wise than the output. Without load, you should see a more stark deviation on the output vs input.
Question: does your meter read True RMS?

Oct 25, 2021

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• #eight

Thanks for your responses. I've been nether the weather condition (cold) since concluding Friday and oasis't worked on the Toshiba flat-screen once again until this forenoon.

Yes, 1 of my meters measures true RMS. I oasis't measured switching dissonance on the unloaded output of the power supply nevertheless, simply I should practice that adjacent. I accept a fairly skillful 200 MHz scope and a differential probe for making accurate switching noise measurements.

Your comment about carbon vs wire-wound resistors got me to thinking more nigh the apparent impedance of a LED, where the voltage vs electric current relationship is not-linear. I took some new measures of the new LED strings this morning. Ane set of readings was at minimum voltage (merely enough to light the bulbs) and a second set of readings at close to maximum brightness. There'southward quite a spread between them when because the required carbon resistor to simulate the unabridged load and the wattage rating needed.

Minimum voltage to each LED yields around .072 watts per LED and an estimated total load resistance of 3,175 ohms at 2.3 watts.
Maximum voltage (as high as I was willing to accept the LED brightness) to each LED yields approx .473 watts per LED and an estimated total load resistance of 531 ohms at 15.24 watts!

I've ordered a 5w carbon resistor assortment and will try several different load resistor combinations when it arrives. Meanwhile I'll employ my telescopic to measure switching dissonance.

I have the growing feeling that the power supply is probably working fine and I'm but being too cautious in trying the new LED's. Don't really want to encounter them get up in smoke.

Cheers again!

Sep 6, 2020

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• #9

HMARK,
Your welcome, and it'due south squeamish to talk with someone who knows their stuff for a alter of pace!
I think you you are just playing it safety, a prudent thing to do, only if you simply have a relatively well thought
out depression intensity range of the LED power, and the power supply comes downwards to a approximate 3V per LED level,
I think information technology would be safe to connect your LED's and relish your repaired Idiot box.
Hey, I go accused all the fourth dimension of over-thinking things, and so as far as I'm concerned, better well thought out than
being a Bozo.
Hope you lot get better fast.
-Beauford
(online proper name or I'd change information technology)

October 25, 2021

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• #10

Thought I should type a last mail to close out this thread. To epitomize, I took in a 48" Samsung with no picture show or sound for a friend. Subsequently attempting a hard reset with the external ability switch, the LED light would blink six times indicating a switch mode ability supply (SMPS) problem.

After some initial troubleshooting I determined the LED string (one series string with 32 LED'southward) was open in at to the lowest degree 2 sections (each row of 8 LED's looped dorsum to the back plug at the SMPS, and so on to the next row). I ordered all new LED sections, so carefully (I've never done this earlier) disassembled the front LCD assembly , replaced the LED'southward and reassembled (after testing them to brand sure they worked). When testing the SMPS I became more than and more convinced that information technology was defective every bit well; at least in the LED current-limiting circuitry. So I ordered a replacement SMPS and installed it. The Telly is now upwards and running!

As a final comment, there are ways to 'simulate' the LED strings using a calculated value value carbon resistor (5W minimum); however I'm convinced no resistive load tin ever accurately' mimic' the extremely non-linear current to voltage relationship institute in a LED string. The best you lot can hope for is to make up one's mind the SMPS is providing some level of current limiting at something close to the desired voltage.

My special thank you to Beauford Savage who provided lots of spot-on advice and encouragement!

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Source: https://forums.tomsguide.com/threads/samsung-48-power-supply-and-led-issues.493286/

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