Imac A1312 - Blown Diode twice in LED inverter Board!!

[fzabkar]

I would think that the R240 resistors would sense the current.

[james_s]

BTW, you can edit an existing post to add more to it instead of posting 4 or 5 times in a row, it keeps things more consolidated and easier to read.

[aqibi2000]

Swap LED strip number 1 with another position (ensuring they have the same number of LEDs)

Then see if D1 still gets hot to the touch....

[reessi]

Hi there, I do not see what difference this can make since all three diodes have their outputs connected together and then that goes to the leds and all the positive inputs are connected together! The diodes do not drive seperate strings.

Also 3 of the strings are all on one circuit board,
There are 2 circuit boards of three on each, left and right half of the screen along the bottom only, you cannot seperate the individual stings and where they plug to the connecter they are all soldered together.

I have ordered a new inverter board now as my friend needs his Imac back, if that one overheats then we know it the LEDS! But I do not see how they can as the inverter is current limiting, or supposed to be!!

Cheers.

[reessi]

Well the new Inverter gets hot as well but takes longer to get there! Can it be the LEDS but I do not understand how as the inverter is supposed to be current limiting, so it should not be possible to overload it just like on a bench power supply with current limiting??

Does anyone one know if these inverters get hot anyway? I do not have it in the case at the moment so maybe should be cooled inside with the fans? Still the old one does the same but quicker, I do not want to give it back and the same diode fails on this new inverter!!

Any ideas? Cheers.

[james_s]

How hot is it getting? I would expect the hotter parts to reach maybe 60C in still air. If it's running hotter than that then I would be concerned.

[aqibi2000]

LEDs gets warm when operating at the specific current rating, testing with a DVM is not sufficient.

The VF could have changed at the differing voltage level == over driving the SMPS?

[fzabkar]

This is the inverter for the same Imac at ifixit.com (you can see the pinouts):



This Amazon listing identifies compatible inverters using the same Apple APP003SN chip:

https://www.amazon.co.uk/TONELON-605-3035-661-5311-V267-601HF-Backlight/dp/B01AZDJFZM

TONELON 605-3035 661-5311 612-0075 V267-601 V267-604 HF V267-601HF Genuine LED Backlight Board for M 27" A1312

Searching for PDFs locates a potential circuit diagram, but the sites looked spammy, so I didn't try.

Edit: Removed links to potential scam sites.

[reessi]

I'll investigate those links.

So I stuck a temperature sensor I have on the replacement board next to the inductors and it was up to 71c degrees after an hour on full brightness...

So I'm guessing it has to be the LEDS?

[fzabkar]

Those spammy URLs are suspicious. I can't believe that "3.8M people liked" a Wordpress inverter schematic. Nor do I understand why there is only an invitation to "register", but nowhere for "registered" users to login. I'm wondering whether it's a credit card mining scam.


Edit: I thought it best to remove the links.

[reessi]

I checked and also believe they are a scam, I recommend that you remove those links so nobody trys to sign up.

So now I moved the temperature sensor from the LCD panel and stuck it to the back of the Inverter and it reads 95C Degrees!!! So the probe I was using wasn't accurate.

No wonder they blow the diode. I am convinced now I need to replace the LEDs but can anyone explain what is happening?

Surly the inverter is current limiting and it sets the voltage! I see that it changes the voltages for the brightness. It is my understanding the leds internal resistance changes with voltage hence you can control brightness with voltage. Can you also control brightness with current limiting?

Under what fault condition are the LEDS to overload the inverter? I can only think that their internal resistance has lowered as they have got old but still this would not matter if the inverter was current limiting!

By the way I spent extensive time looking for schematics before and datasheets for the chips on the board to no avail.

I can buy only refurbished LED strips from Aliexpress! What about me replacing the LEDS, I'm sure can find them on RS electronics or ebay but how will I know the specs of them? I will have look tomorrow for the LCD panel datasheet!

Cheers.

[Rasz]

Its Apple, 95C might be its intended operational temperature.

[fzabkar]

AFAICT the output power to the LED strips should be about 25W maximum. If anything, the inverter seems much too big and complex for the job.

FWIW, here are the specs for an LG LM270WQ1-SLC2 panel that was used in an Imac A1312:

https://www.avnet-integrated.eu/fileadmin/user_upload/Files/Displays/Colour_TFT/LM270WQ1-SLC2.pdf

It uses 4 LED strings with a maximum current of 125mA per string. The max power consumption of each string is 57V x 125mA = 7W.

"The LED Bar consists of 68 LED packages, 4 strings (parallel) x 17 packages (serial)."

[james_s]

I don't think 71C is all THAT unreasonable. It's warm enough that it would be nice to find someone with a similar system to find out how how theirs runs but it's certainly well within the range that semiconductors can tolerate. By the time it's been on for an hour I would expect the temperature to have stabilized.

[reessi]

james_s -> actually it is running at 95 to 98c degrees as the first probe I have which is just a little temperature board and sensor off ebay was not accurrate. I moved the Imacs sensor from the LCD to the inverter and got over 95!

Rasz -> Indeed ! But the inverter cannot take this abuse!!

fzabkar -> actually in the Imacs for some reason they have a different version LM270WQ1- (SD) (E3): - Note the digits in brackets!

It has 6 LED strings running @ average 36v each, the inverter is rated at input 12v * 7.2a Maximum = 86.4w.

Maximum voltage output it is rated at is 42.3v.

I have confirmed this from the wiring and I've counted the LEDS, there is LEFT and RIGHT bar. Each bar has 36 LEDS so TOTAL 72 LEDS total in the Panel.

So there are 3 strings per BAR.

Specs from Datasheet are:

36v per string of 12 so 3v per LED.
12.6w per string of 12 so 1.05w per LED.
So 350mA per LED.

So I am going to look for LEDS on RS electronics.

What I would like to know is how can the LEDS be tested? I already checked each on on diode mode with the meter and they all lit dimmly, I'll check again if they show different readings. They all light up as normal when out of the panel!!

DO I need to replace them all? But also if I only replace a few maybe they are different Lumen output and different colour temperature?

I've attached the right datasheet for these panels.

I wonder why there are panels with different backlight strings on the same model panel!!

I still do not understand why they are causing the Inverters to overheat?

I should point out that when the LEDS are on the Inverter is usually outputing 41 or 42v and the datasheet is saying typical 36v! Is this a clue that the LEDS are failing?
It is like the inverter is running a MAXIMUM all the time!! Hence why it gets so hot and after as week fails!


I just looked on RS and it is difficult to see an exact match. There are not any LEDS at 350mA and 3v and I've no idea what lumen output these are!

[fzabkar]

I realise that the 4-string arrangement is different for the LM270WQ1 panel, but since it is for an Imac A1312, it stands to reason that the power specs for its backlight should be similar to the 6-string version.

Here is the inverter for the 4-string setup:
https://www.sergeraln.com/index.php?main_page=product_info&products_id=117050

Notice that it is rated for 80W, even though the LEDs draw only 28W max.

"LCD Inverter Board Constant Current Source Booster 80W For LM270WQ1 12P Output"

Your panel has 72 LEDs, whereas the other has 68. Therefore I expect that the LED current should be the same.

In short, I believe that your backlight draws 28W, not 84W, and that the LED current is more like 120mA.

[james_s]

Measure the voltage across each string of LEDs, if they are all the same within a percent or two then you can be pretty confident that the LEDs are fine. There are two failure modes I've seen with backlight LEDs, the most common is that one or more LEDs will short, causing the voltage of that segment to be reduced. The other is that an LED goes open causing that whole string to go dead. There is really nothing other than shorted LEDs that could cause the inverter to overheat, it will be a current source so up to a point it should deal with a higher or lower voltage across the string. Many will shut down if the detect a significant discrepancy though.

95C is certainly toasty, not impossible that it was designed to run that hot but that's getting up there. Is the housing set up to benefit from the airflow of a fan, or thermal conduction to the case? It's possible that it runs hotter in open air than it does when it's all put together. Are you sure the inverter you got is genuine and not a low quality aftermarket part that might run hotter?

[reessi]

In short, I believe that your backlight draws 28W, not 84W, and that the LED current is more like 120mA:

In the datasheet for my panel it is saying 350mA, in the datasheet you posted it states 125ma Can it be that apple wanted a brighter panel?

james_s:


I'll do some more testing this evening. The inverter is a used inverter I bought, it is a later version but still overheats the same. The temperature measured is when it was inside as the LCD temperature sensor would not reach outside the unit! So the fans seamingly do not help. I have already cleaned all the fans in this IMAC. I also put a fan on it from an old graphics card and that did barely anything but there is not room for the airflow behind the inverter with a small fan....


It cannot be normal as the back of the Imac is red hot where the inverter is. The only thing left is the LEDS if not the inverter!

I have ordered some LEDS for this panel for £16, since I cannot find the correct replacements individually and need to try to resolve this soon.

Both the inverters run at 95 to 98c!

I thought it should shut down as well and I also thought like you that they're either short or open, but all of them light! So I am totally confused! It does shut down when there are no LEDS plugged in!

[fzabkar]

Do you have something like this?

https://ae01.alicdn.com/kf/HTB10ZxTau6sK1RjSsrbq6xbDXXaJ/HiDANCE-compteurs-de-courant-alternatif-220v-wattm-tre-num-rique-compteur-d-nergie-ue-watt-moniteur.jpg

They cost about AU$20.

If so, then compare the power consumption of the Imac with inverter connected and disconnected.

[james_s]

Apologies if you already mentioned this but is the input voltage correct? If the input voltage is wrong (or has a ton of ripple due to a bad capacitor somewhere) that could cause the inverter to overheat. The only other thing I can think of is a short from the LEDs to the chassis somewhere, or the inverter is being instructed to drive the LEDs harder than it should be. Does the display look unusually bright? Many LCD displays pulse the backlight in sync with the frame clock of the panel. I suppose it's possible that it's stuck on all the time when it's intended to be pulsed. Or maybe there is a common fault with the inverter and the used one you bought happens to have the same fault as the one you had. The thing that would really help here is if you were able to get your hands on another similar model computer to compare to and try your inverters in that known good machine.

[aqibi2000]

Fixed this now?

[reessi]

No but I've got a new set of LEDS from china. Hopefully have time to test this week and then I'll update the results. I didn't see your message because this forum said I cannot use my normal hotmail email account.

Cheers

[reessi]

I forgot to update, this was fixed by replacing one of the led rows! After that the diode did not blow in the inverter!

Cheers