PWM lighting 120hz to dc - Page 1
Forum section: electronics / projects

author: Frederico Acardi
author_rank: Contributor
timestamp: June 04, 2021, 11:05:27 am
content: Hello, I have bought a TV it's been 3 months and I have neck pain and eye strain when I look at it for even a short time (like a movie 2hours).Started looking on the forums and I was told my TV (like most cheap LG/samsung TV) uses a very low PWM led back lighting of 120Hz.I don't know anything about tv but : Can I just add a PWM to DC converter to the leds and have DC lighting ?Will I still be able to dim the back light with the remote ?Where should I put this converter ? are leds strips on TV powered by a simple 2 wires PWM signal ? What voltage ?I can't return it guys and my neck pain are getting so bad I don't use this TV anymore (most of the TV is the 400€ uses very low pwm frequencies).Thank you.MY TV is an LG 43UN73

author: NiHaoMike
author_rank: Super Contributor
timestamp: June 04, 2021, 01:16:16 pm
content: That's a feature to reduce motion blur. There might be settings to adjust it.https://blurbusters.com/faq/motion-blur-reductionYou might want to look at TVs that advertise 240Hz or more. That's assuming the motion blur reduction is the actual problem and not something like the brightness set too low or too high or bad positioning of the display.

author: Simon
author_rank: Unknown
timestamp: June 04, 2021, 01:19:26 pm
content: I don't know much about how the LED control of a TV works, your first problem is to find where you can break in between the PWM supply and the LED's.It may be possible to treat the switched output as the positive switch of a SMPS and add a diode and LC filter to produce a very rough SMPS but this is a very theoretical solution as I have no idea of the practicalities. The frequency is rather low for a SMPS.Typically LED'S are driven from a SMPS running as a current source to avoid large series resistors and be more efficient. So the SMPS is regulating the voltage drop on a small current sense resistor instead of the output voltage with some maximum voltage available.

author: Frederico Acardi
author_rank: Contributor
timestamp: June 04, 2021, 02:00:36 pm
content: Thank you very much NiHaoMike, it might be why they choose 120hz to reduce motion blur from the cheap panel response they use on entry model like mine.But I looked everywhere and it seems it can't be disabled and it the default PWM on this model, see here :https://www.rtings.com/tv/reviews/lg/un7300Thank you also Simon, I found a video on youtube of an LG screen tear down and it seems the use simple positive and negative wire (2 pole) to feed the leds, maybe I can just open the screen and place PWMtoDC module between the Power supply and the leds ?EDIT :IT LOOKS LIKE 4 WIRES ?EDIT :IT'S4 WIRESYES, and the leds are separated in 2 rows (2wires to LedROW1 and 2 other wires to LedROW2)Can you check this video at 3min and then the other side at 9min :

author: Simon
author_rank: Unknown
timestamp: June 04, 2021, 02:22:00 pm
content: I'm not aware of any power converters that convert from PWM to steady voltage based on the duty. Designing one is not too difficult of course. Ideally you use a diode in the positive line followed by a capacitor. This allows you to still sample the PWM duty and use it to define the reference voltage to control the SMPS output voltage as the constant voltage only exists after the capacitor. So using the PWM you control a SMPS that is working at a higher frequency than the PWM frequency. a SMPS at 120Hz is impractical, it will be huge and very inefficient as the inductor will be so huge that you may as well just use a resistor as the resistive losses in such a large inductor will be so high.

author: Frederico Acardi
author_rank: Contributor
timestamp: June 04, 2021, 02:34:12 pm
content: Thank you I don't care about having the same brightness as before. Even if the brightness is lowered (lower DC voltage as a result of the conversion) I am ok with it I just need it to be steady.Do you think It's doable, or should I just give up ?I will check ebay if there is cheap modules doing it as I am not wanting to design from scratch. I too don't think an SMPS would be the solution too...If everything else fail I will just use a 12/19 VDC power brick, and wire the wire to the leds.

author: Simon
author_rank: Unknown
timestamp: June 04, 2021, 02:50:34 pm
content: converting to a voltage will also change the brightness control as the minimum voltage to get the LED's to light must be produced and may happen at a lower voltage than the minimum duty. Because under PWM the voltage was always that of the supply, any duty would turn the LED's on a bit.

author: Simon
author_rank: Unknown
timestamp: June 04, 2021, 02:51:56 pm
content: Quote from: Frederico Acardi on June 04, 2021, 02:34:12 pmBut that won't give you brightness control and implementing it may make a lot of heat. Instead get a power LED driver off the shelf, more efficient and less heat.

author: Frederico Acardi
author_rank: Contributor
timestamp: June 04, 2021, 02:58:07 pm
content: Thank you, I will see what I find. I may post back when I have some news, thank you.And by the way if you want to buy a TV buy an OLED or Sony LCD (the use dc no PWM for the led).Life is hard when you are just barely making it...yet i am happy I can eatEDIT :Anyone knows how much watts is needed to power the LED of a 43 inch panel, I don't know the type of leds and specification these tv uses ?https://www.alibaba.com/product-detail/ES-1064-ORIGINAL-STOCK-led-tv_1600082933734.html6V 2W it seems

author: Simon
author_rank: Unknown
timestamp: June 04, 2021, 04:54:44 pm
content: That is something you are best to measure to make sure whatever you do can handle the power. My 43" screen uses around 75W, I suspect a fair portion of that is the backlight like 50-60W of it but I know very little about these devices.

author: Frederico Acardi
author_rank: Contributor
timestamp: June 04, 2021, 05:14:37 pm
content: Thank you. I think you are right I did not want to open it before I order the part but I guess I will do.You know, I have a lot of leeway, because I use very very low brightness, so I might just get lucky shooting for a 30/35W led drivers. (maybe I am wrong)I just need to find if they are mounted in series or in parallel to decide for a constant-current driver or a constant voltage, if you have any ideas.From what I understand these devices are basically a Phone (ARM chip) mounted on a panel (IPS or VA) whith a backlight.So I think You are right by saying more than half the current goes to the backlight.

author: NiHaoMike
author_rank: Super Contributor
timestamp: June 05, 2021, 01:26:24 am
content: If it's a constant current driver as it most likely is, you should be able to just add parallel capacitors to make it steady. I don't think it's the 120Hz causing the actual problem, old TVs were 60Hz and most seem to have tolerated it.

author: Simon
author_rank: Unknown
timestamp: June 05, 2021, 08:50:49 am
content: If by old TV's you mean CRT they were as low as 50Hz in sync with the power supply and actually refreshed the whole picture at 25Hz by alternating odd and even lines at each sweep. But in the CRT's the phosphor on the screen would continue to glow for an amount of time.LED's don't "glow" they just go out. I once did a little light display board and had to control the brightness, I ended up with over 200Hz if I remember rightly to get rid of the flickering. I suspect PWM has been used instead of constant current to save money as inductors and low ESR electrolytic capacitors are just the start of the parts that need adding to that basic PWM output to control the brightness properly and will comparatively speaking be expensive.

author: Frederico Acardi
author_rank: Contributor
timestamp: June 05, 2021, 03:21:10 pm
content: Never used a CRT monitor, but I used to have old TVs (if this is what you mean by CRT and I never felt like this) with this 120HZ flicker, I feel like I have a point between the 2 eyes at the forehead that gets tighter and tighter as I watch it. At first I was thinking it's because I work too much, but in the end it was the TV. I can stare at my PC monitor (Benq) all day long, but the TV it's impossible... it took me more than three month to find the cause of my head pain, so probably other people feel it but think it's the way it is if you watch tv too much your head hurt, but it's not true.Quote from: NiHaoMike on June 05, 2021, 01:26:24 amThank you. Are you saying I should just add the capacitor between the 2pins on the actual TV 120Hz supply out ?

author: Simon
author_rank: Unknown
timestamp: June 05, 2021, 04:46:59 pm
content: Quote from: Frederico Acardi on June 05, 2021, 03:21:10 pmIf you do that the capacitor will just charge up to the full voltage of the PWM, depending on the value of the cap and the PWM duty you will get some very ripply voltage. The turn off of the LED's will be gentler but depending on how much the capacitor discharges you will get quite a turn on spike up.

author: NiHaoMike
author_rank: Super Contributor
timestamp: June 05, 2021, 05:19:08 pm
content: The LEDs most likely are driven by a constant current source that's pulsed in time with the refresh rate. Find out the peak voltage of the backlight and then we can calculate what capacitor value to try.

author: Simon
author_rank: Unknown
timestamp: June 05, 2021, 05:27:08 pm
content: why would they pulse the LED's if they have already gone to the trouble to current control them?

author: tooki
author_rank: Super Contributor
timestamp: June 05, 2021, 05:37:23 pm
content: Quote from: NiHaoMike on June 05, 2021, 01:26:24 amThat's because that's comparing apples and oranges.CRT TVs had significant phosphor persistence (meaning the phosphor glows longer than it is excited by electrons). The result is a decaying brightness that masks a ton of flicker.LEDs have insanely quick response times, so their flicker is on full display.Finally, because eye motion comes into play, the size and sharpness of the light sources matters. The comparatively large, blurry dots of a CRT image are not nearly as susceptible as the tiny, razor-sharp pixels of modern displays.With that said, as someone who is highly sensitive to flicker, it is a constant frustration explaining to people that no, 60Hz is not enough for PWM. No, 500Hz is not enough. In fact, the PWM frequency needed to fully eliminate flicker in the worst-case scenario has been estimated to be in the tens of KHz. Why? Because a) our peripheral vision is much more sensitive than the center, and b) our eyes move constantly. Large fields of light are less problematic due to the latter, which causes point light sources (including small lights, specular reflections, and small bright dots in images) to appear as dotted lines during eye movement, something called the phantom array effect. This isinsanely distractingwhen driving, where it appears often in the shittily dimmed tail lights of many cars.In my own testing with LED lighting, I can't detect any flicker above 3KHz, even with the most contrived experiments. But since many power supplies will ring audibly at that frequency, I designed my LED dimmer for 20KHz to ensure it was inaudible.But 120Hz in a backlight? Whoever signed off on that should be taken out back and shot.Here's an old post of mine, which cited a study that found that the average person can detect the phantom array effect at PWM frequencies over 10x higher than 120Hz, with the outliers beingfarabove that!Quote from: tooki on March 15, 2017, 11:47:43 amEdit: original link is dead, here's the archived URL:https://web.archive.org/web/20170325051112if_/http://www.essex.ac.uk/psychology/overlays/2013-207.pdf

author: tooki
author_rank: Super Contributor
timestamp: June 05, 2021, 05:38:15 pm
content: Quote from: Simon on June 05, 2021, 05:27:08 pmTo reduce visible blur by essentially blanking the backlight while the panel updates, so you don't see the transition. But it's still lunacy to me. :/

author: Simon
author_rank: Unknown
timestamp: June 05, 2021, 05:41:51 pm
content: Oh I see, I guess with the speed of LED's they could indeed do that.

author: Frederico Acardi
author_rank: Contributor
timestamp: June 05, 2021, 05:43:45 pm
content: Hey guys it looks like it's duty cycle controlled and always at 120Hz. What can you get from this ?Also tooki I feel your pain, but it's no joke PWM really affect you, and at this point of your life when you understand you are sensitive to flicker, you question your whole mental health and tiredness, as it may be closely related to all the flicker you are subject to in your daily life.

author: tooki
author_rank: Super Contributor
timestamp: June 05, 2021, 05:47:20 pm
content: Quote from: Frederico Acardi on June 05, 2021, 05:43:45 pmI don't think it causes me any tiredness, but I find it incredibly distracting. As someone with ADD/ADHD, the last thing ISQUIRREL!need is more distractions...

author: Simon
author_rank: Unknown
timestamp: June 05, 2021, 05:49:51 pm
content: i will not work with anything but 4k monitors preferably IPS. I ended up taking my own to the job I just left and took away with me my own monitor, keyboard and mouse. I will be taking the keyboard (DVORAK) and the monitor to my new job on Monday. My new boss may well offer to buy a monitor like I like it but I will be happy supplying my own as I got really tired working every day on a crappy 27" HD monitor that I struggled to read text on unless I blew it right up. Not much point in spending hundreds of pounds finding out what glasses work for me when I am still ruining my health with a bad monitor.

author: Simon
author_rank: Unknown
timestamp: June 05, 2021, 05:51:18 pm
content: Quote from: Frederico Acardi on June 05, 2021, 05:43:45 pmWhat is that measuring? looks like no current control

author: Frederico Acardi
author_rank: Contributor
timestamp: June 05, 2021, 05:57:15 pm
content: I got this graph from here :https://www.rtings.com/tv/reviews/lg/un7300Quote from: tooki on June 05, 2021, 05:47:20 pmI understand

author: tooki
author_rank: Super Contributor
timestamp: June 05, 2021, 06:01:08 pm
content: Quote from: Simon on June 05, 2021, 05:51:18 pmI'm not sure whether they measured brightness optically, or the LED voltage, but it's (unfortunately) common for CC driving to be PWMed. Take a look at figure 12 in this CC LED driver IC datasheet:https://www.ti.com/lit/ds/slvs538b/slvs538b.pdf?ts=1622859127927

author: Frederico Acardi
author_rank: Contributor
timestamp: June 05, 2021, 06:07:57 pm
content: From this youtube video led strips on TV are definitely Current control :And from this video at 1:30min the voltage seems quite high almost 290Vbut it's a samsungEDIT: probably not relevant it's a different model:

author: NiHaoMike
author_rank: Super Contributor
timestamp: June 05, 2021, 07:47:13 pm
content: Quote from: tooki on June 05, 2021, 05:37:23 pmSo a carefully selected capacitor value would make LEDs do the same, basically making them emulate a CRT. Probably worth a try over making the LEDs constant since there will still be some reduction of motion blur.Quote from: tooki on June 05, 2021, 05:38:15 pmThere are even time domain multiplexed color displays that use a RGB light source scanning through all 3 colors very fast. It's common for DLP since it's easy to get the transitioning fast enough. It has also been done for LCDs to cut down on backlight power usage but getting those to transition fast enough is hard and such fast transitioning tended to undo the power saving benefits. Taking that a step further, there was a tablet that could operate in monochrome mode with the backlight off or color mode with the backlight on, as a competitor to Eink tablets.Quote from: Frederico Acardi on June 05, 2021, 05:43:45 pmMaybe figure out which current sense resistor to increase in value to reduce the LED current and then set the brightness to maximum since that pretty much turns off the blur reduction.

author: tooki
author_rank: Super Contributor
timestamp: June 05, 2021, 11:08:14 pm
content: Quote from: NiHaoMike on June 05, 2021, 07:47:13 pmYep, I’m obviously aware of this, since as a flicker-sensitive person, I’m consequently also very susceptible to the rainbow effect on crappy DLP projectors. Newer, more expensive ones tend to be a lot better, not only having 2 segments each for RGB on the color wheel, but adding in additional segments like pure white, and spinning the color wheel faster, so that each segment is used multiple times per frame. This gives effective frame rates of e.g. 240Hz, which is fairly good for video. I still don’t like DLP for computer projection, though, especially white on black backgrounds.

author: james_s
author_rank: Super Contributor
timestamp: June 06, 2021, 12:57:34 am
content: I'd probably try putting a capacitor across the output of the LED driver. Not to turn it into smooth DC, but to reduce the instant off-on flicker of the LEDs, it should roughly emulate the "softer" flicker of a CRT.*edit: Just read the rest of the replies and Mike suggested almost exactly the same thing.

author: Simon
author_rank: Unknown
timestamp: June 06, 2021, 05:44:46 am
content: So the current control is just there to make the driving more efficient not to allow a constant steady supply of power. As it is a constant current control putting a capacitor in parallel starts to sound like a really good idea. As a constant current source it will limit the current. If it was just PWM of some voltage a capacitor would just negate the control of the brightness as it would just make the power switch work harder by drawing short circuit current at each turn on to charge back up as the LED's are also powered. This would in the extreme mean that the capacitor would hold enough charge to keep the LED's on regardless of brightness and at low duties overload the power switch.But with a constant current being PWMed you actually have a nicer situation where the average power drawn cannot be increased by the capacitor stealing extra power during the on to release when the LED's should be off because the current source will hold the power level down.So what will happen is on the start of each rising edge the voltage will not suddenly increase as the capacitor has to be charged, the capacitor will act as a short circuit at first and the voltage will rise linearly until the LED's come on, the current to the capacitor will then reduce as the LED's take more and more current as the voltage steadily rises. When it comes time to turn off the LED's the capacitor will hold some charge and continue to power the LED's until it discharges to a voltage that is too low to do so. and so it will repeat. You will though still get flicker of some sort. I don't know if ramping up and down will help your brain versus the fast on off as there will still be a 120 Hz cycle to it all. You would still get the off time during the display update to reduce blur.In the extreme if the capacitor(s) is big enough you will just get a smoothed DC supply to the capacitors but the control will be very sluggish and the light could take some time to turn on and that will be a tone of capacitance and this assume the TV is not monitoring the current/voltage and gets upset by different circumstances.Alternatively as I said earlier, diode, huge capacitance. Right now make a new driver that will look at the PWM before the diode and will switch the LED's at the same duty but higher frequency so you don't get a headache but the brightness control works the same as before.

author: Marco
author_rank: Super Contributor
timestamp: June 06, 2021, 03:00:32 pm
content: LCL filter can be made a little faster than pure capacitive.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: June 06, 2021, 08:06:14 pm
content: I fail to understand what the relationship is between a sore neck and PWM back lighting.

author: Frederico Acardi
author_rank: Contributor
timestamp: June 07, 2021, 02:42:29 pm
content: Hi guys I will simplify the requirements :- I do not care about being able to control the backlight with the remote (even an external pot is good enough)- A DC signal with a small ripple is good enough- I don't need and internal supply (I am ok having the thing in a box aside)- I would love to keep it under 50€ (or even less)Is it ok to assume that right now a capacitor of high capacitance (I know turn on on the led will be slow but as long as it's under 5min) able to discharge slow enough to remove the sharp square wave ?There is pretty shallow pulse at 0% luminosity, is it safe to assume that there will will an optimal brightness setting (on the tv to allow good duty cycles) to charge a capacitor .Quote from: Doctorandus_P on June 06, 2021, 08:06:14 pmIt's probably the nerves of the spines that branches to the brain... But I am not looking for a relationship, I just describe what I feel. I would to be able to just watch it without pain and get my life back.