I solved it. Turns out that removing the transistor and connecting the Pi directly to the dim pin gets rid of the unexpected behavior; I can now change frequency without changing the brightness.
Since I don't know, and since I'm not an EE, I'll assume that it was my use of the transistor that was somehow improper (it was included in a generic Amazon/aliexpress Pi kit, marked "8050"). I just tested my circuit wit a single LED before hooking it up to the backlight. Don't know why the transistor appears to have changed the effective duty cycle of the signal . Should I have...
ohren
Senior Member
Last Activity: 04-17-2025, 06:30 PM
Joined: 03-10-2015
Location: Sweden
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Oh yeah, you are right — TPV101AD.
I'm always taken by surprise whenever modern products have service manuals, and discovering that not everyone has gone full Apple. Maybe I've been brainwashed. Thanks for the reminder.
Anyway, I didn't have more luck finding the data sheet, and instead mostly found more threads on here that were also looking for it. Even so it might still not have explained what I'm seeing, so I'll have to conclude that I'm blind without a scope....Leave a comment:
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I think you might be right in your identification of the driver.
If the datasheet describes a similar driver, then it confirms the behaviour that I fail to observe:
It's starting to seem difficult to understand this without a scope......Leave a comment:
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I just tried 1 and 2 kHz at 1% duty and the power draw just kept climbing — 122 watts at 2 kHz, which is 1.2 W per diode. I've no guidelines with regards to how much heat they can dissipate, or survive, so I think I'll not push them further for the time being.
Nice of you to look it up and sharing the pdf! Unfortunately it looks like the two drivers are under soldered down heat sinks so I couldn't confirm. All I could determine was that they're eight-legged and that the dim pin is connected directly to the two drivers' pin 1. I'll take a look at the data sheet.
(The...Leave a comment:
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Yes, 15 to 600 Hz was the test range.
Really? I did search for what TVs and displays use in their backlights and it was suggested that many of them used way lower frequencies even than the 10 kHz generally accepted as sufficient for indoor lighting, i.e. around 2000-6000 Hz, or even lower for "strobing" displays which use the same frequency as the video signal (if I understood that concept correctly).
Since I haven't reverse engineered the power board and don't know exactly what the dim pin is connected to, I didn't dare ask anything close to...Leave a comment:
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Thanks for your reply!
Ok, so I take it this means that I need an oscilloscope if I want to know why this happens? That's a shame, I was hoping it'd be some common phenomenon.
Regarding the Pi being responsible for this behavior, I'll have to take your word for it since I don't know what's going on. I'll investigate that path, thanks.
I understand what R_J wrote, but I don't understand what they're hinting at by mentioning it, considering that I'm well within the mentioned frequency range in my test — I just don't know how to use that info...Leave a comment:
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Ok, thanks, but I'm not sure I understand!
I'm already using frequencies between a few Hz and 50 kHz in my tests. I don't understand why brightness increases with frequency in my test when using identical duty percentage.
Why do I need to use Arduino? Is a Pi not sufficient? Its PWM at least worked well enough when testing with a single LED when using a 1000 ns period.
Why do I need to use 3.3 V instead of the power board's existing 3.5? Does it really matter?
I'm not quite sure what you mean when you say "write yours script steps Freq as...Last edited by ohren; 12-05-2024, 08:41 PM.Leave a comment:
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Trying to control Philips backlight manually with PWM — brightness increases with frequency...?
I raised the question of whether PWM on the power board "dim" pin would be a reasonable assumption for controlling the backlight of a Philips TV (55put6101/12): [URL]https://www.badcaps.net/forum/troubleshooting-hardware-devices-and-electronics-theory/general-electronics-technical-discussion/3517245-how-would-i-manually-dim-an-led-tv-backlight[/URL]. I didn't receive any insights there so I went ahead and tried it.
The setup is basically:
Rpi3 hardware PWM → 220 Ohm → NPN transistor → dim pin
Transistor is switching the 3.5 V supply on the...Last edited by ohren; 12-05-2024, 11:24 AM. -
How would I manually dim an LED TV backlight?
Hello! I thought I'd repurpose a 55" 4k Philips led tv backlight for something else (7 strips, ~45 V), and thought I'd get the dimming working without the logic board.
Measuring the PSU "dim" pin upon startup (I assume 100% brightness here) gives 3.3 V, and after it's dimmed down a bit I get less voltage. Giving that pin a constant 1.8 V seems to result in full brightness, so I'm assuming it uses PWM.
1. Is this a reasonable assumption?
2. If yes, what are reasonable values for frequency and duty cycle? Does it matter what I use, or can I experiment... -
Is there any additional info I can provide to help someone lend their intuition about the pattern?
Here are some clues:[LIST][*]The visual artifacts occur on every fourth pixel column[*]They seem to always occur on the same columns between reboots[*]They occur over the boot image[*]The boot image seem to affect the appearance of the artifacts (what looks like the image's bounds become discernible — imperfect black background color in interplay with the fault?), i.e. what the picture is supposed to look like seems to have an effect on the artifacts[*]The artifacts differ between...Leave a comment:
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Samsung UE32D6515WS — boot loop + what does this pixel pattern suggest?
I'm trying to determine the nature of these faults and whether they're coincidental or in fact connected.
Symptom 1: boot looping, showing logo and then going blank, backlight turning on for 2 s or 14 s intermittently, and arriving at logo again after approx 1 minute. Never appears to boot fully.
Symptom 2: vertical lines across whole screen while logo showing. Every 4th pixel column.
Backlight seems to work fine and is lit while logic board is disconnected. 12v, 5V seem stable and only dip when the system resets.
System still seems to reboot... -
Re: Upgrading a Samsung ue55nu7096u with logic board from an ue55ru7105k -- good idea
Thanks for your input. At this point it sounds as if it's not an idea worth pursuing. Too bad! Would've been a fun project if the chances were higher.
Regarding full panel numbers, I wasn't trying to avoid posting them, but since I don't know how to properly identify the numbers I was unable to determine whether I already had old photos of them or whether I had to open up the working set (undesirable) just to get at them.Leave a comment:
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Re: Upgrading a Samsung ue55nu7096u with logic board from an ue55ru7105k -- good idea
Thanks for the suggestion. Now, there are three stickers on the chassis alone, of e.g. the RU (see attached images). How do I know which number refers to what? I never figured that out in my first repair, and I don't know the format of full panel numbers....Leave a comment:
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Upgrading a Samsung ue55nu7096u with logic board from an ue55ru7105k -- good idea?
I hope this is the right forum to post in despite not technically being a repair, but still a question of swapping boards.
I thought I'd explore the possibility to upgrade an existing working Samsung ue55nu7096u with the logic board (and power board if needed) of a newer model (ue55ru7105k) of the same size and appearance (and resolution). The reason being I'd like the extra inputs of that board including component in, an extra HDMI port and bluetooth. Its panel is broken so I have no use for the board otherwise.
The models launched in 2018 and 2019 respectively.... -
Re: Post your Samsung NU model series backlight failures and diffuser melt-downs here
Has anyone of you ever swapped out the logic board in a NU (or other Samsung) with a more feature-rich board from another model (with a broken panel)?
I'm kind of curious about doing that, using a board from a RU, but I don't know if it's likely to be able to drive the LCD. The tvs look pretty much identical except for the logic board of course (and slight differences in the PSU). Do you think there's a chance?
—
And to not stray from the topic: this...Leave a comment:
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Re: Why did my MOSFET blow up?
Yes, exactly, I've triple checked. Drain and source are not shorted to the gate. It might have been, though — here is a photo. A piece is missing nearest the gate pin....Leave a comment:
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Re: Why did my MOSFET blow up?
They did not, unless they subsequently failed open.Leave a comment:
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Re: Why did my MOSFET blow up?
It shorted drain to source. Does that bear any relevance to what you're talking about?...Leave a comment:
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Re: Why did my MOSFET blow up?
Pasting did not preserve the link target, sorry:
[url]https://www.manualslib.com/download/1250393/Harman-Kardon-Mas-101.html[/url]Re: Why did my MOSFET blow up?<br />...101.html[/url]Leave a comment:
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Why did my MOSFET blow up?
I had a switching transistor and PWM IC blow up as part of a repair, where a big chunk of each went missing. Now I would like to understand the likely turn of events, and maybe determine if I did something stupid. If nothing else, I should at least extract some lessons from it.
"IC10" can be found on page 169 in the service manual:
[url]https://www.manualslib.com/download/...n-Mas-101.html[/url]
The entirety of the repair is basically:
1. Device doesn't power on (IC10 supply voltage unstable)
2. Replaced C18 and C66
3. Device...Last edited by ohren; 04-30-2023, 07:06 AM.
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