The thread about electrical problems

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Re: The thread about electrical problems

Post by zyssai »

mctom wrote:
Fri Mar 29, 2024 5:12 pm
I find it fascinating that after so many repairs and projects you still didn't get a bench top power supply. :)
Unless you do have a Smart Power 3 already, but I still hold onto my 30V 5A Korad PSU for special tasks like this.
You're absolutely right, I am missing some crucial equipment, I actually have this power supply, honestly sufficient most of the time.
It is designed for smartphones repair, that's what I do the most, with a maximum voltage of 15V.

But yes, a huge one should be a good idea :)

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Re: The thread about electrical problems

Post by zyssai »

I retrieved my 24V power supply so.i will be able to test my amplifier.

But now I have a professional LCD screen, used on some sort of CNC machine. No video input, only rs232 and USB interfaces, so I assume there is a basic OS as I barely see some menu, but display comes totally white once I power it (can take 2/3 seconds when it is cold).

I have found many different voltages (5v, 3.3v, 1.8v, 1.3v), I checked backlight IC and it seems to work as I have signal at EN pin (thanks to the oscilloscope :D ), and 9.8V at FB pin, with something like 2ohm.

I did not found any shorted cap or anything else strange.

ImageImageImageImage

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Re: The thread about electrical problems

Post by mctom »

zyssai wrote:
Sun May 05, 2024 5:00 am
and 9.8V at FB pin, with something like 2ohm
You sure about this? That's about 4 Amps flowing through that resistor. ;)
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Re: The thread about electrical problems

Post by zyssai »

mctom wrote:
Mon May 06, 2024 2:54 am
zyssai wrote:
Sun May 05, 2024 5:00 am
and 9.8V at FB pin, with something like 2ohm
You sure about this? That's about 4 Amps flowing through that resistor. ;)
So, LED driver is marking code DQ3BA, I have found it is SY7201.

Your're right. I mesure 0,179V at FB pin.
I mesure 9.5V at the left pin of the connector going to the panel, so it seems this is a good working condition.

Image

Edit: I add video of the problem:


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Re: The thread about electrical problems

Post by mctom »

I think the backlight is fine, and so is the video data transfer, assuming it's digital. The traces on the small board with two ribbons suggests it's RGB666.
The image slowly fading into white cannot be a result of digital data corruption.

It would be helpful to determine the exact model of the display panel and find its datasheet. The model may be written on a sticker underneath the metal housing in which the panel is seated.

In fact, I think there is something wrong with that small board. It has a lot of resistors, I'd like to see their markings better. I suspect this board translates digital RGB666 into analog signals using resistor ladder, and this is where things may go wrong.

This may be a matter of poor ground connection between this small board and the main logic board, for example.
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Re: The thread about electrical problems

Post by zyssai »

I love your analysis.
This is a logic I do not know how to proceed.

I took some pictures, not easy to have a clear reading of the resistor values.

I have just found this unique reference about the panel.

ImageImageImage

From bottom to top, left connector is going to the panel.

Image
Image
Image

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Re: The thread about electrical problems

Post by mctom »

https://www.elecok.com/pl/data_sheet/36 ... y_(PL).pdf

So I didn't guess it entirely right, this displays accepts digital data in parallel RGB888 format. It comes as RGB666 from the board, but it gets "converted" to RGB888 completely passively.

But this display also has some weird "gamma correction voltage reference", for which that bunch of resistors and capacitors is used.
PDF page 28 (Chapter G1) shows a schematic of that gamma correction circuit. The resistors you photographed match this schematic, except the resistor values are different.
I suspect that VDPA isn't 5V or VDNA isn't -5V.

Try measuring voltages of these 1uF capacitors and make note of the highest and lowest values you will find.

Also measure VCOM voltage (C29 if I'm not mistaken) and try to find out where does it come from. Changing this voltage may also affect image brightness.

Using display datasheet (chapter C1: Pin Assignment and C4: electrical DC characteristics) you can also verify all display power supplies.
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Re: The thread about electrical problems

Post by zyssai »

So thanks again, it is really helpful.

VDPA is 5.02V and VDNA is -4.978V.
VCOM pin 1 is -2.26V, coming from 5V by U4 IC marked 59303, cannot find any corresponding SOT26. Value seems correct according to datasheet.

From V1 to V10 I have something around 10% of the required values, so I assume it is also correct.

So there's probably something weird inside the panel itself.
I will see if I can disassemble it.

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Re: The thread about electrical problems

Post by mctom »

I don't think you will find anything repairable inside the panel, I wouldn't open it.

If VGH and VGL are also within specified limits then I'm out of ideas. :(
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Re: The thread about electrical problems

Post by zyssai »

mctom wrote:
Tue May 07, 2024 5:32 pm
I don't think you will find anything repairable inside the panel, I wouldn't open it.

If VGH and VGL are also within specified limits then I'm out of ideas. :(
You have already been a great help :)
I will keep investigate and update if I have something new.

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Re: The thread about electrical problems

Post by zyssai »

Anyone recognize this brand?
I searched even with image search but cannot find anything about it.

Image

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Re: The thread about electrical problems

Post by mctom »

No idea what this might even be. There's no polarity marking, so not a diode I guess? Unless it's bidirectional TVS.
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Re: The thread about electrical problems

Post by zyssai »

Thanks, there's no polarity but there's a polarity printed on the soldermask.
I removed this component and the short is gone.
This is a security outdoor camera and I suspect it from going out of order after a bad weather with lightning.

The "fun" fact is that the power adapter is always working :evil:

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Re: The thread about electrical problems

Post by mctom »

Then I think it was a TVS that fulfilled its role - got damaged and became short circuit due to transient overvoltage event.
The device should work without it, but you'll better add a new TVS in its place. Doesn't have to be exactly the same, especially that we cannot identify the brand.

The package looks like SMA, SMB, or SMC depending on its size. Depending on the nominal voltage on the red wire (12V?), pick a TVS that gets triggered above that voltage.
A unidirectional type would be better, so it will protect against negative voltages too.
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Re: The thread about electrical problems

Post by zyssai »

So to get it work with a minimum of security I will replace by SD15C-01FTG as I ordered some for replacement of a diode on the N2.
I will order a TVS asap :)

Thanks!

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Re: The thread about electrical problems

Post by mctom »

If you wish we could review your TVS choice together, before you order. :)

From designer's perspective, it's easy to forget or disregard components that do nothing during normal operation of a device. Space Industry job taught me to take care of that. It's easy to speculate that 1/3 of components in space designs are protection :D
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Re: The thread about electrical problems

Post by zyssai »

With pleasure :D

So it seems to be SMA package, in fact I don't know anything about it, just that it seems to be this one regarding the size and the appearance of the pins.

DC power is 12V, no need to protect for negative voltages I think because once adapter is plugged in there's no matter about polarity.

This is a security cam given for 1A at 12V, powered 24/7.

I just have a doubt now about my cheap DC adapter, as I don't understand why it could have made a surge after 2 years and works normally after that. I'll probably replace it.

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Re: The thread about electrical problems

Post by mctom »

Great, now go and find a diode that matches your description. :)
SMA, SMB etc up to SMF is a package family that looks like this, they differ in size. Mostly for diodes, but I think some tantalum capacitors also can have this package.

This protection is against very short spikes, like below 1us. Those can occur as a result of sudden changes of voltage in AC grid (for example, when old fridge compressor turns on :) ). Or a nearby thunder strike that induces current in all long conductors.

This diode should be unidirectional, because spikes can have either direction, and negative voltage is just as destructive as too high voltage.
Bidirectional diodes are used for lines that nominally also have negative voltages, such as speaker outputs or AC voltage input.
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Re: The thread about electrical problems

Post by zyssai »

Thanks, always learning something here :D

So I have found SMAJ12A that seems to suit my needs, but I know nothing about all the different voltages (reverse, breakdown, and clamping) :lol:

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Re: The thread about electrical problems

Post by mctom »

That's a good choice. :)

There are many numbers because there is no single voltage above which diode starts conducting, as we like to think about them. In fact, all diodes conduct at any non-zero voltage.
When voltage is low, we simplify by saying the diode doesn't conduct, but in fact it does a little. This is undesired and thus often called "leakage".
Above some voltage, diodes start conducting much better. This is what we assume to be 0.6V or 0.7V for silicon diodes. By convention we say diode starts conducting when it reaches 1mA at some voltage. This is why many diode testers in multimeters have 1mA test current.

The diode you selected is guaranteed to leak no more than 5uA at 12V. We can simplify by saying it doesn't conduct at 12V.
Then, the datasheet says that the diode will pass 1mA somewhere between 13.3 - 14.7V. This is a point that by convention we call a threshold, when diode starts conducting.
And at the maximum supported current pulse of 20.1A the voltage drop of a diode will reach up to 19.9V. This gives us information that when it is fully conducting, it may have resistance as low as 1 Ohm! At the same time, it will dissipate some 400W of heat, so obviously it cannot last longer than a few milliseconds.

So the takeaway is that at nominal voltage of 12V it will not conduct significantly, so it won't do anything, as we expect for a 12V power supply rail.
But when voltage radically exceeds 12V it will try to do something about it.

It may be surprising that a protection device with 19.9V mentioned in its rating is even effective. But it is designed to clamp very short spikes of voltage/current, and not protect against a PSU with too high voltage.
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Re: The thread about electrical problems

Post by zyssai »

You have an amazing electronic knowledge, and know how to explain with easy words, thanks for that!
Give us a donation link for this :D

But can I ask something again? :)
mctom wrote:
Tue May 14, 2024 2:03 pm
Then, the datasheet says that the diode will pass 1mA somewhere between 13.3 - 14.7V. This is a point that by convention we call a threshold, when diode starts conducting.
So assuming that, what if my power adapter is 11.8V?

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Re: The thread about electrical problems

Post by mctom »

This is a TVS voltage-current curve. Of course the drawing is not to scale but presents the idea.

At Vrm=12V we've got Irm specified, 5uA. At voltages below 12V, the leakage will be even lower, and the diode still considered not conducting.
Vbr is the threshold, as typically measured when Ibr = 1mA. Above this point, it starts conducting much better.
Vbr is given as a range (13.3 - 14.7) because it depends on temperature and randomness of manufacturing.

For completeness, Ipp = 20.1A and Vcl = 19.9V.

By the way, you can also observe what happens when negative voltage appears: At some Vf it will start conducting too, thus protecting against negative voltage spikes.

EDIT: In other words, 11.8V PSU is fine too. :D
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Re: The thread about electrical problems

Post by zyssai »

Oh I see it now, I was confused by the word conducting, but as it is mounted between GND and VCC, and not between VCC and VCCOUT, that makes sense :D

Thanks again for your demonstration, it helps a lot!

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Re: The thread about electrical problems

Post by zyssai »

I received SMAJ12A, replaced it and it works :)
Just replaced adapter because I do not have trust on it anymore (it seems to be cheap).

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Re: The thread about electrical problems

Post by mad_ady »

Remember my no-name BT speakers that were making a buzzing noise when connected to my H3? Don't worry, I don't remember them also, since they were in the first post, 1.5 years ago! Anyway, both of them died long ago, and were on my todo list, for when I have time... I still don't have time, but now I have the need :lol: Kid #1 has a music project in school, and needs a portable speaker (hopefully to initiate his peers in the wonderful music that is The Floppotron (https://www.youtube.com/watch?v=kCCXRerqaJI), with which kid #1 is well accustomed to). :roll:
The biggest issue was getting inside the case. I tried gasoline on the joints, but nothing. So I used a hacksaw, on one speaker, to let it know I mean business. Half-way through I figured out they had a fake bottom glued on, and beneath the glue there were 3 screws... :evil: Unit 2 came off more easily...

The speaker would not turn on at all when power was applied, there were no leds on. Inside I was greeted by a swollen and dead battery. 0V... I disconnected it and tried to power up via SP3 and took a chance and supplied 5V. It powered up, and seemed to work fine. With the battery disconnected, I also tried to connect its power connector, and this time it started and seemed to work ok. So, no need to solder anything to the battery connector.

https://imgur.com/a/L3TUTVb

My goal for these speakers is to be able to use them as wired devices (which don't require a battery), but also I'd like to be able to use them wirelessly, by connecting them to an external battery pack.

And here comes the first problem. When idle, or when playing back low-volume stuff, the speakers draw very little current (0.01A at 5V). Modern usb battery packs usually turn off automatically at low current, because they're designed for charging. Is there any easy-to-do hack that would allow me to power the speaker from a portable battery pack? I can think of adding some sort of a resistor to a second port of the battery pack, to draw more power and trick it to remain on (assuming both ports are controlled together). Perhaps there is a more elegant way that I am not aware of?

I've noticed some instability in bluetooth mode - where the speaker restarts when playing some audio streams, but I hadn't tested BT when it was in one piece, so maybe it's been like that from the factory...

Also, a fun fact - I learned my speakers have a FM radio, and I can listen to (mono) radio! Sweet!

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Re: The thread about electrical problems

Post by mctom »

So your speaker works decently sipping 50mW? Sounds weird to me.

Yes you can increase current draw by adding a resistor in parallel to DC voltage input. The resistance follows Ohm's law, at 5V and whatever current you think is necessary to keep the powerbank active.
Don't forget about power dissipation of this resistor, it must be properly sized!

Other than that? Hmm.. How about you place a new battery in there? Anything with the same chemistry should work. It was Li-Ion most likely.
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Re: The thread about electrical problems

Post by mad_ady »

So your speaker works decently sipping 50mW? Sounds weird to me.
You're right, I meant to say it idles at 50mW.

Tomorrow I'll try something nonstandard: powering it from the M1S ups, which doesn't have the low current cutoff, like regular power banks...
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