GS8208 12V pixel strings - A wildcard third option for the 5V vs 12V WS2811 question?

AussiePhil

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I get 2.6-2.7Ω (ohms) for 2x 50 pixel strings connected together. This is with the V+ and gnd of pixel 1's injection pigtail twisted together and the measurement taken between V+ and gnd at the opposite end (after 100th pixel).
The total resistance indicates that they used Aluminium wire between the chinese 18 and 20 sizing. As the voltage drop you measured below shows that the wire sizing and type becomes less critical. getting these made with copper would make them even better.
As far as voltage on 100% white, what starts at 11.91V drops down to 11.36V at the end of the 100th pixel (2x strings of 50).
That seems in line with back of the envelope calcs based on the wire resistance and per node draw
Thanks for the data
Cheers
Phil
 

AussiePhil

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what are the options? from what I could see of the spec sheet you guys found of the one Ray was using was good for up to a 30v input, so a 2v drop would get you quite some distance and still give you more than 10v output?
* that's 30v absolute Max, test conditions for the 05 only go to 20v.
* 24v is likely to be the sensible working limit
* the 6mA quiescent current is an issue in strings of pixels
* add in the 2.5mA used by the 8208 you have 850mA per 100 pixels before you create colours
* the quiescent current of 6mA would kill any benefits your thinking of getting with higher v

Having said that you would actually need to use a reg higher in value than the 05 as Vdh on the 8202 is specced for 9v to 15v.

All of the above is just to drive the IC itself, next you need to deal with the 15v limit (Vds)on the IOUT pins for the LED. you could use the same regulator as it has 100mA MAX rating.

This would mean a 78L12 is used.

from what i could make of the data sheet, the higher the output voltage, the higher the input could be for the same device

Edit this was one discussed being used by Ray https://pdf1.alldatasheet.com/datasheet-pdf/view/22687/STMICROELECTRONICS/78L05.html

for 12-15v output up to 35v input... sweeeeeeeeet
As stated above, that is a MAX rating, 24v or maybe 27v would be the logical supply voltages

The whole idea breaks down once you add the Constant current draw from the LED with the standing quiescent currents which means you actually use more current and whilst this may mean longer runs between power injection points if needed it increases the overall power used and likely increases the number of power supplies you need.

In my opinion this is self defeating in the long run noting i am a huge fan of local point of use DC-DC conversion.
 

MichaelF5

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Sooo, we are balancing a few issues

1, the regs will get hot as unless oversized
2, the reg inefficiency is going to cause each pixel to draw more current which leads to a huge voltage drop on the wire

I assumed that as the voltage increased the amount of current being drawn would drop relatively proportionally, you are saying however efficiency drops

is the solution going for a bigger reg that has a higher voltage range and or higher efficiency... assuming such a thing exists (I can't find anything much more efficient than around 86%) ? this is for driving the GS8208's as an example.. something that goes as high as 40v and 100ma rated...

Thoughts?
 

David_AVD

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A linear regulator (like the 78Lxx series) will have an efficiency of < 50% if the input was 24V and the output was 12V.

If it's 24V in and 5V out, that drops to < 20% !
 

Kent

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Don't forget the total pixel cost we're talk circa $0.50 for the led, driving chip, manufacturing the thing, supplier trying to actually make money. The switching controller also needs supporting components like the inductor, etc.

"Tell him he's dreamin' "
 

PermaLights

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I do not think that the GS8208 maintains a constant brightness across its colors. Rather its PWM generators are only trying to create a linear relationship to a colors command value and the current in that colors LED where of course 100% (binary 255 command) is the full LED current. A human eye's light intensity response is not linear; logarithmic I think. And to increase the complexity, the eye is more sensitive to some colors than others. I also do not think LED lumen output is linear with current.

The links to the chip specs and previous Forum Threads detail why the current per pixel does not sum with more than one color turned on as in a WS2811 for example. A GS8208 works very similar to the WS2815 except that the WS2815 is a strip only while the GS8208 can be incorporated into a bullet pixel.

Without all the detail, the GS8208 RGB LEDS are in series while a WS2811's are in parallel. This means the LEDs used in a WS2811 and GS8208 must be different. That is a WS2811 LED would have the RGB and a Common Anode connections. A GS8208 would (going by the the GS spec sheet) a +R, -R, -G, and -B. Still 4 pins but definitely different internal connections in the LED itself.

As a side note: I really like the WS2815s mostly due to the lessor amperage draw per pixel. I really wish GS8208s would be available from US vendors as bullet pixels. I would immediately move away from the WS2811s.
If you are looking for GS8208, check out PermaLights.co. We have stock on G28208 in Square and LED Pixels.
 

TerryK

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I checked the web link. The company is based in Utah, USA apparently. A bit pricier than WS2811 12 Volt pixels; slightly less than US $22 per 50. Wire is clear and I sent an email regarding wire gauge and pixel spacing which is not yet known.
 

TerryK

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Permalights responded to my email. Pixel spacing is 4 inch center to center. Wire AWG is 20. Lead-in and lead-out wire length is 5 inch. They also added these specs to their product's web page. Researching their website in more detail, it's mentioned that they use WLED and have a WiFi controller that will drive 1200 pixels.
 

Martin Mueller

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Hmm. Oops. I missed those in the datasheet. Typical value for the resistor, minimum value for the capacitor I think.

I would not rule out the divider theory but am somewhat skeptical as the internal constant current source should handle the LED load and the internal 7805 the logic portion. I'm more inclined to revert to my first thought, filtering of switching noise. However without knowing more of what's inside the IC it is mostly making educated guesses.

Short circuit failure might be a problem. Twelve volts across 30 Ohms works out to a little under 5 Watt. Something will warm up rather quickly.
That works out to Pixel == fuse. It will burn out quickly.
 

Martin Mueller

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An additional thought on these things. If you use the 1Mhz mode then the number of pixels per frame at a given frame rate goes up by 25%
 
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