Hi there, power injection, not FL directly related, so tell me if it is not the right forum for my subject. I know this has been discussed so many times but I can’t find answer to those questions: 1) is there a precise technical rule about how many leds should you reinject power? 2) It seems a good idea to have power injection at both end of the strip to avoid voltage drop and get more balanced light, is that the answer. Okay I will push a example soon on what I’d like to achieve, but any reference, discussion or weblink on this would help to take the right direction. Thx 
Every 128 LEDs at most (64 is better) I’d say, but it really depends on the size of the connection between each pixel.
That’s if you care about full brightness. Less will be fine otherwise.
For all my designs, I did some simple trial and error and added power as necessary
Mmmm okay thank you Marc for your experience on this.
it seems @Yves_BAZIN injects voltage every 3 five meters strips (30 leds/m) in his geant panel (https://hackaday.io/project/158268-5904-leds-panel/details) with a common connexion to the ground at the seconth end of each strip. It means an injection every 450 leds was successfull. I also found that illustration on SIRS videos, where injection is done every 170 leds (5m x 34 leds) So the answer seems not easy … Also here each end is connected both to ground and +5V .
Any experience is / Welcome /
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@Derrick_Giscloux Here’s my ‘technical rule’ about 5Vdc power distribution and large LED assemblies…
I will never let the voltage drop below 4.5Vdc anywhere in my setup at full brightness… That’s it !!!
Because not all strips are built the same I would do a test of a single strip at full brightness white ( or use whatever will be your maximum brightness level… IE: max current draw for that strip ) with exactly 5.0Vdc applied at one end only and I would measure the voltage at the far end and if it is under 4.5Vdc I would find a way to re-inject power at the far end of that strip.
The reason for 4.5Vdc is that is the minimum supply voltage specified by device manufacturers (ws2812b, APA102 and I think most others… ) for all their timing and electrical characteristics. That does not mean your LEDS wont work with voltages below that, just that the manufacturers will not guarantee the functionality.
Also, 4.5VDC is already 10% lower than the nominal voltage and for brightness consistency, I would not accept anything below that.
So logical, indeed, thx @JP_Roy . But still, if for example the 170th led of a 3 meter 60 leds per meter strip has a drop under 4.5V should I reinject straight in that led specifically or I should do at the end ?
@Derrick_Giscloux keep in mind that ultimately it’s all linked to the size of the copper trace between pixels which is different on different kinds of strips. Hence trial and error not being so bad
@Derrick_Giscloux wrt your 170th led question… yes in that specific case, supplying voltage at 2 points, at the beginning and end of that 3 meter strip length should work well. Keep in mind that your ‘far end’ wiring may be up to 3 meters in length (if your strips are all straight and not looped around somehow) and that wiring may need to be a heavier gauge.
@Derrick_Giscloux the length of my panel is 4m long not 5 (unfortunately). And I inject power on every strip so one injection every 120leds.
I use one data line for 3 rows (360leds).
Unless you use full brightness I do not see difference between start and end of the strip.
@Yves_BAZIN @Marc_MERLIN @JP_Roy many thanks guys, all this help me very much. Great community, great forum. Best.
You ask one question and then you get 4 specialists who argue with one another on what the real answer actually is 
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