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Forum: Analog Circuits Voltage drop consequences in led strips


von fuberator (Guest)


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Hi

I would like to understand how voltage drop in a power cable affects led 
pixels.
Is there a way to figure out when they start fading and when the logic 
drops out?

I am using 12v pixel control strips with 144px per meter. I will feed in 
extra power every meter, but I would like to keep my power lines as slim 
as possible (1mm) so the strip and power lines fit in slim alumninum 
extrusions.

LED chip gs8202 datasheet: 
http://www.normandled.com/upload/201805/GS8208%20LED%20Datasheet.pdf

Strip manufacturers recommendations: 20w  12v  1.6A per meter,
My total length per strand: 7m plus 3m extension.

A total of 140W @ 12v = 11.2 A

I am using an online calculator and these are my results:

- 10m @ 1,5mm² drops 23,15% to 9.22v
- 10m @ 1,0mm² drops 34,72% to 7,83v

So assuming that this accurate enough, what are the consequences for the 
last meters that now have to make do with for example 9.22. Some fading? 
Logic malfunction?

Thankful for any knowledge on how this can be derived from the 
datasheet, or any experience on the subject

Ciao

fuberator

von Laurent (Guest)


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Hi!
Let's considere a LED. A single one, fed by an adjustable power supply 
(capable of of both constant current or constant voltage). LEDs are 
powered by a current (20 mA for the cheapest ones), that's how you can 
modify their brightness. But there is a minimum voltage so they can 
light up (Vf, forward voltage). Too low voltage, nothing will light up 
(NO conduction) and too high (leading to a too high current) will fry 
the LED.
So for a known and stable voltage, a series resistor is inserted:
R=(Vsupply-Vf)/I
If Vsupply is 12V on one end and 9V on the other end, you can either:
-change R in series with your LEDs along the entire path (requires 
-de-soldering of SMT components). High R close to the power supply, 
lower R away from it. Err... not easy.
-use larger wires (Rwire=rho*length/section). Errr... not easy.
-use "dual end feeding". Run a pair of electrician wires, as the ones 
used for a lamp(2.5 mm2?), from your power supply to the remote end. 
Near end and far end power cables will share load-> reduced current in 
each->lower loss->brighter remote LEDs. Easy and cheap !

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