Author Topic: LED strip light  (Read 1656 times)

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Offline rupesh

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LED strip light
« on: April 22, 2015, 01:25:26 pm »
i am new in designing so please go through this and help me .I want to make LED strip light using SMD 2835.The data sheet is provided below.The forward VI characteristics shows 75mA at 3.3 V.The relative luminesence intenisty at 75mA is 110%

12 V,1 Amps DC is used as the source .I have 3 LED's in a single strip, and a resistor to limit the current flow to 75mA.(but as per the available resistance 63.6mA flows through it) resistor calculation
The resistor value is 28 Ohm and the nearest high rated resistor is 33 Ohms.

input power=12W
power abosrbed in single strip =(12*63.6=0.7632W)
The maximum parallel strip i can connect to 12V,1A DC supply =(12/.7632) =15 strips which means 48 LED and 15 resistors in total.


from the economical point of view can i limit to 12 strips by allowing 80 mA current to flow through it by a 39 Ohm resistor

So anyone please go through these and guide me,

Offline PSR B1257

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Re: LED strip light
« Reply #1 on: April 22, 2015, 01:50:39 pm »
allowing 80 mA current to flow through it by a 39 Ohm resistor
If a 33 Ohm resistor gives you 64mA, how can a 39 Ohm resistor result in any more current  ;)

You should drive the LEDs at 100% (or less) nominal current. No need to go any higher than this because your eye (and everyone else human eye of course) can't really thell the difference anyway.
Test it for yourself. 1/4 of the nominal/initial LED current looks just like 1/2 of the nominal/initial brightness.

In theory, there is no difference between theory and practice. But, in practice, there is.

Online mariush

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Re: LED strip light
« Reply #2 on: April 22, 2015, 01:59:52 pm »
The absolute maximum rating for those leds is 90 mA.  All the measurements in the datasheet are done at 60mA, so you should probably run those leds at around 60-70mA, don't push them higher. 

Also, note that you'd need some space around the leds to act as heatsink or design double sided pcb with lots of vias from the top side to the other side to transfer heat from the led side to the copper on the back.

Using resistors to limit the current is the most basic way of driving leds, there is some power wasted in the resistors  ( p = i^2 x r = 0.063x0.063x33 ohm = 0.13w so you'd need at least 0.25-0.5w resistors)... with 15 strips, you'd waste 15x0.13w = 2 watts on the resistors alone... with a 12v 1A power supply that means you have about 80% efficiency.

From the way you wrote your message, it feels like you're a beginner so resistors is probably easiest for you.
If you're willing to experiment, I'd suggest removing the resistors and using led drivers to have finer control over the current going into the leds, and better efficiency. 

There are boost led drivers that would allow you (for example) to put up to about 12 leds in series (as long as the total voltage doesn't go above around 36-38v) and drive them from low voltage like 12v at current you want without using resistors, achieving 85-90% efficiency this way. You'd need an additional inductor and maybe a couple of diodes and capacitors but in some cases it's worth it.


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