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Design of series of 8 LED Driver circuit

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4tuty

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I am using 8 green LED View attachment ife93 (2).pdf ($5.4/LED) in series for my project, I need a driver circuit with forward Voltage of 3.1V and 20 mA. can any one help me with schematics.


is it ok to use this circuit or this circuit schematic_leds.png
 
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You need to specify what is the color of the LED.
Each one have different drop voltages.
 

andre_teprom said:
You need to specify what is the color of the LED.
Each one have different drop voltages.
Click to expand...

all 8 LED's are green colour i've attached the datasheet also for your kind perusal.
 

The circuit above seems wrong due to the lack of the reference. You should use the circuit at this link, and the formula for exact 3,1v ( unfortunately the datasheet do not make any mention to the min/max range )
 

you need 8*3.1V = 24.8V for 8 leds in series
and to limit the current you can use 100 ohm 1/4 W resistor (need 0.02 A * 100 ohm = 2 V)

you can Adjust the LM317 o/p at 24.8+2 = 26.8 V by usin 240 ohm between Vout & Adj , and Pot between Adj & GND

you need 30 V Dc as i/p the adjust o/p using pot

then connect 100 ohm between Vout & LEDs.
 
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ahmed saad said:
you need 8*3.1V = 24.8V for 8 leds in series
and to limit the current you can use 100 ohm 1/4 W resistor (need 0.02 A * 100 ohm = 2 V)

you can Adjust the LM317 o/p at 24.8+2 = 26.8 V by usin 240 ohm between Vout & Adj , and Pot between Adj & GND

you need 30 V Dc as i/p the adjust o/p using pot

then connect 100 ohm between Vout & LEDs.
Click to expand...

thanks for the reply, i need to use 9Volt battery to make circuit simpler and do i need heat sink for LM317?
 

if you use lm317 and "only" 9v battery, then you must put the leds in 4 strings of 2-in-series, and add a resistor in series with each of the two's.
You will never be able to do it totally accurately, because of course, you do not know the exact vf of the leds......so from a 9v batt, you would need 4 lm317's to do it accurately. (then if with 4 lm317s, you wouldn't need the series resistor, just the resistor that goes with the lm317)

so in fact, if you were going to use a single lm317, from just a 9v battery, then you might as well just use 4 strings of 2-in-series and add dropping resistors in each string, and forget the lm317
 
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4tuty said:
i need to use 9Volt battery to make circuit simpler and do i need heat sink for LM317?
Click to expand...

You have to pay the price of each solution. The LM317 IC is widelly available on electronics market and you can purchase it form several manufacturers, however it set the LED brightness based on the voltage output in an open loop control, whereas if you use some other circuit which perform an output control based on current sensing you would be able to drive precisely the amount of power required with a minimal of waste, therefore could consider reducing or elliminate the haatsink.
 

in case of 9V bat use four set in parallel
you can adjust Vout to 7V
two Led in series need 2*3.1 = 6.2V
Led series resistor 39 ohm (to limit current)
 

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thanks to everyone to solve the problem 20151126_224037.jpg. is this circuit is correct? or any other improvement in the circuit is needed? guide me
 

you may need a capacitor between Vout & GND
as datasheet says "CO is not needed for stability, however, it does improve transient response".

I think you can do it now

BR
 

You might be aware that the brightness will decrease a few with the battery discharge, even considering that the alkaline batteries have a soft slew rate on its voltage during discharge.
 
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andre_teprom said:
You might be aware that the brightness will decrease a few with the battery discharge, even considering that the alkaline batteries have a soft slew rate on its voltage during discharge.
Click to expand...

thank a million sir, one more help how to include 8 photo darlington in same circuitView attachment op560a.pdf

if 9v battery not suitable i will go for 12v adapter
 

Capture.PNG sorry forget to add schematics

please consider the npn darlington as photo darlington op560aView attachment op560a.pdf.

is it possible to to add both circuit? and can it be driven by 9volt battery?
 

why not have the 9v rail and use 4 led strings of 2-in-series, but have a BJT regulator in each string.....you know, the one where an opamp with a reference into one input, feed the base of the regulating bjt, and a low side resistor in series with the leds and bjt feeds back to the other input of the opamp and it regulates the led current.........the opamp forms an error amplifier.

- - - Updated - - -

Here is an LTspice sim solution to driving 8 leds at 30mA each from 9v. Ltspice is free.

- - - Updated - - -

here is another one (ltspice sim), this time a switch mode circuit...use the free eagle pcb layout to make it on a pcb..

- - - Updated - - -

to run them, change the files to .asc instead of .txt, then open ltspice, open the file, hit runningman icon, and it runs for you
 

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So far it appears to be a poor mismatch of wavelengths to the sensitivity of phototransistors , no daylight blocking or stray light blocking in the design and low efficiency in driving LEDs.

WHat is the purpose of your project? How will you control stray light and the threshold ? Is this for logic level out?

Too often young designers fail to grasp the importance of defining the requirements before implementation begins and the project fails in meaningful ways,
 
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just read SunnySkyGuys reply and realised these are not normal leds, these are leds which form an optical link and need to be on/off'd quickly....as such, I would use the transistor regulator and also arrange for the transistors to be turned on and off too, as well as operating them in linear mode when on.

For the switch mode circuit, I would use a led driver chip that has a pwm dimming input and use it to on/off the leds...by turning on / off a series transistor. I would ensure the driver froze the compensation voltage when the leds were off, so that there was no overshoot or delay when coming back on....also, I would make sure there was nothing to draw current out of the output capacitor when the leds were off.
 

Define the specs for testing its acceptable performance to interference and range.

Too many offer advice and forget the application specs need to be told.
 

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