please help me design a constant current regulator for led

Ok, lets just think about this a little bit. If you want a constant current source to still work with the identical source voltage as the load voltage, then your regulator circuit must be capable of having an impedance of 0V/300ma= 0 ohms. A little difficult to achieve at room temperature. So accept that your lowest battery voltage will drop out of regulation range at some slightly higher voltage than the LED's rated voltage. (I suspect this will be fine option.) Or you could accept an LED output voltage that is less than 3.0 volts like maybe 2.5 volts. The LED will be dimmer but the battery should last longer.. Lastly and the most complicated, you could design a boost buck switching voltage regulator that will provide a constant voltage as the battery drops in voltage.

thanks sir,i got ur point.i just thought of minimising the voltage drop in the circuit so that i can get almost same voltage across led.this will help increase the battery life, i thought.anyway thanks for ur help.

i've another thought.why not use a n channel jfet in series with led and let the gate voltage be monitored by connecting it to the emitter of a bjt which comes in parallel with the led.would this work?

That and a whole host of other linear arrangements can be made to work. But to actually maximize battery life you should use a switch mode configuration. This way the power draw from the battery remains constant and not the current draw.

Instructables might be a good place to start:

http://www.instructables.com/id/Circuits-for-using-High-Power-LED_s/

A GA as to the point technically.

I like to point out:

. Why to to use LeadAcids .... Why not the Li-Ion or latest type ... Costwise better propsition and also maintenance-wise a better choice. Less charging time ... In Power shutdowns areas.

Regards

I've reread the OP problem and I don't see where he/she is asking for a battery charger circuit. I interpreted the question that this was for some form of a rechargeable battery driven LED sign. This would explain the desire for such a high wattage LED instead of the tiny wattage that one can achieve with an indicator LED like a T 1-3/4. Depending on the number and configuration of the LEDs one might even be possible to pulse the LED's so that the apparent brightness can be much higher than the actual brightness due to the latency effect in the human retina. But this gets into a lot more detail than what the OP has offered on how this is going to be used.

hi,

i'm sorry that i didn't get to the detail of my problem.actually i'm using this led for home lighting in villages where power outage is very common.now i thought of having a battery driven led with more lumens compared to incandescent so that all the light that is required could come from a low wattage device which obviously saves cost and gives the luxury of having light anytime they want.U were correct in guessing that this led would be driven by rechargeable battery.now to charge this battery we thought of having a chopper.this chopper would be connected to solar pv and also mobile phone charger output(which is already connected to ac mains).this is to make it easy for user to switch between two source if anyone is not available.i hope i've made it clear about my problem.right now i'm very much concerned about the current regulator than the battery charging part.so,it would be a great help if u can help me with this.

thank u,

vishwanath

There are a multitude of switch-mode LED circuit diagrams available on the web. Many IC manufacturers provide very good application notes along with complete schematics and occasionally circuit board patterns. Look and you shall find.

One thing that can significantly save you power is to change your design goal from a constant current source to a pulsed current source. You'll be using the natural latency of the chemical reaction that happens in the eye while saving power by not having the LED ON all of the time but still flashing faster than people can see.

If the LED is just an indicator you might consider running it at a lower voltage. I tested a lot of LEDs to select the best load resistor and found that most LEDs with a suitable resistor will operate below their rated current at 32V and yet still operate with good brightness and much reduced current at 4V. Get hold of a bench dc power supply and a selection of resistors/LEDs and find a pair that does what you want; select-on-test used to be the way we made up circuits many moons ago!.

If you read the manufacturers specification you will see that LEDs are fairly short lived if you run them at the rated current so this is another consideration - how long will your piece of kit be used and what do you want from the LED.

Hi from Sleepy.

I do not know your precise requirement.

I would not accept a requirement for both a constant voltage and a constant current.

An LED is , in efffect, a diode and a look at the V/I Characteristic and the lumens vs Amperes and lumens vs voltage characteristic reveals that the output lumens will be dependent upon the current through it, the voltage will have a considerable effect but it will be difficult to control luminosity and life through voltage, use that as just a an absolute maximium. but it will be easier to stabilise the LED with a constant current drive.

LEDs often require a constant curent drive in order to maintain stability of lumen output; and to keep the LED below the rated maximum and a life rated maximum - ask the supplier. Read the Philips information if nothing else is available.

Good Luck

Sleepy

Dear all,

use a LM317 regulator. Connect 6V to input terminal. between output terminal (pin 2) and the third pin connect a resistance

(1.25/0.35)ohms about 3.3 ohms of 0.35x0.35x3.3 watts.

This gives constnt current. Resistance may vary in the commercila products but small current variations are tolerable with LEDs. This is a practical circuit used by me.

written by

shreeramachandran,

AGM Telecom(REtd)

04428474099

sudarsanajyothi@gmail.com

http://www.ledsmagazine.com/features/4/8/4