I have in mind a pure theoretical clue, but I doubt it will work in real life. Suppose I connect the led to the supply without anything else, and measure voltage and current used. This allow me to calculate the "equivalent resistor" of the led. After I connect a P channel mosfet on high side with the calculate resistor to ground and the gate in parallel with the switching N mosfet. In this way the load seen by the PS should be constant.
Anyway I really doubt that will work, and even if is working, will be a waste of power and money. A dimmable PS is the right way to go!
DrDiettrich:
If these are really simple LEDs, you need an adjustable current source. A FET or transistor by itself, driven by PWM, will not act as a current source.
Have a closer look. OP is using a mains powered 300mA constant current source.
With a string of 10 LEDs (should be 11 or more for this supply).
The supply is likely to have a fit during PWM-off periods.
OP should have used a constant voltage supply, e.g. 12 or 24volt, and PWM-able constant current drivers.
Leo..
Thanks all
Leo i have a 24v 1A switching supply can i use it?
If yes, how my setup should be changed for that?
As I googled ,can i use lm3404hv with unos pwm?
Is there a way to change that constant current led driver to constant voltage supply?!
abrisham:
Leo i have a 24v 1A switching supply can i use it?
If yes, how my setup should be changed for that?
As I googled ,can i use lm3404hv with unos pwm?
Is there a way to change that constant current led driver to constant voltage supply?!
Yes. Two strings of five (or six) LEDs. 300mA through each string.
You need a constant current driver with PWM input. I use the PT4115. There are many others.
Complete boards are available on ebay. This one can probably be modded if you know what you're doing.
Don't know. Never used that chip.
Probably not. Small chance if you post the circuit diagram.
Leo..
1).If i split that string in to 2*5 led the current will drop to 500mA and voltage to 4.8 v for each led is it correct?
2).If yes what happens to led with that extra current (200mA) and voltage about 1.5 v??
3). if one of 2 fails suddenly the other ones could be damage by heavy amp and volt... is there any way for protection?
No. 24volt supply, two strings of five, two 300mA constant current boards, two Arduino PWM pins.
Those LEDs have a Vf (working voltage) of about 3.3volt each.
300-350mA is all a 1watt LED can take.
No. If one fails open circuit, the whole string will be off. If one is shorted, the constant current board will still send 300mA through the string.
Leo..
after all your valuable help i decided to go with LM3404HV for controlling brightness by my UNO ...
but i need your help again..
i have read that data sheets of LM3404hv
first one was manual and second one was Evaluation Board...
in Evaluation board paper a table and schematic and its parts was presented and of course a part for changing current ... in addition i found a online calculator for that ic ...But the problem is parts i found in that calculator is totally different from what i saw in evaluation paper...
as you know i have a string of 10*1w led
Vf = 3.5
I = 300mA
i planned to buy a new voltage supply 48V 1A...
please help me for right pars and design..
again thanks to all
How do you intend to build your circuit? You can buy 2 evaluation boards, or all components to build your own current regulator circuit. If you build your own circuit, you are responsible for making it work properly.
