how to conversion constant current to constant voltage led driver for pwm control?

[Amindarvish]

hello to all
i want to control brightness of led series with constant current power supply.
but when i connect the power supply output to pwm controller and led series, i cant control the brightness because power supply has not fix voltage on output.
while pwm output is 255 (high) led work fine, but when the pwm output is 200 or less (low) my led start to blinking.

i think conversion constant current to constant voltage led driver for pwm control, but i havent any info for this.

please help me.:?::-(

is there any way for solve this problem? any idea?:roll:

thanks for help
best regards, amin

 

[barry]

Your constant current power supply doesn't have a fixed voltage output? Why does this surprise you, IT'S CONSTANT CURRENT!! Why are you using a constant current power supply to begin with, shouldn't you use a constant voltage supply?

What does "pwm output is 255" mean? 255 what? parsecs? doorknobs?

What do you mean about converting constant current to constant voltage, and what does this have to do with controlling the brightness of an LED?

I think you need to define your problem a little more clearly, I'm not sure you have a grasp of what you are really trying to do; I sure don't. And I have no idea what that picture is supposed to tell us.

 

[Amindarvish]

sorry for ask bad question.
this is schematic of my controller with atmega8 avr:

Your constant current power supply doesn't have a fixed voltage output? Why does this surprise you, IT'S CONSTANT CURRENT!! Why are you using a constant current power supply to begin with, shouldn't you use a constant voltage supply?

my lighting project based on constant current and i bought many pcs of this driver.
now, i want to control my leds with constant current by pwm. i cant change driver to constant voltage, because my leds in series circuit.
not economical for change it.

What does "pwm output is 255" mean? 255 what? parsecs? doorknobs?

my purpose was change pwm duty cycle, like this:
while duty cycle is 99% or less my leds start blinking

What do you mean about converting constant current to constant voltage, and what does this have to do with controlling the brightness of an LED?

my purpose was add other circuit on output for fix voltage of led driver and just in my mind, i dont know this possible or impossible !!

thanks for help me barry, again sry for ask bad question:grin:

 

[barry]

Ok, I think I understand now. First of all, I'm not sure this will work. Your driver is trying to put out a constant current-what happens when you break the current path (turning the mosfet off)? I would guess the voltage out of the device jumps to maximum in an attempt to maintain the constant current. Then, when the mosfet turns on again, the voltage will drop as the current increases. What is the response time of that current supply? My guess is you don't know, and I sure don't. (BTW, I'm not sure what that bipolar transistor does, but that's just my limited analog skills).

Maybe you can try this: put the mosfet in PARALLEL with the LED chain. (Put a resistor in the path so you limit the maximum current). Thus, you will steer some of the current away from the LEDS and through the mosfet+resistor. As the duty cycle goes up, less average current will flow through the LEDs.

 

[Amindarvish]

hi, thanks for help
i did it this video is result:
https://youtu.be/QaHoUy2UO4c

any idea for this?
thank you barry

 

[barry]

I'm not sure what that's showing, other than how to give somebody a seizure. It looks like you are turning the power on and off about once a second. What am I supposed to determine from that video? Are you using the circuit in your original schematic? What's driving your mosfet?

 

[Amindarvish]

thanks barry for reply

Are you using the circuit in your original schematic? What's driving your mosfet?

yes,and my mosfet switch the negative wire of constant current to leds.

please tell me how to change my schematic for work my project, any idea on your mind?

thanks a lot

 

[barry]

I think I already suggested putting the mosfet in parallel with the LEDs. See post #4

 

[Amindarvish]

I did it and dont work fine.
thanks for helped.bye