Hi everyone
I have project for my vivarium that i want to make a power led arrays (10*1w) to simulating sun rise and sun set effect by PWM form arduino uno.
I have used irf512 and tip122 …
every thing is OK till the PWM value decreases to 0-15 range from 255 step, in that range leds start to flashing (rapidly on & off ) …
i have tested the setup with a simple 13-18*1w driver …
Please help me…
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Hi abrisham,
You need to provide a lot more information about your system before someone can help you out.
The IRF512 and TIP122 are transistors and not stand alone current control devices for LEDs. You make mention of a simple LED driver but you don’t say what it is and how you have it connected to your Arduino. Show us exactly what you are using and how you have it all connected as well as how you believe it should be operating. We can go from there.
-Bill
What is your PWM frequency? It sounds like it is too low.
/mike
thank you all
i will attach a schematic of setup and photo of driver label…
hopefully it helps 
the other option i have found is using LM3404HV IC but i have problem in design and parts value…
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 
This not a good or safe way to control the brightness of your LEDs. By effectively breaking the circuit with your transistor, you interfere with the driver’s current control loop. This can have the side effect of high voltages being present on the LED string which can be dangerous if you don’t know what you’re doing. Without knowing the architecture of the LED driver (isolated vs. non-isolated, high-side vs. low-side current sensing, etc.), you could also be causing excessive current through the LEDs since you have another GND connection through the Arduino which may be bypassing the current sensing resistor in the driver.
What you are seeing at the low end of your dimming range is most likely the result of the LED driver trying to return to regulation after being interrupted by your transistor. The time constant of the driver’s startup time is likely much longer than the period of your PWM which is causing the odd behavior.
I would highly recommend you not try and dim your LEDs in this manner and look for an LED driver that has an external dimming input. These typically send a signal directly to the driver IC which allows the system to be dimmed in a controlled and safe manner. It does not appear that external dimming is supported by the driver you have.
-Bill