I got some nice 12V led strips for my aquarium.
and I want to have them automatically fade on and off.
I was thinking of using a transistor (P2N2222AG) and the PWM functing of the arduino to be able to do this.
however other people who build similar systems seem to use constant current drivers.
Can someone explain to me what is better and why that is? How would you control brightness with a constant current driver?
You did not give the specifications (current rating and voltage drop) required for a specific answer, but since you said it is a '12v' strip I am assuming it is an automotive type LED string.
In which case the answer is YES... and No.
Yes, your Arduino can do this.
Just not with a 2N2222, rated at only 1/2 watt. Even a very short LED strip may consume 2.5 watts of power, while a 6 foot strip may consume about 15 watts.
The result would be a Krispy Quad Two.
A logic level power FET would be a much better choice of transistor.
Since you will be using a much more stable power supply than an automobiles so-called 12 volts, the constant current driver is just an extra expense and complication.
How would you control brightness with a constant current driver?
By controlling the amount of current the supply delvers. A constant current supply provides the set current independent of the impedance of the load (up to a pre defined maximum voltage limit). So you control the "set" current. You can also PWM the current, that is the on / off cycle time.
Constant current supplies are needed for high power LEDs because the traditional current limiting resistor value would be too small to allow a stable current to flow due to the large thermal variations in the on voltage doped across the high power LED.
The result would be a Krispy Quad Two.
Um, no, not necessarily. You've misunderstood the ratings and how they're used.
That 1/2W isn't about the switched load: it's about dissipating heat from the transistor, itself.
Taking your first example of a 2.5W LED strip: that's about 200mA. Which would mean dissipating about 150mW (200mA * .7V) from the 2N2222. That wouldn't even require a heat sink if there was some air flow around the transistor.
The 2N2222 can, theoretically, handle up to about half an Amp, though it's not a good idea to operate it near that limit if you're not an expert (personally, I'd start thinking about moving up to a heftier transistor somewhere around 200-250mA. But I'm a wuss when it comes to having the electronic parts of my projects getting too hot to safely touch: my fingers blister easily. ;D I also hate desoldering parts when my calculations of peak power are slightly off, and the magic smoke leaks out of the transistor).
Ran
Thanks
There was no current specification, but they are no cree or other supper powerful led's. below is a picture of the strips (not much use without specs but it gives an idea anyway)
I want 2 of the 6 led strips like the picture and 2 similar "strips" with only 1 led each.
together maybe 250-300mA.
I guess I try the resistor since i have that already. and ordering led drivers will late time and $$.
