Given these LEDs cost around $30 I understand your concern. The amount of drift is somewhat exagerated however, but if you come back with a power supply we can design around it with a constant current chip.

I recommended you read this article, the difference between a 300ma LED and a 20 ma LED is basically one of scale, the basics are the same. I still recommend you read it for that reason. It will answer most of your questions about LEDs, I think.

LEDs, 555s, Flashers, and Light Chasers

 

Hi,

[...] Led driver circuits have high price about 5-6 dollars for 1.4 watt power led driver. i cant understand that if it was so easy why people buy special led driver circuits? i cant solve this.

How much do you have to pay for a ~4µF Class-X non-polarized capacitor (which is the single most expensive part you'd need for an off-line capacitive voltage dropper supply)?

 

1-is there any cell phohe charger which can give more than 300 ma output?
if there s like this cell phone charger so i can ask more.
forexample i have 6V, 9V or 12V cell phone charger.
my led properties: 3V nominal voltage, 350ma current.
So how can supply constant current? cause resistance of led will be decrease soon later.

 

Again, read the article! LEDs do NOT have resistance, they drop a constant voltage. It is a defining characteristic of LEDs, the parameter is called Vf. You have your head full of apparent facts, most of them wrong from what I can see. Otherwise I and others are going to be giving you this information piecemeal, and it doesn't look like your going to believe what we say anyhow.

It would help if you started from a position of common knowledge, then we could discuss this from there.

 

1-is there any cell phohe charger which can give more than 300 ma output?
if there s like this cell phone charger so i can ask more.
forexample i have 6V, 9V or 12V cell phone charger.
my led properties: 3V nominal voltage, 350ma current.
So how can supply constant current? cause resistance of led will be decrease soon later.

There are thousands of them. If you include MP3 player chargers also, you get them starting from about 3.6V at 150mA to 1500 and above at over 8V. You can get branded, chinese or local ones. For higher voltages you will need to go for chargers of other devices like torches or even can use transformer based adapters. For a 3V led it is better to use a 3.6V charger at near 350mA and use a current limiting resistor for minimum heat dissipation.

 

For a 3V led it is better to use a 3.6V charger at near 350mA and use a current limiting resistor for minimum heat dissipation.

I'd have to disagree with that statement. You need at least 5-6VDC, to have enough excess voltage to control and regulate. Think in terms of a LM317 for example. Otherwise you will have to somehow boost the voltage or current. Remember, the typical Vf for these kinds of LEDs is 3.4-3.8V, with a lot of variation within the family being the norm.

I agree that most chargers meet the spec of 350 ma easily though, this is not a problem. Wall warts and chargers are both common and cheap.

 

I'd have to disagree with that statement. You need at least 5-6VDC, to have enough excess voltage to control and regulate. Think in terms of a LM317 for example. Otherwise you will have to somehow boost the voltage or current. Remember, the typical Vf for these kinds of LEDs is 3.4-3.8V, with a lot of variation within the family being the norm.

Oh, I meant that voltage just in case you are using a resistor for the drop to make things simpler. If it was regulation, we need 3+ Volts than Vf. If he needs precision load regulation with regulators it is a requirement to have 6-8V input. But when we use resistors alone, we try to keep the voltage as low as possible to Vf to minimize loss, right?