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Topic: PWM 3V 200mA High Power Led from 3v soure (Read 1 time) previous topic - next topic

cio74

Mar 18, 2013, 10:03 pm Last Edit: Mar 18, 2013, 10:05 pm by cio74 Reason: 1
Hello,

Besides an raduino board I have a 3v 600mA power source and a 3v 350mA high power LED. I plan to run the led at 2.5v only and 200mA using PWM. Not sure if 2n2222a is okay to drive the LED,  would it run hot?

From what I have read I can't use any MOSFET i.e. IRF510 or 2n7000 due insufficient Vgs voltage.

Any suggestion to how to drive the LED

Thanks.

Grumpy_Mike

Quote
I plan to run the led at 2.5v only and 200mA.

You do not have any choice at what voltage you run an LED at. It all depends on the materiel is is made from. For a power LED like this you need a constant current supply. You can set this to 200mA if you want.
So first of all you need to know what the Vf ( forward voltage ) of your LED is to see if 3V is enough even to start it glowing.
Anyway do you mean 3V or 3.3V which is more normal.

cio74

I have a tested the LED already, it's fine at 2.5V it takes 200mA and it's bright enough. At 3.3V it takes 700mA, don't need it.


Grumpy_Mike

Quote
I have a tested the LED already, it's fine at 2.5V it takes 200mA and it's bright enough

So you are planning to run it without any current regulation?
If so please know that this is very wrong and bad and you should not do it.

cio74

I would definitely use current limitation, at this moment my concern is how to PWM drive it from the 3V source.

Grumpy_Mike

PWM is trivial you just apply it to the constant current driver.
Your first job is to build / buy the constant current driver.

Start by looking here:-
http://www.pcbheaven.com/userpages/LED_driving_and_controlling_methods/

cio74

Not sure if I asked it properly...

Is there any way to drive a 3V led sinking 200mA from a 3V power supply using a microntroller which can at best source 40mA?

Apparently the only way is using a buck converter to ste-up the voltage and then use a CC driver?

Thanks.

Grumpy_Mike

Quote
Is there any way to drive a 3V led sinking 200mA from a 3V power supply using a microntroller which can at best source 40mA?

Yes there are many ways as shown in that link.

You need some external components you can not do it without.

These components will be transistors and resistors and possibly an op amp.

There is no need for a buck converter if your maximum voltage needed is less than the supply voltage you have.

Headroom



Apparently the only way is using a buck converter to ste-up the voltage and then use a CC driver?

Thanks.


A buck converter is a step down converter, not a step- up converter.
http://trippylighting.com

http://ledshield.wordpress.com/

cio74

Any example that I can use as a startup point, on that web page all is pure theory.

Thanks.

mjkzz

Sorry, read the wrong data sheet ... but take a look at MOSFET FDS6690A datasheet, Fig 1 and Fig 2. It is a SOIC 8 package, however.
Search for "mjkzz" on eBay :-)

cio74

FDS6690A looks fine at 3V, I have to do some research in here, thanks for the tip.

Grumpy_Mike

Quote
Any example that I can use as a startup point, on that web page all is pure theory.

Now you have me puzzled. I don't know what you consider pure theory but that site is exactly the opposite.
As a start you can take the last diagram on this page:-
http://www.pcbheaven.com/userpages/LED_driving_and_controlling_methods/index.php?topic=worklog&p=5
and from the collector of that transistor connect it through a 1K resistor to a PWM output of your arduino.

cio74

Mike, the trouble I have is due the formula:

VM = VDD - Vf_LED - 0.7

now, Vdd=3v, vf_led=2.5v

VM=3 - 2.5 - 0.7 = -0.2v

How this would ever work? This is the reason I have asked in first place if it's possible at all. I think I'm missing something in here?


Grumpy_Mike

Sorry but where is that formular from and what is VM?

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