[dorkbotpdx-blabber] ARRGGHH - Please check this circuit

[yiyi]

Are you powering 3 1w leds in serial off a 5v rail like in the
circuit? If so, then in my experience each 1 watt led is about a 3.5v
voltage drop, which means you actually need about a 10.5 volt rail
(more likely 12 volts with a resistor). if you only have a 5 volt
rail, then once they get about half on, they don't get enough power
any more to shine.

the reason for you burning them out when you tried with a 12v power
supply before was perhaps an inadequate r3 which would end up allowing
too high of a voltage to run through the leds.

also, i'm no maffs star, but this calculator says you should be using
5.6 ohms instead of .56 ohms:
http://led.linear1.org/led.wiz

On Mon, Jan 12, 2009 at 10:04 PM, Ron Jackson
<Ron at fascinatingelectronics.com> wrote:
> Hans,
>
> The problem is probably the transient that occurs when the LED's fire. With
> a fast scope and short ground connection you will probably see a short,
> maybe sub-microsecond, voltage drop that causes your microcontroller to lose
> it.
>
> I think ATX supplies have a STANDBY+5 supply that is independent of the main
> 5 volt rail. I would try using that to power the microcontroller and
> associated logic and use the high current supply to drive the LEDs.
>
>  -- Ron
>
>
>> Hans Lindauer wrote:
>>>
>>> Yes, R3 is really .56 ohms.  I'll try checking the rail voltage as I
>>> increase the PWM - that's a good idea.  I have a friend who's going to let
>>> me borrow a LabView DAQ so I can try to figure out better what's going on,
>>> but the DVM should give me some idea.
>>>
>>> One of the reasons I wanted to explore using 12V, was so that I could
>>> place a 7805 regulator between 12V and the logic portion of the board, with
>>> the idea that it would help to isolate it further while providing lots more
>>> overhead.
>>>
>>> It seems like the power sags more when I strobe the LEDs to full power
>>> than when I leave them on, so maybe the PSU has too slow of a response?  I
>>> can definitely see the Dorkboards' power LEDs go dim when the LEDs flash.
>>>  To address Mykle's concern, I am pretty much right at the upper limit of
>>> current draw for this PSU.
>>
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