[dorkbotpdx-blabber] ARRGGHH - Please check this circuit

[yiyi]

oh, also the 5.6 ohm resistor  referenced below was based on a 3.5
volt current drop and 300 ma, not sure exactly what yours is.

On Mon, Jan 12, 2009 at 10:30 PM, yiyi <hijinx at x0r.org> wrote:
> 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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>>
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