> I spec'd a 12VDC (nominal) wall wart, which can be
> as high as 16V with a light load.
> ...

Ahhh... this I did not know.  I would've just dropped
a zener in there or something, so I'm dealing with
less unknowns in the ckt.

Cheers,
-Neil.



-----Original Message-----
From: pic microcontroller discussion list
[mailto:PICLIST@MITVMA.MIT.EDU]On Behalf Of Spehro Pefhany
Sent: Tuesday, July 16, 2002 8:08 PM
To: PICLIST@MITVMA.MIT.EDU
Subject: Re: [OT]:20ish led lamp


At 05:21 PM 7/16/02 -0500, you wrote:
>Spehro,
>
>5 * 1.15V = 5.65V.
>9V - 5.65V = 3.35V across the resistors.

Hi, Neil,

I spec'd a 12VDC (nominal) wall wart, which can be
as high as 16V with a light load. Since the actual
output voltage is not really known very well, and
the LED Vf isn't known for sure, I chose to burn
at least half the power off in the resistors (not
much, really, the wall wart will lose much more than
that from core losses), and run the LEDs at a
conservative current. That way if the wall wart puts
out 15V (quite plausible), you have 50% over nominal
current, but no smoke and it works reliably, assuming
it's not mounted in a black housing under the Phoenix
sun. Given a regulated supply, and a data sheet for
the LEDs that included tempco of Vf, I would call it
much closer, but probably still burn 30% off in the
ballast resistors unless active current regulation was
used. I'd go higher on the nominal current, assuming
the maximum Ta wasn't too bad.

It's just a place to start that won't burn out another
100 of his LEDs, assuming it's connected right. ;-)

Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the
reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com
9/11 United we Stand

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