At 02:04 AM 9/13/2005 -0700, you wrote:

>On Sep 13, 2005, at 12:34 AM, Russell McMahon wrote:
>
>>Iled ~= (Vdd - Vled - Vgs)/ 220r
>
>Huh?  Vgs doesn't enter into the LED current.  Vds does, but the FET
>is basically ohmic if Vgs is past threshold:
>         Iled ~= (Vdd - Vled) / (220 + Rfet)

Yabbut, Vgs(th) for only 1mA can be anything from 0.8V to 3.0V,
and *typically* to get 100mA anode current you need at least 4V (see
curves on datasheet). Some current will flow with a red display, but the
display will be very dim and probably very uneven.

>But we're in violent agreement.  My initial reaction was that it was
>just wrong, and then on further thought I decided it would probably
>work anyway (but don't put a blue display in there!)

I suspect that a blue LED display will give much more consistent results.
;-)

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
->> Inexpensive test equipment & parts http://search.ebay.com/_W0QQsassZspeff


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