I just checked the TI & Fairchild datasheets -- interesting that you work with 
the "absolute maximum rating" of 35mA.   Though the Fairchild does not 
specifically state a normal high and low output current, the TI datasheet 
specs this as "+/- 6mA output drive", but also states an absolute max of 
35mA.

I asking this because it's piqued my interest for future use.  I usually have 
a self-coded PWM routine to allow dimming of the LEDs from 1/9 to 9/9 
duty-cycle, and on a newer experiment, I'm trying to see if I can easily dim 
different LEDs to different intensities.  If I can swing the current I need 
when at 100% duty cycle, I'm trying to think of how intense the data output 
(from the PIC to the shift register) could be to do software dimming.  I also 
see it has an output-enable pin, which could be used for global dimming (at 
the cost of 1 extra pin), but not for multiple-level dimming.  I'll have to 
think about this some more...

Cheers,
-Neil.



On Friday 27 May 2005 01:54 am, phil B scribbled:
> The Fairchild 74HC595 datasheet specs a max current of
> 35mA per pin though I wouldn't get anywhere near that
> with out a pretty gentle duty cycle.  The dip package
> has a 600 mW limit, and the SO's is 500 mW.
>
> I've run a number of tests on it because I was
> concerned.  With all 8 outputs constantly driving LEDS
> at 10 mA for 24 hrs there was very little heat
> increase (a couple degrees F) so the 500 mW limit may
> be conservative.  This was with both Fairchild and ST
> parts.  Any kind of <100% duty cycle is going to help
> here as well.
>
> One thing to be carefull of is inrush current and
> spikes on the clock and serial in pin.  I use a 1K
> limiter and 100K to ground on both RCLK and SERIN.
>
> Phil
>
> --- PicDude <picdude2@narwani.org> wrote:
> > On Thursday 26 May 2005 04:30 pm, phil B scribbled:
> > > My favorite answer is a 12F509 + 2 74HC595s (to
> >
> > drive
> >
> > > up to 16 LEDs) but I suspect thats not what is
> >
> > being
> >
> > > looked for.
> > >
> > > Phil
> >
> > Doesn't the 74HC595 not have the current output
> > capability of a PIC?  IIRC it
> > was something like 5-6mA for the 74HC595, which IMO
> > is not much for driving
> > LED's properly.  But of course, it depends on the
> > specific LEDs and
> > application of the OP.
> >
> > Cheers,
> > -Neil.
> >
> >
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