> -----Original Message-----
> From: Olin Lathrop [SMTP:olin_piclist@EMBEDINC.COM]
> Sent: Wednesday, September 03, 2003 12:39 PM
> To:   PICLIST@MITVMA.MIT.EDU
> Subject:      Re: [PIC]: 4 7-seg displays
>
> > What's a good transistor to use to drive Common Cathode 7 segment
> > displays?
>
> Almost anything will do since you're only asking for 20mA (probably, check
> specs) and 5V.  You want NPNs for the cathodes and PNPs for the anodes.  I
> use 2N4401/4403 for these kind of jellybean applications.  You can get
> bags of 100 cheaply from Jameco.
>
The current depends on the number of segments lit, and how bright they need
to be.  For all segments lit, a 20mA common cathode current is only 20/7 =
2.8mA per segment, which seems very low, especialy if the displays are
multiplexed.

Mike




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