At 05:04 PM 9/2/2003 -0600, you wrote:
>What's a good transistor to use to drive Common Cathode 7 segment displays?

With n-digit CC displays the entire digit current flows through each digit
drive (sink) transistor for (1/n) of the time, assuming straightforward
multiplexing. For example, if you're driving 15mA/segment, plus a DP,
that's 120mA with all segments (and the DP) on.

You thus need a decent NPN transistor or MOSFET (eg. 2N/MMBT3904, 2N/MMBT4401,
2N7000/2N7002). Be sure to give it enough base current/voltage to keep it
well saturated/low Rds(on). Vce(sat) is typically guaranteed at a forced
beta of only 10.

You may be able to drive the segments directly from a PIC, or pretty
much any other PNP transistor (2N/MMBT4403, 2N/MMBT3906) or P-channel
MOSFET ** will do. I prefer to use *Common Anode* displays because CMOS
chips including PICs are better (lower voltage drop) at sinking current
than at sourcing it, and that gives you more brightness with minimum
parts count.

** There's not as much competition in the market for small p-channel
discrete MOSFETs, so the price is much higher for similar specs to the
2N7000/2N7002 types, which are now under a nickel a reel at a time.

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

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