Re: Newbie transistor question
- From: Tim Wescott <tim@xxxxxxxxxxxxxxxx>
- Date: Mon, 29 May 2006 09:17:37 -0700
realexander wrote:
Hi,A 0.6 or 0.7V drop from base to emitter is to be expected -- if you're seeing 4.3 or 4.4 volts on the emitter (assuming 5V on the base) and rounding down then your transistor is working as expected.
I'm having trouble getting a transistor to work the way I thought it
should, and I hope someone could help me.
I'm using a PIC to drive common anode 14-segment LEDs. Since the PIC
I/O line can't supply enough current, I'm trying to use a 2N2222
transistor instead. The PIC directly drives the 2N2222's base (no
intervening resistor), the collector is tied to +5 and the emitter to
the common anode. The problem is that, while the LEDs light, they are
very, very dim - only visible in a dark room.
I thought maybe that the PIC wasn't saturating the transistor, so I put
in a pull-up resistor on the base. No difference. I connected the base
directly to +5, and the LEDs were still dim. The emitter shows +4V (5
volts in the collector, 4 volts out the emitter??!!) If I tie the LEDs
common anode to +5, they light up nicely, but of course I can't
multiplex a bunch of LEDs with the anodes tied to +5.
Am I using an inappropriate transistor? Am I using it wrong?
Thanks,
Bob Alexander
What are you doing to limit the current of your LEDs, and how are you switching the cathodes of the LEDs? I suspect that with the voltage drop on the transistor your current limiting resistor is now too large, and your LED current is insufficient. You can probably get your current back up either by using lower resistance current limiting resistors, or by using a PNP transistor in common emitter. The PNP in common emitter will only drop 0.2V or so -- connect it emitter to +5V, collector to the LED, resistor between PIC and base, and remember that pulling the base _down_ will turn the transistor _on_.
--
Tim Wescott
Wescott Design Services
http://www.wescottdesign.com
Posting from Google? See http://cfaj.freeshell.org/google/
"Applied Control Theory for Embedded Systems" came out in April.
See details at http://www.wescottdesign.com/actfes/actfes.html
.
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