Re: Newbie transistor question


"realexander" <realexander@xxxxxxxxxxxxx> wrote in message
news:1148916666.693697.248590@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
Hi,

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


The emitter being at 4V is about right. It can never reach the collector's
voltage in the configuration you've got it in (common collector).

If 4V isn't enough to light the LEDs then it may mean that either the LED's
current-limiting resistor is too big, or (somehow) there are two LEDs (in
series) per segment.

If you rearrange your circuit to be common emitter configuration then you
can get the emitter and collector voltage to be closer together, during
saturation. If you use an NPN, in the common emitter config, then you'll
have to make sure that the PIC can source enough base current (through an
appropriate series resistor). If the PIC can't source enough current, but
can sink enough current, then you'll need to use a PNP device.

It seems that you do understand the basics. You just need to dig a little
deeper to find the root of the problem.

Bob


.

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