Catching up on this thread, and I have used the 2N7000 as a replacement f=
or=20
the PN2222A to drive the LED commons.  With this arrangement, I eliminate=
d=20
the resistor from the PIC output to the transistor base.

But now, I'm wondering if I can, and if it would be wise, to replace anot=
her=20
NPN transistor with this...

- Existing arrangement is 12V signal (on or off) to 10k resistor to base =
of a=20
PN2222A, emitter to ground, and collector to PIC input, using internal we=
ak=20
pull-up resistor.

- Proposed arrangement would be 12V signal directly to mosfet base, sourc=
e to=20
ground, and drain to PIC input, also using internal weak pull-up.  I'm a =
bit=20
uneasy about the 12V directly to the mosfet base, but it would be great t=
o=20
eliminate the resistor if it works well.  Switching frequency is <100khz=20
IIRC.

Cheers,
-Neil.




On Wednesday 09 July 2003 11:31, David VanHorn scribbled:
> > I
> >understand the heating during changeing but it should be better than a
> >regular npn or pnp transistor, right?
>
> Yes and no.
> The FET can have an extremely low on resistance, but only if it's turne=
d on
> completely. This requires sufficient gate voltage, and varies with the =
S-D
> current.
>
> > The info I found on the web says that fets are faster than regular
> > transistors.
>
> Not really. Again, you have to drive the gate hard enough, or the fet's
> speed won't matter.
>
> > I should not need a to220 package, but maybe a to92. How do I pick on=
e
> > that will work with logic? I use 5v or 3v supply for the PIC.
>
> Look at the gate threshold voltage spec.
> The 2N7000 is a popular device with 5V logic drive.
>
> 3V will be more challenging.

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