PIC pin via resistor to base of 2N2222

Emitter of 2N2222 to ground

Collector of 2N2222 via resistor to base of PNP power transistor

Emitter of PNP power transistor to +V

Collector of PNP power transistor is your output

Driving the PNP into full saturation your output voltage will be +V - Vcesat

Select R's to ensure full saturation of both transistors

You may also want a resistor between PNP base and +V to ensure it remains in
cutoff when 2N2222 isn't driven.

Bob Ammerman
RAm Systems


----- Original Message -----
From: "Dave King" <kingdws@SHAW.CA>
To: <PICLIST@MITVMA.MIT.EDU>
Sent: Tuesday, February 12, 2002 7:50 PM
Subject: Re: [PIC]:interfacing power transitor


> >Do you have a higher supply voltage available?  Most FETs - even logic
> >level FETs - will not fully enhance below 4V.
> I actually compromised and went with 3.3V on the board. I thought that
> was a good thing as the per board cost dropped from about $47 to about
> $7. I didn't think I was going to give myself a stroke getting these #$%
> transistors to behave.
>
> >You can use a Darlington bipolar transistor: one NPN part I use is
> >TIP121.  Feed the base with a 220R resistor (Ib ~ 10 mA if Vdd=3.3V).
The
> >collector will handle 2 Amps easily - but with a fairly nasty saturation
> >voltage of about 1V.
>
> I'll see if that would work with this. I'm sure there is some part that
> will but
> I've not stumbled across it yet.
>
> >If you DO have a higher supply available, use it to turn on a MOSFET.
For
> >example, where is your 3.3V derived from?  Can you use that?
>
> I have a smart smps as I don't know exactly what my power is going to be
> out in the field. So all I can count on is the 3.3.  I've been debating if
> I need
> to scrap the 3.3 and go back to 5vdc but then I need to start adding back
in
> parts.
>
> TNX
>
> Dave
>
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