> Thanks for a very informative answer Russell,
>=20
> I'm driving 12V LED load using Arduino PWM. While looking for transistor
> suggestions and before reading your post, I used MPS2222A (Motorola TO92
> 2222) from by parts bin.
>=20
> The current draw from the LEDs (with PWM) was 135mA @12v. I determined th=
e
> wattage to be 1.62W. Ta < 25C.
> I used a 1K base resistor and at 100% duty cycle, the base current was
> 3.8mA (hFE ~100)
> At 100% duty cycle the collector current was 136mA
> At 50% duty cycle the collector current was 67mA
> http://i.imgur.com/E8IjHhq.jpg
>=20
> Although MPS2222A in a TO92 package can only sustain 625mW dissipation, i=
t
> was not running hot. I ran it >24 hours and I saw no heat issues. So you
> are right that small transistors (TO92) like BC337 can be used.

You have to understand that the wattage you are stating (1.62W) is over the=
=20
LEDs and not the transistor. The current through the transistor is the same=
 as=20
through the LEDs but the voltage over the transistor is much lower. The vol=
tage=20
over the transistor is the saturation voltage between the collector and the=
=20
emitter and it should be less than 0.5V. 135mA * 0.5V =3D 67.5mW. The volta=
ge is=20
likely even lower. Just make sure that the transistor has enough base curre=
nt=20
to keep it saturated.

/Ruben



=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D
Ruben J=F6nsson
AB Liros Electronic
Box 9124
200 39 Malm=F6 Sweden
www.liros.se
Tel +46 40142078
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D

--=20
http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist
.