> If you have no experience, you get some by measuring typical parts.  If
> this is a "one off" for yourself, guess a little, calculate a little,
> measure a little, fix a little.  If this is for commercial purposes, all
> the answers are different.


This is for hobby purposes, but I'd like to learn how to do it as close as posible
to a "production unit".


> >?Is there any way to know this from the curves or other parameters?
> >Also, to be sure there is not excess current I should assume a minimum
> >Vce(sat) and not a maximum ?is this right?
>
> Yes. The data sheet usually has enough information to get a working design.
>  But sometimes not.  You must supply additional information such as what is
> the range of temperatures it will work in, what range of voltages, etc.


Well... I'll rather try to get a "not so good" production unit :)


> the next consideration is what the temperature rise is of any of your
> parts that dissipate lots of heat.

This is another thing I'm concerned with.
How is power calculated in a transistor? Something like...?:

    P~=Vce(sat)*I=0.25*0.24=0.06W ???

Asumming Q2 dissipates 60 mW, How can I get a rough approximation of temperature rise?
In page 1 of the datasheet there's a suspicious parameter called:
Thermal Resistance, Junction to ambient = 357 :C/W

0.06W * 357:C/W = 21.42:C

Did I calculate something useful (eg: temperature rise)? :)



All your answers were very helpful. Thanks!

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