At 12:46 AM 3/25/04 +0100, you wrote:
>    I'd like to control the LED backlight of an LCD using the PWM output of a
>PIC,
>but I know very little of electronics and need some help to get the correct
>values
>for the circuit.
>
>First, to calculate R3:
>
>    R3=(9-Vce(sat)-Vled)/0.24
>
> First problem: How do I know Vce(sat)?.

Experience.  ;)

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.  You cannot have production people do all those
"little" things on each unit or it will cost too much.  You cannot have
failures so you must decide which things are important and bias your
circuit in a way that works with all expected ranges of parts.

>?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.

>And I should also assume minimum Vled voltage...
>
>Assuming
>    Vce(sat)~=0V
>    Vled=3.8V
>
>    R3=(9-0-3.8)/0.24~=21.7 ohms
>    and the power disipated would be P(R3)=1.248 W
>    So a 24 Ohm, 2W resistor will be ok, ?right?

Probably good numbers for a first test.

>Now, for saturation of Q2 the minimum base current should be:
>
>    Ib~=Ic/hFE=0.24/50=4.8mA
>
>Again, I took the minimum hFE in the datasheet as it depends on some
>parameters...
>How should I get it?

Bracket. Ideal part: zero volts. Typical worst case: 1 volt.  My guess
here: 100mv to 250mv. Depends on other things I don't know.

>And how do I get Vbe?

Theory.  Approximate typical Vbe: .6 to .7 volts; typical worst case
design: 1 volt.

>Well, enough for a post. I think I can get the other values If I know the
>answer of
>what I already asked.

Your work will be tested based on providing enough backlight to do the job
- probably a wide range is ok. Especially if you can adjust it.  If you do
this, the next consideration is what the temperature rise is of any of your
parts that dissipate lots of heat.

You are doing an excellent job of starting this design.  Take notes and
measure the results so next time you will have that magical thing: experience!

Good luck!
Tom

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