Re: how to control LED array? (follow-up)

"Ban" <bansuri@xxxxxx> wrote in
news:maSle.952901$b5.41512298@xxxxxxxxxxxx:

> Did Mr. Fields also advise to take these 56 ohms base resistors?

Now that I'm looking back - He used different values.

> They
> will consume more current than the LEDs.

The idea is that there'd be 15ma going through the 56ohm transistors,
saturating the transistors. (I believe they're saturated at 15ma?) Did I
mess up somewhere in my calculation?

> You can use 470 ohms for them.

But unless I'm looking at something wrong - that wouldn't saturate the
transistor, and if it's not saturated my understanding is that you can't
accurately predict voltage and current through it.

> with 8mA you can drive easily 100mA collector current in case
> you multiplex.

It will need to be able to drive 240ma - 8*30ma = 240ma. I want to run
these at max brightness due to the 1/8 duty cycle.

> You have to calculate the voltage across the resistor,
> not the transistor. when you have 4.5V control signal (typical PIC)
> and 0.75V Ube, then R= 3.75V/8mA = 468 ohm. You can even take 1k if
> you use transistors with beta of 150 or more.

Oops - you're quite right about that. Not quite sure where my head was
there.

> Uce sat will be a bit lower than 0.4V, maybe 0.15V, so it is better to
> use 47 ohm for 30mA.

Oh damn - just realized that Vcesat is not constant - it varies
depending on collector current. That's no good... I'm not quite sure
what exactly to do about that as collector current should be anywhere
between 0 and 240ma - which presents a fairly wide range of Vcesat
voltages.

> But if you always multiplex them and the program
> doesn't get stuck with one LED always on, you can probably even put
> 100mA through the LED with a duty cycle of 1/8. It would be better to
> make a current source for the Leds then you do not need these
> resistors at all and can easily adjust the brightness.

Could you advise on how to make such a current source?

Thanks,

-Michael

.

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