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

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

Michael, in the first place choose different transistors for 240mA current.
I had a look at the Zetex site and found these for example:
NPN
ZTX1047A Vbe<70mV @1A Ib=10mA
ZTX1048A Vbe<45mV @0.5A Ib=10mA
PNP
ZTX717 Vbe<17mV @0.1A Ib=10mA
ZTX788A Vbe<35mV @0.1A Ib=2mA
You can also get a ready made current source BCR402R, you can switch it off
putting pin1 high.
http://www.google.com/url?sa=U&start=1&q=http://www.infineon.com/cgi/ecrm.dll/ecrm/scripts/public_download.jsp%3Foid%3D18469%26parent_oid%3D22765&e=10313
.. It is also possible to make a current source with 2 transistors that can
be switched by a logic voltage. But I do not draw it now.
--
ciao Ban
Bordighera, Italy


.

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