Re: Size of Resistors

I can confirm that the 250ma is correct

The QTLP690C is not the device that will be used. I just added it to
the design for illustration. I need to drive the LED at a range of of
constant currents to check the beam profile.

My design is two bootstrapped current sources, hence having to use
14BJT.

I accept that I might be over complicating the issue but I can't seem
to find anything in the literature.

thanks for the help so far.

g
PeteS wrote:

Jonathan Kirwan wrote:
On 25 Aug 2006 08:31:34 -0700, "GraemeC"
<graeme.cunningham@xxxxxxxxxxxxxx> wrote:

<snip>
This circuit provided a constant current through the load LED (D1) when
Vi1 is greater than approx 3v
<snip>

I've only just begun to look at the arrangement. You have 14 BJTs in
order to provide a constant current into an LED!?? Can you explain a
little more about why?

Jon

I ditto this reponse.

If you are trying to provide 250mA into an LED as a constant current
(which seems awfully high), it can be done with only a single
transistor and a few other parts.

Apart from that, the part you specify (QTLP690C) is listed as obsolete
http://www.fairchildsemi.com/sitesearch/fsc.jsp?command=text&attr1=&attr2=&t=0&i=sitemap+id&ia=1&text=QTLP690C+&as=1&render=1&w=

Please tell us the application requirements :)

Cheers

PeteS

.

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