"Perfect" LED current limiting challenge

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MrAl
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Re: "Perfect" LED current limiting challenge

Post by MrAl » Tue Feb 07, 2006 5:44 am

Hello again,

stephen:
I agree that you cant count on the HFE of the
transistor to limit anything because it's able
to change too much over temperature. I thought
i mentioned this a while back in this thread
but i bet it got overlooked :)

Gorgon:
Oh ok, now i see what you are talking about,
and you are certainly right...this is a very
important design issue and should be looked at
carefully.
What i was assuming was that he would be using a
high brightness LED with a forward drop of maybe
3.5 volts, and that once the PIC went down due
to battery voltage drop even a direct connection
to the battery wouldnt burn out the LED. Still,
this may not be a good idea anyway and something
else could be installed to prevent the LED from
turning on in case the PIC stalled with the
power pulse in the 'on' state keeping the LED
turned on when it shouldnt be anyway.
There are many ways of attacking this potential
problem, and one is to limit the drive. If
the PIC itself cant be used to detect undervoltage
then perhaps a transistor or comparator chip to
do that. There are lots of pre-programmed ic's
that are also low cost made especially for
detecting undervoltage and can be used to kill
the drive to the power pulse stage.
These cheap ic's dont even require any external
resistors as they are set to a certain voltage
already (5v, 4v, etc.). If this sounds like
something he wants to do i'll look up some part
numbers or else we'll come up with something
else to make sure this problem doesnt occur.

In any case, thanks for bringing this up!


Take care,
Al

<small>[ February 07, 2006, 05:47 AM: Message edited by: MrAl ]</small>
LEDs vs Bulbs, LEDs are winning.

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