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I had not looked at the schematics, just the parts
list. The LED's are, in fact going back to 12V/14V.
Agreed, this is not a good idea but starting a fire
is not an expected result either. At least 40 years in electronics shows me
that.
Based on the LM3914 spec sheet, the LED current
comes out to 23ma. 520 ohms in parallel with 10K on 1.25V. Paralax error
maybe?
With a 100C limit,an ambient of 40C and 55C/W, the
allowable disipation is 1.09W. With all LED's on at 230ma, that allows 7V on the
LED's.
A simple solution would be to have the 5V
regulator with a heat sink feed the LED's. . . . . what I assumed in
the first place.
I like offering a fix rather than just
crtiticism.
Wolfgang
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Wolfgang,
Unless you have access to a
secret source of information not available to the rest of us, your
comments are completely baseless. Have you actually looked at the
schematic posted on BakerAircraft.com's website at http://www.barkeraircraft.com/files/AOA_rDisplay.pdf ?
Have you actually understood it?
If you actually study the circuit
on page 6, you will see that the LEDs are connected to the "+14V In" input
voltage and they are set up to run at 24mA not the "typical 10mA" that you
pulled out of thin air. At 13.8V input and a drop of 2.2V on the LEDs, the
chip has to dissipate 11.6V at 24mA or 278mW. At a Theta-J-A of 55C/W,
this leads to a 15C increase per LED that is actually on.
Next time
please study the subject matter before
posting.
Regards,
Hamid
Wolfgang wrote:
The home made AoA indicator from BarkerAircraft.com has a
5V regulator. With that running the electronics and a 2.2V typical
forward V @ 10ma on the LED's, that leaves 2.8V at 100ma total
dissipated by the chip with all LED's illuminated. The total dissipation
of the chip comes out to about 0.28W. I don't see that raising the chip
temperature to any where neat a 100C
limit.
Wolfgang
------------------------------------------------------------------------
From:
Hamid Wasti <hwasti@lm50.com>
Sender:
<marv@lancaironline.net>
Subject: Re: [LML] Do-it-yourself AOA
Project
Date: Mon, 11 Jan 2010 19:41:18 -0500
To:
lml@lancaironline.net
It is an interesting
product with some clever ideas, but a very poor implementation. It is
unlikely that it will keep working reliably for long in the
real-world/real-aircraft environment.
The system does not take
into account flap deployment. Anyone here use flaps?
The "normal"
position of the probe is with the Hall sensor at the zero point of the
magnet. Anyone concerned about the fact that the system could report
"normal" output if the probe or the magnet falls off?
Even the
most rudimentary analysis of the Hall sensor's output as its temperature
varies from -20C to +50C would reveal a potential for some worrisome
inaccuracies. You could stall several LEDs before you expect to, or you
could be several LEDs away from a stall when the system says you should
be stalling. Would you be more or less likely to actually stall in these
situations?
The design does not take into account the fact that
each LED that is on will cause the main IC's temperature to rise by
about 15C. A little slow flight practice with a 6 LEDs on and the IC is
well past its 100C limit, generating a burning smell and possibly smoke.
Same would happen after an engine out as you glide down at best glide
speed. Can you think of a worse time to find smoke in the
cockpit?
This is just a partial list of issues, but I think it
gets the picture across. The only thing worse than not having something
is having something that is not
reliable.
Regards,
Hamid
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