Message-ID: <376F1168.E03016BE@calweb.com>
Date: Mon, 21 Jun 1999 21:30:32 -0700
From: "Hamid A. Wasti" <hwasti@calweb.com>
To: lancair.list@olsusa.com
Subject: Re: annunciators
References: <9906219300.AA930001629@inet001.cardell.com>
Mime-Version: 1.0

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> I have not been satisfied with them
> because the LEDs seem to burn out quickly.

Leeds have high infant mortality, but that is WAY too high.  Looks like they
are being over-driven.

>  I believe the story goes that the factory decided to
> remove a resistor in each unit in order to increase the brightness of the
> LEDs.
> Perhaps the LEDs are not liking the increased voltage they are seeing.

LED's are non-linear devices (they are light emitting "Diodes") and are driven
by controlling the current through them.  All devices are rated for a certain
current, typically 25 or 30 milliamps for the large 5 mm T1-3/4 package
devices.  This is done by putting a resistor in series with the LED so that the
voltage across the resistor/LED pair allows the right amount of current to flow
through the LED.  The formulas for this calculation have already been given by
Mar in his comments to this e-mail, so I won't repeat them.

Most of the time, the resistor is external to the LED and is sized
appropriately for the drive voltage and the desired intensity.  There are Leeds
with integrated resistors available in the market.  Using these saves the
hassle of adding s series resistor.  The down side for aviation & automotive
use is that there are none available for a working voltage of 13.8V.  If a LED
with an integrated resistor sized for 12V is used in a 13.8V system, the LED
will see about 20% more current than intended.  Over-driving Leeds on a long
term basis is dramatically shortens their life span.

I am merely speculating here, but this is what may have happened -- a 12V
LED/resistor pair being used in a 13.8V system.  You do have a 14V system and
not a 28V system?

My suggestion would be to replace the Leeds with other Leeds, but with
appropriately sized current limiting resistors.
they bought these units for 12V application and it is being used in a 13.8V
system, the Leeds are seeing a 20% overcorrect, which will shorten their life
quite a bit.

Another factor to keep in mind when selecting a LED is the viewing angle.  The
light emitting portion of the LED is actually quite small.  The rest of stuff
in the LED package is optics that spread this concentrated source of light.  It
is totally up to the optics how wide the LED can be viewed.  Viewed from beyond
this angle, the LED will appear very dim.  Just a quick scan of the DigiKey
catalog lists Leeds with viewing angles from 16 degree to 140 degrees.  For an
enunciator application, neither a very narrow nor a very broad viewing angle
would be suitable.

Hamid


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