Message-ID: <38D5123D.BD3CED57@regandesigns.com>
Date: Sun, 19 Mar 2000 09:45:33 -0800
From: "Hamid A. Wasti" <hamid@regandesigns.com>
To: Andrew Plunkett <aplunkett@home.com>, lancair.list@olsusa.com
Subject: Re: 10 LED position sensor
References: <000901bf9173$afb981e0$80b10c18@bvrtn1.or.home.com>
Mime-Version: 1.0

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Andrew Plunkett wrote:

> Has anyone made one of these sensors yet?  I made one and I can't stop
> melting the 7805 IC.  It litteraly catches fire within 5 seconds.  The LEDs
> work until the fire.  I know nothing about electronics, but want a couple of
> these things real bad.

A few possibilities in no specific order:

1) Do you have the correct pinout of the 7805?  As you face the chip, with the
pins on the bottom, mounting tab on the top and the part number facing you so
you can read it, the input pin is on the left, the ground is in the middle and
the output is to the right.  A full datasheet for the chip is available at
http://www.national.com/ds/LM/LM7512C.pdf including a graphical description of
what I have described on the first page.

2) If your answer to the previous question is "what tab" then your problem is
that you have the wrong part.  Are you using a 7805 and NOT 78L05.  The 78L05 is
the low power version of 7805 (the 'L' in the middle) and will not be able to
handle either the load current or the power dissipation.

3) Are you sure that the current set resistor is 1.2K and not a smaller value
like 120 ohm?  Have you verified it with an ohmmeter?  A lower resistor will
increase the current consumption significantly.

4) Is there a low resistance short (not a dead short -- the LEDs would not
operate otherwise) from 5V to ground?

The following two are long shots at what may be causing the chips to blow, but
are worth the consideration:

5) Have you considered replacing the 2.2uF capacitor?  One of the failure modes
of a capacitor is a low resistance short through it.  However, in that case I
would have expected the capacitor to catch fire and not the 7805.

6) What is your input voltage?  The circuit is barely adequate at 13.8V with a
worst case power dissipation of close to 1W (0.11A*(13.8-5)) which will cause
the 7805 case temperature to rise more than 50C above ambient.  With no heat
sink and an ambient temperature of 45C (I have personally experienced much
higher in the cockpit of a Lancair IV in Phoenix) you are limited to about 1.5W
maximum dissipation on the IC.   In your test setup, if you are driving it with
an unregulated 12V wall mount power supply, you are actually putting close to
17V into it.  That results in a 1.3W dissipation in the 7805, putting it close
to the red line. But then, you are most probably not testing it in 45C ambient
conditions either, so this is unlikely to be the cause.

Best of luck and keep us posted on what you find.

Hamid


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