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Thread: Bullet Proof Circuit...
face BY : Lawrence Lile email (remove spam text)

Here are some additional rules for "Wagners 10 golden rules"  (Excellent,

Often I design low cost  PC boards with 120Vac and 10 amps on the board.
These wierd conditions require some extra care:

1.  Traces:  Try to use the fattest trace with the widest spacing that you
can.  Just because your board house can print 8 mil traces with 8 mil
spacing doesn't mean you have to use them.

2.  UL requires 0.064"/1.6mm spacing between 120Vac traces.  Use 0.080"/2mm
where possible.

3.  Watch for ways that high power sections of the board can short out.
This is especially a problem on vertical PC boards.  Overheated solder
joints drip!

4.  Add a thin trace going into the power section of a high power board.
Hourglass it down to 0.013"/0.3mm.  If you get a short on the board, this
will act as a fuse.  Only use such a crude technique if you can't afford a
real fuse.  Make sure that you don't put much current through it!

5. Power resistors heat the board less if they are standing on end, rather
than parallel to the board.

6.  Never overstress a capacitor more than 80% of it's votlage rating.  50%
is better.

7. Wattage calculations are about as good as weather calculations or
predictions of the end of the world.  If you have the chance, glue a
thermocouple on a component and measure the temperature in the actual
conditions.  Then go ask your electronics teacher why wattage calculations
are never right.

8, Poor connections make lots of heat, when high currents are flowing.
Often crimped or riveted connections will fail, due to creep or poor
technique.  You'll see a big burnt spot around the connection on the PC
board.  Spade terminals are a common culprit.  A loosely crimped spade
terminal can get so hot as to melt solder!

9.  There are charts out there purporting to show "maximum" current ratings
for specified trace width - always too conservative.  There are other charts
showing TEMPERATURE RISE for a certain current and trace width.  These are
the better ones to use.  Here are two good ones:

http://www.geocities.com/CapeCanaveral/Lab/9643/TraceWidth.htm   Nice online

http://www.aracnet.com/~gpatrick/  useful charts for trace width and a lot
of other PCB design rules

-----Original Message-----
From: Wagner Lipnharski <wagnerlspamspamEARTHLINK.NET>
Date: Thursday, October 21, 1999 8:09 PM
Subject: Re: [OT] Bullet Proof Circuit... was: Survey... What is an
acceptable failure rate?

>No books about it AFIK, but some tips perhaps,
>here goes "Wagner's 10 golden rules":

<004101bf1cad$1127bc80$5301010a@6840cck4k019.toastmaster.com> 7bit

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