Homebrew PCBs

Hello,

The ""bible" for pcb design is "Printed Circuits Design" by Gerald 
Ginsberg. Published by McGraw Hill, I paid 5 bucks for myt copy of 
the '91 edition. You can pick up a copy inexpensively at ABEBOOKS.com 
or similar.

Another decent book is Tim Williams, "The Circuit Designers Companion"

Ginsbergs book has several nomographs, one of which answers what 
you've asked. But there ar additional considerations. Is the trace by 
itself--means away from other traces? How much temperature rise can 
you allow? Is the trace on an internal or external layer? What is the 
copper "ounce" rating? 

From page 40, the three nomographs will get you where you need to be. 
Assuming external layer, nothing REALLY close by, and a worst case 
temp rise of 60 degrees C. 7 amps requires a 70mil cross section. 
With one ounce copper, that's a trace about .053 wide... With 2 oz., 
you need .028-.030 width.

At 20 degrees rise, you need 150 cross mils. So that's a trace 
about .120 wide in 1 oz. copper. In 2 oz., it's only .060 wide.

Hope this helps,

Ballendo

P.S. Did you try putting "required PCB trace width" into a search 
engine?

FWIW, there is a formula, but the nomographs are easier to use. The 
formula is in ANSI/IPC D-949 

--- In Homebrew_PCBs@yahoogroups.com, Anthony Toft <toftat@c...> 
wrote:
> is there a formula for how wide tracks should be? I need to make 
> tracks capable of carrying 7+A at 12v How fat should they be?
> -- 
> Anthony Toft <toftat@c...>