Current on a PCB
Dan Michaels email (remove spam text)
Brian Kraut wrote:
>There used to be maximum currents for various trace widths on the AP Circuits
>web site. http://www.apcircuits.com
Looking at those nomographs in a couple of books I have [one being H.
Johnson's "High-Speed Digital Design", p. 214] shows curves for 5degC -
100degC temp rises vs current in trace. Curves are linear on a log-log
plot. Roughly speaking, going from the 5degC curve to the 100degC curve
involves "only" a 3x increase in current. This is roughly the spectrum
from no problem to fusing the trace [or at least boiling water].
Thus, it would seem good practice to determine the worst case situation,
and then allow a good sized safety margin [2-3x min] in trace width
selection. This is fairly easy to do with pcb traces, since situations
requiring large currents generally involve physically large switching
components, like relays and transistors, so there is usually plenty of
room for wide traces. Traces can also be doubled, top and bottom of pcb,
and of course, heavier copper can be used for more severe cases.
- Dan Michaels
See also: www.piclist.com/techref/pcbs.htm?key=pcb
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