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From:	"David W. Schultz (david DOT schultz AT earthlink DOT net) [via geda-user AT delorie DOT com]" <geda-user AT delorie DOT com>
Subject:	Re: [geda-user] Maximum current for thermals?
To:	geda-user AT delorie DOT com
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Date:	Wed, 15 Jan 2020 13:24:39 -0600
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On 1/15/20 9:47 AM, Richard Rasker (rasker AT linetec DOT nl) [via
geda-user AT delorie DOT com] wrote:

> Still, I wonder what the approximate current capacity of these thermals
> is? I know from experience that a few amps is no problem at all, but I'm
> pretty certain that they'll fail upwards of 25A or so. If possible, I
> prefer thermals for easier soldering, so a more accurate estimate of the
> limits would be appreciated.
> 

Tis a complicated question.

This might help:
https://www.amazon.com/PCB-Trace-Via-Currents-Temperatures/dp/1530389437

Too rich given my level of curiosity.


A paper by that books author that I found discusses fusing current.

t = 0.0346 * (A/I)^2 where A is area in square mils.

Looking at a TO-220 thermal I see a flared trace but the center is about
20 mils so that is the starting point. Assuming 1 second then I is about
5A. Since I have both top and bottom pours that gives 8 connections and
a limit of about 40A. But that equation ignores all heat loss. With a
short trace connected to a large area of copper and the device lead this
is a lot. So fusing current should be way more than 40A.



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