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Subject: Re: [geda-user] Maximum current for thermals?
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From: "Richard Rasker (rasker AT linetec DOT nl) [via geda-user AT delorie DOT com]" <geda-user AT delorie DOT com>
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Date: Wed, 15 Jan 2020 21:56:47 +0100
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Hello Dave,

Op 15-01-20 om 20:24 schreef David W. Schultz 
(david DOT schultz AT earthlink DOT net) [via geda-user AT delorie DOT com]:
> 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.

Nick Bowler's calculations suggest to me that 100A probably causes 
problems (0.3 watts per 0.04 mm² thermal spoke), but that 25A is fine.

But it is indeed rather more complicated than I thought. I think I'll go 
brush up on my thermal conductivity calculations (did some of that long 
ago with Peltier cooling) and see where that gets me.

Thanks, best regards,

Richard