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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 21:49:50 +0100
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Hello Nick,

Op 15-01-20 om 18:28 schreef Nick Bowler (nbowler AT draconx DOT ca) [via 
geda-user AT delorie DOT com]:
> On 1/15/20, Richard Rasker (rasker AT linetec DOT nl) [via
> geda-user AT delorie DOT com] <geda-user AT delorie DOT com> wrote:
>> [snip]
>>
>> 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.
> Resistivity of copper ρ is about 1.7 × 10^-8 ohm*m at 20°C, with
> temperature coefficient α about 3.9 × 10^-3 / °C.
> [snip calculations]
>
>    25A: each thermal will dissipate 20mW or 80mW altogether.
>    50A: each thermal will dissipate 80mW or 0.3W altogether.
>    100A: each thermal will dissipate 0.3W or 1.2W altogether.

This is about as far as I got too...

> How hot the thermals will get is a system design question of
> how well you are heatsinking that generated heat away.  They are
> presumably connected to a large copper plane which will help.

And this is the main problem. Even a few dozen milliwatts can be too 
much if the heat isn't conducted away as fast as it's generated. But 
indeed we're talking about a large copper plane, so the heat can spread 
out pretty fast -- and I never actually experienced anything below 100 
mW giving problems in any way.

Based on the numbers so far, I'd say that 25A isn't a problem at all, 
but 100A will probably cause trouble, and pretty fast, at that. And I'll 
also have to look at inrush current (two terminal blocks connect to a 
pretty beefy 1 kVA SMPS).

OK, thanks, it seems that these thermals are more resilient than I 
thought, and 16A should not be a problem at all.

Best regards,

Richard