[sdiy] Trace width and via diameter/hole size

[Tim Ressel]

Bert,

That's a complicated question. I wonder what that
program conciders when it makes its determination.

There is the fusing current, which the current at
which a conductor becomes a fuse. Any current below
that is fine, but you get voltage drops and thus heat.
Conductors are usually choosen  to limit the voltage
drop along its length. Ever wonder how they can get
140 amps through a HexFet in a TO-220 package? It's
because the length is short enough to limit the
voltage drop. 

If I calculated right, your trace has 0.06 ohms/inch.
At 640mA the voltage drop would be 38.4mV, the power
dissipation would be 24.6 mW/inch.

I hope this helps.

--TimR


--- Bert Schiettecatte <bert at percussa.com> wrote:

> Hi all,
> 
> I'm still working on a pcb here in Eagle, and I've
> been using the
> length-freq-ri.ulp program to find out how much
> current can go through
> my traces. It says 640mA through a 0.2mm trace. Is
> this correct? It
> sounds like a lot of current through such a tiny
> trace. The copper
> thickness is 0.035mm. 
> 
> This program tells you how much current can go
> through the traces, but
> what about the vias? How do you figure out how large
> the via hole needs
> to be to carry the required current? I searched and
> searched with google
> but couldn't find anything about via size, only
> trace width and
> clearance. 
> 
> Any ideas?
> 
> Thanks,
> Bert
> 
> 



	
		
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