[sdiy] Trace width and via diameter/hole size

[Bert Schiettecatte]

> Hi Tim,
> 
> > 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.
> 
> Hmm.. How do I figure out if the traces will light up or what 
> Simple rule can I use to decide if I'm still ok with the PCB 
> Trace width? Is there a PCB guidelines table somewhere that 
> Tells you the minimum track width for a certain current?

Ok an update, I've used traceres.ulp in Eagle which puts Current 
Numbers on the traces on all layers. You can specify a temperature
Rise allowed (50 degrees celcius being standard??) and then you get
The currents allowed through the traces. If the ambient temperature
Is at 25 degrees celcius I find a rise of 50 degrees pretty scary 
But maybe that's just me. 

I am still clueless how to calculate the allowed current through 
A via. 

bert