Re: Potential heating problem with standard patch Cat5 panels
Yair,
I have enclosed a chart of Fusing current for Copper conductors, which is like a
time limited limit. I have extrapolated the data into the lower end of the
graph. The table comes from ITT press "Reference Data for Radio Engineers" I
have translated their numbers based upon wire gauge to trace width and
thickness.
Sterling Vaden
Yair Darshan wrote:
> Jack,
> I understand your calculations, thanks.
> Is there any data for peak currents limited by time?
>
> Yair.
>
> > -----Original Message-----
> > From: Jack Andresen [SMTP:jandresen@xxxxxxxxxx]
> > Sent: ב מאי 07 2001 20:41
> > To: Yair Darshan
> > Cc: stds-802-3-pwrviamdi@xxxxxxxx
> > Subject: Re: Potential heating problem with standard patch Cat5
> > panels
> >
> > Yair,
> >
> > Correction: A 0805 SMT resistor is 2 X 1.6 mm, which translates to 79 X
> > 63 mils.
> >
> > If we assume that the effective convection/radiation area is only that
> > area (i.e., no conduction to the board nor to the traces) you have
> > .07/.005 or 14 watts/sq in. That would clearly burn up. Hence, you need
> > to take into account nearby conduction. If we allow .25 in. spacing per
> > device in an array and assume uniform conduction/radiation, we still get
> > .07/0625 or 1.1 watta/sq in. That translates to over 100 degrees C rise.
> > Still a problem.
> >
> > Concerning my numbers, I did make a mistake that I only realized when
> > trying to rationalize your numbers. I forgot that my model was for a
> > pair, yielding twice the wattage. Thus, my calculation should have shown
> > a 19 degree C rise. Still not a problem.
> >
> > Note. At 25 degree C a .1W/sq.in generates 15 degree C temperature rise
> > while 1W/sq.in. gives a 100 degree C rise
> >
> > Jack Andresen
> > ******************************
> > Yair Darshan wrote:
> > >
> > > What is the worst case of the two cases:
> > >
> > > Case 1: PC board trace with 0.07W/inch generating 12C rise
> > > Case 2: 0.07W dissipated on 0805 SMT resistor rated to 125mW and its
> > area is
> > > 8mils x 5 mils.
> > >
> > > I know that case 2 is being used without failure reporting. Since in
> > case 2
> > > the power density/area is much higher than case 1, it seems that it is
> > not a
> > > real problem.
> > >
> > > What others think about it?
> > >
> > > Yair.
> > >
> > >
> > >
> > > > -----Original Message-----
> > > > From: Jack Andresen [SMTP:jandresen@xxxxxxxxxx]
> > > > Sent: ?, ??? 03, 2001 11:21 PM
> > > > To: stds-802-3-pwrviamdi@xxxxxxxx
> > > > Subject: Potential heating problem with standard patch Cat5
> > panels
> > > >
> > > > A customer mentioned panels burning up when one of his customers senr
> > > > power down the network.. I did some canculations and found a potential
> > > > problem.
> > > >
> > > > Jack Andresen << File: DTE power.doc >>
Fusing_Current.doc