# RE: Potential heating problem with standard patch Cat5 panels

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Jack,
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 >>

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