# Re: [802.3af] 45Kmin instead of 70Kmin ?

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Hi

I am worried that this is a 'major' change we have 'messed' with this so many
times that at this stage of the game, this would require revisiting the details
all over...
so unless we have some good data on this, i am against it, the Lucent hazard
matrix that sold us this in the first place relied on the 70k number....

my 2c.
roger

At 09:44 AM 1/8/02 -0800, Dave Dwelley wrote:
>
>Yair, John -
>
>As I recall, the 70k number was intended to prevent two PSEs from detecting
>each other if they were erroneously connected together. Rick showed a
>presentation in St. Louis (if I remember right) that showed a possibility
>of detection if the source impedance was less than 60-some K. At the time I
>ran his numbers and agreed with them, but I'd have to look back at my notes
>to see how we got to to that number.
>
>We do have text that allows any source impedance if there is a diode in
>series (so it's always 1st quadrant). 45K works fine this way.
>
>I'd oppose this change, at least until I get a chance to look back at
>Rick's presentation.
>
>Dave
>
>At 01:54 PM 1/8/2002 +0200, Yair Darshan wrote:
>
>>Hi guys,
>>We specified that the high impedance option for the detection circuit
>>requires 70K min.
>>We also specified that the voltage across the port is 2.8V to 10V.
>>Now, if we want to get the 10V when we have the following conditions: 70K
>>series resistance, Rsig=23.75K (25k - 5%)
>>the probe source voltage need to be 10*(70k+23.75k)/23,75k= 10V*3.94=39.4V.
>>We have limited the open circuit voltage to 30V max.
>>So to get the 10V max we need to reduce the 70k min value to (30V*23.75K -
>>10V*23.75K)/10V = 47.5K.
>>Having 5% margin we get: 45K min.
>>In case of PSE to PSE connection the worst case power dissipation over this
>>resistor would be: 57^2/45k=72mW which is <125mW.
>>
>>Thus to keep all numbers consistent with the intentions I suggest to change
>>from 70K min to 45K min.
>>
>>Do you see problems with this change?
>>
>>Thanks
>>
>>Yair.
>>
>>
>>Darshan Yair
>>Chief  Engineer
>> > PowerDsine Ltd.  -  Powering Converged Networks
>> > 1 Hanagar St., P.O. Box 7220
>> > Neve Ne'eman Industrial Zone
>> > Hod Hasharon 45421, Israel
>>Tel:  +972-9-775-5100, Cell: +972-54-893019
>>Fax: +972-9-775-5111
>> > E-mail: <mailto:yaird@xxxxxxxxxxxxxx>.
>> > http://www.powerdsine.com
>> >

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