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I am 100% agree with you. The PTC can serve only as secondary (and at higher level) protection. If we want to utilize the full capacity of the cabling infrastructure, we should be close as possible to the maximum current allowed - 350mA- and for that we will need to use some more accurate current limiting hardware.
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From: Rick Brooks [SMTP:ribrooks@xxxxxxxxxxxxxxxxxx]
Sent: Wed, August 09, 2000 8:02 PM
Subject: current limit on 48 VDC
Welcome back Roger.
Since we are putting in our share of opinions, here is mine.
If we assume that 350 ma per pair, 175 ma per pin, is the maximum continuous current that we are allowed to have.
Then for PTC protection alone, we would have select a PTC with a lower current value than 350 ma,
since these devices are not particularly sharp, just as they are not fast.
So, we would have to choose a PTC with (I'm guessing), say 250 ma limit at low ambient temperature.
This choice would drastically lower our guaranteed output power, especially for higher ambient temperatures.
We may only be able to get 5 watts to the PD.
Has anyone worked out an example with real numbers?
So, I think that we will need some form of electronic current limit on each port, so we can run close to the 350 ma limit.
If we use PTC's at all, it will be to prevent fires, in the case of when the electronic current limit fails.
we need more dialog...
anyone for discovery? tennis?