Re: current limiting of 802.3 power
Mike,
PTC's don't cut off sharp enough.
Only an active shunt regulator will work. However, I don't see where we need any PD
control.
Jack Andresen
Mike_S_McCormack@xxxxxxxxxxx wrote:
> Let me be clear, I am not lobbying to not have the source limited, I just want
> to limit it with practical technologies.
>
> The 350ma limit is from the transformers, the limit that has been discussed for
> patch panels has been 250 ma per conductor. The patch panel number has been
> considerably anecdotal, and I don't imagine we can really expect solid
> information.
>
> What I was trying to say with the last e-mail is that I believe it is
> impractical to expect the PSE to cut off at exactly 350ma. The exact limit can
> be more easily enforced by the PD engineers at design time, they must ensure
> that it will not draw more than 350ma on a sustained basis. The PSE is
> responsible for cutting power if the PD faults (or something else faults) and
> draw more than the legislated limit, but there has to be some slop above 350ma
> to make it practical for the PSE to do so.
>
> I don't know that we need to say the over current requirements can not be met by
> PTCs at this stage of the game, at least I can not say I know enough to dismiss
> them. PTCs are cheap and effective in my experience, but this project is way
> beyond my limited experience in the power arena.
>
> Finally, it is not safe to assume that the device that determines the minimum
> current is flowing can also measure the current. Nortel discussed in LaJolla
> using transformer saturation characteristics to know that at least a small
> amount of current was flowing, but the system they discussed, I do not believe,
> was well suited to measuring the actual current draw. An exact cut off may not
> come for free.
>
> Mike