|Thread Links||Date Links|
|Thread Prev||Thread Next||Thread Index||Date Prev||Date Next||Date Index|
Unfortunately, that does not work.
The problem is not that of a connected cable charging up, but that of having a pre-charged cable (with no built-in resistors) suddenly being plugged into the RJ-45 connector.
What you get is a sub-nanosecond rise time pulse with several hundred millijoules of stored energy. This pulse is coming from a resonant cable so what you see is an exponentially decaying damped oscillatory discharge. 5 kV on a 25-foot cable, 700 to 900 V on a 100-m cable.
If you are lucky, both connector pins mate at the same time and you get the common-mode rejection of the magnetics helping you by not passing some of this through to the PHYs. A worse case is if only one pin of a pair mates, and you get differential transfer through the magnetics.
We (and PHY vendors here) have seen as much as 60 volts appear on the PHY pins.
In any case a bleeder resistor that would absorb this would have to be such a low value that it would interfere with normal signals. We have had good luck with diode clamping networks that have a dynamic (clamping) impedance on the order of 1 ohm.
This is, fortunately, a comparatively rare phenomenon that usually only occurs when a cable is being plugged in in a new installation. Careful network installers would make sure that they discharged all cables BEFORE plugging them in, dont'cha fellas?
We have had a few Environments from Hell where a dry climate, combined with moving belts on assembly lines (opportunistic Van der Graaf generators!), have caused constant, repetitive failures. You know who you are out there....(grin)
From: Yair Darshan [SMTP:YairD@xxxxxxxxxxxxxx]
Sent: Saturday, September 16, 2000 2:41 PM
To: 'Dieter Knollman'; stds-802-3-pwrviamdi@xxxxxxxx
Subject: RE: Static Discharge
The 2kv cap is not suppose to leak.
Assuming that the energy level develops on low capacitance value (few
nano-farads), than a "bleed" resistor located between the positive and the
negative feeding lines of 10 to 100K can discharge it (lower value - better
Better way is to clamp the voltage developed from excessive charges by using
low impedance path that activated at a certain voltage that fits
the system requirements at normal working conditions.
Chief R&D 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
> E-mail: <mailto:yaird@xxxxxxxxxxxxxx>.
> -----Original Message-----
> From: Dieter Knollman [SMTP:djhk@xxxxxxxxxx]
> Sent: ו, ספטמבר 15, 2000 6:53 PM
> To: stds-802-3-pwrviamdi@xxxxxxxx
> Subject: Static Discharge
> My background is telephony. I'm totally new to 802.3, so please excuse
> my ignorance.
> One thing that appears strange is the lack of an intentional discharge
> path for the potential on the cable.
> The only means that I have found is breakdown of the common mode
> termination capacitors.
> Are these caps with 2 kV rating intended to be leaky?
> On POTS line interfaces the Tip lead is typically biased around ground
> and serves as a discharge path to earth ground for Analog Sets. Do the
> IP Phones require a discharge path via the LAN?
> Dieter Knollman