Re: [uCsimm] HV capacitors on the LAN transformer

From: Tom Walsh (
Date: Wed Aug 15 2001 - 04:23:24 EDT

Daniel Haensse wrote:
> Am Mit, 15 Aug 2001 schrieben Sie:
> > Folks, who can answer this...
> >
> > There is a LAN transformer on the ucsimm (VALOR- PULSE
> > ENGINEERING part PT4153S). On most Ethernet designs,
> > there are 2 1kV 0.01 capacitors connected to this
> > transformer on the ethernet side. On the ucsimm these
> > transformer pins are left unconnected. Question: what
> > these capacitors normally for, why they are missing
> > and is there any penalty for not having them?
> Those capacitors will discharge to gnd if the voltage is above 1kV.
> So if the LAN transformer can't stand the voltage anymore, on the uCSimm
> it will discharge to the uCSimm and may blow the ethernet controller or
> anything else that the high voltage enjoys to destroy.

While it is true that a sustained potential above 1KV will *eventually*
cause the capacitor to fail.. The EMF pulse will have destroyed the
electronics on the other side of the magnetic long before the cap will
have failed. No, the cap voltage rating is so that it will withstand
the voltages placed across the wiring by standards organizations such as
the U.S. Underwriters Laboratory. I had a circuit that had to comply
with a U/L spec. that required a sustained application of 600volts
potential without catching on fire. The standard was for industrial
wiring installations where it was possible that the wiring could be near
440volt three phase supply lines and may be shorted against that wiring
(due to accident / incompetance).

There are far more effective ways of achieving the protection of EMP
(lightning) and electrostatic discharges that burning a cap out! These
range in effectiveness from: spark gaps, MOV, and transil + transorb
arrangements. You have to keep in mind that the EMF potential of a
pulse is very high and of short duration (in most cases) and would need
a very high speed clamping circuit to absorb the pulse. The clamping
circuit response time is usually in the sub-nanosecond range (as in the
case of a Transil). A garbage-variety capacitor offers no such

One of the most effective EMP clamping circuits that I have designed
with is a Transil, resistor, and Transorb:

Device >---------------\/\/\/\/\/\---------------> Line
                 | |
                 | |
                 | |
                 | |
                 | |
                transil transorb
                 | |
                 | |
                 | |
                 | |
              ------- -------
              / / / / / /

The Transil is a low voltage device, typically withing the ABSOLUTE MAX
ratings of the device to be protected. The resistor is a 10 ohm device,
and the Transorb (gas tube, similar to neon), has a firing voltage
rating of 70 volts. The circuit works as: when a pulse arrives, the
transil clamps the overshoot to ground causing a voltage drop across the
resistor, the transorb will fire at the 70 volt level and continue to
fire (conduct) as long as current is present. The worst case is that
you will blow the resistor off the board! The transorb is capable of
withstanding 1500Watts/second.

Nope, the cap is not for overvoltage protection...



Tom Walsh - WN3L - Embedded Systems Consultant,
"Windows? No thanks, I have work to do..."
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