Re: [uCsimm] HV capacitors on the LAN transformer

From: Daniel Haensse (daniel.haensse@fhk.usz.ch)
Date: Wed Aug 15 2001 - 04:51:12 EDT


As I remember, I've seen some special caps
on some ethernet controllers. They look like a normal ceramic disc capacitor,
but there is a split in the top of capacitor. This will allow the voltage to
break through and jump to gnd, if the voltage is above the rating.
Your right some clamping diodes are much better, but also more expensive.
While thinking about it, the capacitor would work as filter as well as clamping,
so I guess we are both right ;-). Interesting.

Dani

>
> 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
> protection.
>
> 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...
>
>
> Regards,
>
> TomW
>
>
>
>
>
> --
> Tom Walsh - WN3L - Embedded Systems Consultant
> http://openhardware.net, http://cyberiansoftware.com
> "Windows? No thanks, I have work to do..."
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