> > I have one question. You mention using zeners in several places
> > for spike suppression. Have you any experience using transzorb
> > TVS devices [low-inductance, fast-breakdown diodes] to protect
> > I/O lines? Should be superior to std zeners there.

> Re the transzorbs, no I don't think I have ever used them.
> I would guess I've replaced some of the dead ones in TVs with
> big zeners though! ;o)

That probably isn't such a hot idea. Although a tranzorb/transil has
very zener-like characteristics, their application is quite
different.

The zener-resistor-cap is ideal for I/O and signal lines because
normally there isn't much current flowing and the influence of the
resistor is negligible. When something like a load dump comes along,
that form of protection allows the (outside) signal line to go as
high as it wants with the inside being protected by the clamp. The
power is disipated by the resistor but it is limited by the value of
the resistor which can be set fairly high.

However, if the same circuit is applied to the power supply input (or
a relatively high current load switch) the drop across the resistor
in normal operation costs too much in voltage overhead so it can't
be used. That means that when the surge comes along, there is nothing
to limit the clamping current so much more will flow. The transil can
cope with these types of currents while a zener will be vaporized.

Obviously a zener is cheaper and generally smaller and you are
getting a bit of input filtering at the same time. If you can't
afford the impedance then a tranzorb is required.

One thing that is particularly impressive about the zener protection
is that it covers the spectrum from low speed, high energy
disturbances through to high speed ESD pulses.

Steve.

======================================================
Steve Baldwin                Electronic Product Design
TLA Microsystems Ltd         Microcontroller Specialists
PO Box 15-680, New Lynn      http://www.tla.co.nz
Auckland, New Zealand        ph  +64 9 820-2221
email: steveb@tla.co.nz      fax +64 9 820-1929
======================================================

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