Path: utzoo!news-server.csri.toronto.edu!cs.utexas.edu!rice!uw-beaver!milton!whit
From: whit@milton.u.washington.edu (John Whitmore)
Newsgroups: sci.electronics
Subject: Re: surge supressor diodes
Keywords: diodes surge-suppressor
Message-ID: <17812@milton.u.washington.edu>
Date: 6 Mar 91 08:47:02 GMT
References: <1837@ole.UUCP>
Organization: University of Washington, Seattle
Lines: 52


In article <1837@ole.UUCP> ssave@ole.UUCP (Shailendra Save) writes:
>
> What are surge suppressor diodes, and how do they work?  What
>configuration is used? ie, are they in series with the power supply?
>Or are they like zeners and in parallel?

        There are all kinds.  Some are pairs of Si rectifiers to
clamp a signal near ground (telephone 'varistors' are of this type),
some are Si zeners (aka TransZorbs) with ratings for high surge
currents (and relatively loose voltage specifications).  Some are
not doped Si at all, but largeish chunks of semiconductor with
contacts on two sides (aka MOV, metal-oxide varistors, ZNR).
This last type sometimes looks like a disk capacitor and is
usually the type found in surge-protection devices.
I have seen data sheets for yet another type of surge protector,
a 'breakover' type which integrates a zener and an SCR: it
has the breakdown voltage of the zener and the clamp voltage
(when 'full-ON') of about 1V.

        And there are some exotic devices; neon bulbs are a
VERY good clamp, and some radioactive-electrode gas-filled tube
surge suppressors are available (or used to be) that combined
extremely high current capacity with very low stray capacity
and negligible leakage current.  For higher voltages, a simple
spark gap (like the neon light, only with nitrogen/oxygen...)
is effective and cheap.  Your color TV probably has a half dozen
of these around the CRT neck.

        Usually the big surge protectors are placed in the AC line
(because that is presumed to be the source of surges) for any
plug-in-the-wall item.  Because an input capacitor is a
pretty good surge snubber, many manufacturers think they
needn't put in any other device (so people in some parts of
the country have to get a lot of TV repair after a thunderstorm).
An input transformer, like in a linear power supply, is a
pretty good surge suppressor in its own right; it'll absorb a 
LOT of high-frequency energy in a quick spike, and a longer
spike will saturate the transformer (well, at least the OUTPUT
won't burn up...)  Of course, no one uses big input transformers
these days.

        In various forms, diode clamps for surges are ubiquitous.
After the first year of manufacturing 7400 series TTL circuits,
it was found that the logic lines would 'ring' in some
configurations, and that this would degrade the input transistors.
Since that effect was found, TTL circuits have ALL had input
protection diodes (shunt to ground).  I have also seen some
op amps (LM308 for example) with protection diodes to keep
the input transistors from becoming saturated.

	John Whitmore