RE- Varistors (was Info and requests for book)

To: "'Tesla List'" <tesla-at-poodle.pupman-dot-com>
Subject: RE- Varistors (was Info and requests for book)
From: Tesla List <tesla-at-stic-dot-net>
Date: Fri, 19 Sep 1997 00:14:11 -0500
Approved: tesla-at-stic-dot-net

From: 	Robert Michaels[SMTP:robert.michaels-at-online.sme-dot-org]
Sent: 	Thursday, September 18, 1997 4:50 PM
To: 	tesla-at-pupman-dot-com
Subject: 	RE- Varistors (was Info and requests for book)

        Take note.  Varistors are vaguely like fuses.  They are
        not designed to function repeatedly.

        Fuses are good for one shot.  After that they're dead.
        Permanently.

        Varistors can function multiple times.

                Think of it as being similar to the difference
                between men and women -- organismically speaking.

        After each "hit" Varistors degrade somewhat.  After a dozen
        or so hits, they cease to function at all. Varies with the
        brand, rating, and intensity of the hit.

        Remember, Varistors are thin-film semiconductors.  "Varistor"
        is a proprietary term for Metal Oxide Variable Resistor.

                I.e.:  Thin-film metal oxide -- as in MOSFET,
                Metal-Oxide Semiconductor/Field-Effect Transistor.

        Would you want to use a MOSFET as a substitute for a
        spark gap in a Tesla coil?  (MOSFETs and Varistors aren't
        the same thing -- but they're horses of a similar
        color).

                                        Physics again, damn it!
                                        from -- Detroit, USA

                                        Robert Michaels

TL>>>2.      Has anyone tried using varistors (transient surge absorbers) inste
TL>>>of spark gaps for transformer protection?  I realize they my need to be
TL>>>isolated with chokes due to their capacitance but in quantity they are
TL>>>fairly inexpensive (1800V 4500A $1.88ea from DigiKey) and they can be easi
TL>>>chained together to reach a desired voltage.  I would think they would giv
TL>>>much better protection than a spark gap.
TL>>        They are worth investigating, but I have my doubts.  How do they
TL>>perform at RF frequencies?  Can they switch on fast enough?  Will a
TL>>resistive voltage divider work once these things start to turn on?  Peak
TL>>kickback can be more than three times the peak A.C. output voltage of the
TL>>transformer.
TL>>