PROTECTING YOUR ELECTRONIC EQUIPMENT

         How to Use MOV's (Metal Oxide Varistors) for Surge Protection

         by  Harry Steele, SysOp, BCS/Atari BBS (617) 396-4607
                                  The Boston Computer Society
                                      One Center Plaza
                                      Boston. Ma. 02108

              A lot of attention should be paid to protecting computers
         and other electronic equipment against costly damage from
         power line transients. Voltage transients include surges of
         voltages above the specified normal, voltage sags below, and
         instantaneous voltage spikes that leap far above the normal
         level.
             The solution to this problem is to install a device called
         variously a "Voltage Spike  Protector", "Surge Suppressor",
         or "Transient Eliminator". These devices are available
         commercially at prices from $20.00 to $100.00 and are based
         on a component called a metal oxide varistor (MOV).
             MOV's are voltage dependant non-linear devices. When a
         voltage lower than its conduction threshold is applied across
         it, the MOV appears as a non-conducting open circuit. But if
         the applied voltage becomes greater than this set point (when
         a transient hits), the MOV begins to conduct, clamping the
         input voltage to a safe level. In effect, the MOV absorbs the
         transient and dissipates the energy as heat.


           The following instructions show you how to wire the MOV's
         into either a multiple outlet strip or the normal wall outlet.

         Parts:
              3- General Electric V130LA10A MOV's
                 (Radio Shack part# 276-570) light duty  $1.59 ea.
                 (Radio Shack part# 276-568)    med duty  $1.69 ea.
              1- Power outlet strip that can be readily opened is suitable.
                              or
              1- 120v AC "duplex outlet" with ground.

         POWER STRIP MOD:
              Open the power outlet strip to obtain access to the
         internal wiring between the sockets. There will be three
         wires connecting the outlets to one another and the power
         cord. They will generally be colored black, white, and green
         (hot, neutral, and ground). Remove sufficient insulation from
         the wiring between the sockets so that the three MOV's can be
         soldered across the three pairs of wires as follows:
         (1) Green & black  (2) green & white  (3) black & white. It
         may be necessary to insulate the solder connections and the
         MOV's with electrical tape or insulating tubing. Replace the
         cover and the job is completed.

         OUTLET BOX MOD:
              Do "NOT" attempt to work on the outlet box with a "live"
         circuit! Remove the wall plate. Remove the two screws that
         hold the outlet to the box. NOTE: Insulating tubing will be
         necessary on the MOV leads to prevent shorting. Loosen the
         screws as required and connect a MOV from the white lead to
         the black lead. Connect another MOV from the black lead to
         ground. The third MOV goes from the white lead to ground.
         Carefully dress the MOV's against the outer shell of the
         outlet. Examine your work carefully for any exposed wire that
         may produce a short circuit. Replace the outlet into the wall
         box. Restore electricity.

           You have provided outlets with surge and transient protection
         for a cost of less than $5.00 each, with the assurance that
         these transients and surges can be "clamped" within a
         response time of less than 35 nanoseconds. More than
         sufficient to save your valuable equipment.

         References:
           (1) Ciarcia, S. (1983), Byte 8(12)
           (2) Kanter, E.  (1985) Computer Digest, Oct.
           (3) Natural Toxins Research Center Bulletin # 2907