Scan Times V.10

1. Sector 3 linking will be off line for 37.5 seconds starting 07-05-23, 1753 hours.
2. New work cell motto: "Silicon is pure. Carbon is flawed." (Repeat 2 times0
3. Sector 7 elementary reading assignment changes - week 22-23, Low Byte and the Seven Bits; week 24-25, Binary calculus Vol.2.

SOA History File Update

John V. Atanasoff (1904 - 1995)

In 1939, Professor Atanasoff, while at Iowa State University, developed the first computer prototype using vacuum tubes. His invention was overshadowed for 34 years until 1973 when a Federal judge ruled the professor's computer design "reduced to practice" as the first electronic computer. The Smithsonian displayed his prototype in 1989 and President Bush gave him the National Medal of Technology in 1990.

Agra-artist Earth Contest 27^thRound Results

Scan Times Editors' Choice Award for Automation Progress

The Metal Oxide Varistor (MOV)

By now, this device has saved the world trillions of dollars in damage from spikes, arcs, backlash, RFI/ EMI, surges and over voltage events. Be it a complex computer system or a lowly relay contact, the MOV does its job by conducting and absorbing any over voltage of either polarity that left unchecked would be damaging to any electrical system.

The MOV looks like a simple disk capacitor. Its size is related to its voltage protective range. Dime size MOVs can protect low voltage communication, phone and modem circuits. Quarter size units are found protecting 110 and 220 VAC equipment. Slightly larger units can protect 440 and 880 volt systems.

MOVs have two common applications. Most are used to protect electrical and electronic equipment from damage by spikes and surges originating from some other source. Those multi outlet power strips advertised as "surge protective" use MOVs. Not enough are used at the potential source of the arc or transient to suppress and prevent it from happening in the first place!

Except for transients from radio transmitters, static discharges (lighting) and solar bursts, the primary source of destructive electrical spikes can be traced to the electromagnetic coil. The coil, a great invention hundreds of years old, is a method of converting electrical energy into useful mechanical energy. Without the coil there could be no relays, solenoids, electric valves, motors, atom smashers or doorbells.

It's interesting that it's not the use of coils that cause damaging spikes but the disuse or disconnection of the coil from its electrical power source that results in a high voltage spike of energy. I blew the front end of a nice oscilloscope trying to measure the spike on a relay coil. Where's a MOV when you need one? Lesson learned. I applied only 7 volts to a coil normally rated at 110 volts. At the moment of disconnection I measured over 700 volts across the coil, over a hundred-fold increase in original voltage! This increase is very brief and will depend on number of turns of copper wire, the wire diameter, and the make up, size and position of the core or rotor or plunger. An AM radio 12 feet away gave a loud pop at every disconnection. Congratulations, I just reinvented the old spark gap transmitter.

With 7 volts applied and a MOV across the coil terminals, each disconnection measured less than 170 volts

And the radio complained with only a muffled click. I did not have a low voltage MOV for this test or the

RFI and the voltage spikes would be much lower.

How Is the Spike Formed?

When a coil is energized, it's an electromagnet with an electromagnetic field of force all around the outside and through the core of the coil. At the moment of disconnection this field does not dissipate into thin air. It collapses back into the copper wire as a surge of electrons all charged up with no place to go except backward arching across the ever-widening air gap of the disconnecting contact causing pitting and wear. Some of this energy also radiates into the air as an electromagnetic pulse.

How Does the MOV Work?

The metal oxide varistor is made like a parallel plate capacitor with fused zinc oxide filling as the dielectric. If the voltage potential is in the normal range, the MOV acts just like a disk capacitor across the coil leads or the power lines. However, zinc oxide has semiconductor properties, so when a voltage surge occurs the zinc oxide will conduct to absorb the excess voltage converting the voltage to heat. Lucky for you and the MOV,

these spikes and surges last only microseconds. One hundred-ten volt equipment and the MOV will "cook up nicely" if someone attempts to connect it to a 220-volt power source. For the typical spike of thousands of volts at thousands of amps, the MOV will do its job over and over again.

Contact life is extended by a factor of 30, RFI noise reduction, electronic protection. The MOV is now a sung hero. Thanks go to the Harris Corp and Intersil for tech specs.

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VOL 10 END.