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June 1975 Popular Electronics
 Table of Contents
Wax nostalgic about and learn from the history of early electronics. See articles
from
Popular Electronics,
published October 1954 - April 1985. All copyrights are hereby acknowledged.
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As is usually the case,
John Frye uses his monthly "Mac's Service Shop" story to entertain whilst proffering
a valuable lesson in the field of electronics. Mac is famous for his superb knowledge
of electrical principles, and for his ability to troubleshoot and solve just about
any problem put before him. His sidekick technician, Barney, is a young Ham operator
who often needs the admonishment and/or wisdom provided by Mac. In this 1975
Popular Electronics magazine piece titled "Taming Static Electricity," Barney
is saved by Mac from the wrath of office girl Matilda after she received an electrostatic
discharge (ESD) shock intentionally administered by the young man. Prior to the
advent of microelectronic circuits, everyday ESD was generally not a huge concern
in the electronics realm because components were naturally robust enough to survive
the high voltage, but as things miniaturized, ESD began causing damage and/or failure.
Micron-dimension gate widths in ICs are easily destroyed by even a couple hundred
volts applied for a split second. By 1975, special instruments and test procedures
were being developed to study ESD in electronics.
Mac's Service Shop: Taming Static Electricity
 By John T. Frye, W9EGV, KHD4167
Through the open door of the service department, Mac watched Matilda typing in
the outer office. She did not hear Barney, Mac's assistant, come in from the bright
freezing weather outside. He quietly removed his snow boots and stealthily walked
across the floor, sliding his feet on the carpet, until he was standing directly
behind the absorbed girl. He slowly reached out a forefinger towards the nape of
her neck, and she suddenly let out a shriek scattering the papers she was holding.
Barney beat a hasty retreat to the service department with Matilda in hot pursuit.
"Let me kill him," Matilda begged, trying to dodge around Mac and get at Barney.
"He stuck me with a pin."
"I did not," Barney denied, grinning down maddeningly. "I didn't even touch you."
Static Electricity Is to Blame
"He's right. I saw the whole thing," Mac said. "He gave you a shock with static
electricity, and that gives me an excuse to continue a discussion of several months
ago. Then we talked about the uses of static electricity for such things as smoke
precipitation, ore separation, spray painting, and flocking. These are examples
of static electricity on its good behavior - which it usually isn't! Most of the
time it's causing shocks, unruly hair, clinging clothes, lightning strokes, and
explosions; or it's fouling up printing and manufacturing processes or destroying
IC's and transistors. Astronomy has been called the wise child of a foolish mother,
astrology. In the same way, static electricity might be called the mischievous,
annoying parent of a hardworking son, current electricity, which provides us with
light, heat, telecommunication, and power. So now let's talk about how we can take
the mischief out of static electricity."
Custom Materials' Model CMI -7777M Static Meter.
"Amen!" Matilda said soulfully, rubbing the back of her neck.
"You both know from your high school physics or our previous discussion that,
when certain substances are placed in firm contact and then separated, electrons
transfer from atoms of one substance to atoms of the other. Atoms which lose electrons
become positively charged ions, or cations; those gaining an electron become negatively
charged ions, or anions. Both a potential difference and an electrostatic attraction
develop between the separated, oppositely charged surfaces. In fact, if only one
electron in 100,000 atoms of a surface is exchanged, that surface is very strongly
charged. The substances vary widely, but one or both is usually a poor conductor.
Some combinations are: glass and silk, wool and hard rubber, paper and a printing
press roll, a fabric belt and a steel pulley, a rubber tire and the pavement, a
cold dry stream of particle- bearing air and an airplane wing."
"Or the soles of a pair of size eleven clodhoppers sliding across a nylon carpet,"
Matilda injected tartly, glowering at Barney's feet.
"Electrostatic attraction is what makes your hair follow the comb on a snappy
winter day such as this," Mac hastened to continue. "The passage of the comb leaves
one type of charge on the hair and the opposite on the comb. But since the individual
hairs have similar charges, they repel each other after the comb is taken away leaving
you with a Phyllis Diller coiffure. Dampening the hair provides a conductive path
for the charges to leak off the ends and permits you to comb it down.
"Here we have one rule for the control of static electricity problems: provide
a comparatively low resistance path for the charge to leak off as fast as it accumulates
on a nonconductor or on the surface of an insulated conductor. I say 'comparatively
low resistance path' because a resistance as high as one megohm will ordinarily
be adequate for preventing a potential buildup. When the relative humidity is raised
above 70%, most insulators become conductive enough, either through absorption or
hygroscopic action, to allow a static charge to leak away as fast as it is formed.
Where static electricity causes the adhesion or repulsion of sheets of paper, layers
of cloth, fibers, etc., humidifying the atmosphere has proved to be a solution in
some industrial applications. Unfortunately, this sometimes brings on new problems
with both materials and personnel.
"There is another way we can put moisture to work as a conducting path. That
is by loading a plastic with a 'detergent' hygroscopic material or by spraying this
material on a fabric or rug. This material will attract and hold water molecules
forming a conductive path that prevents static buildup. Tomorrow, Matilda, I'll
spray that rug out there with Anti -Shock Spray, marketed by Bigelow-Sanford, Inc.,
Greenville, S.C. Then you should be safe from Barney's shocking behavior until the
humidity takes over in the summer."
"Then high relative humidity doesn't prevent the 'generation' of static electricity;
it simply interferes with the accumulation of a charge high enough to be noticed,"
Barney observed. "Incidentally, how high a stored charge is 'high enough' to cause
trouble ?"
"Consider what happens in explosions of gases and vapors ignited by electrostatically
produced sparks. The ability of such a spark to produce ignition depends on its
energy, which is some fraction of the total stored energy. In a nonconductor, in
which the charges are tightly bound, this fraction is low because only a small area
can contribute to the spark; but with a conductor, through which charges move easily,
the total charge contributes to the effect. That's why sparks between two nonconductors
generating a charge seldom produce hydrocarbon gas explosions directly, but they
induce charges in nearby conductors which are quite capable of producing such explosions.
Diagram shows how meter to measure static buildup works.
"Stored energy is a function of both voltage and capacity. It can be calculated
from the formula: E = ½CV2 x 10-4; where E is the energy
in millijoules, or thousandths of a watt - second; C is the capacity, related to
the surface area, in picofarads; and V is the potential across the spark gap in
volts. It has been found that about 0.25 millijoules of stored energy is required
for ignition of optimum mixtures of air and saturated hydrocarbon gases and vapors.
Sparks arising from potential differences of less than 1500 volts are unlikely to
be hazardous in these mixtures because of the short gap and the heat loss at the
terminals. Plugging these minima into the formula, you come up with 222 pF, which
is slightly less than the capacitance of a large man. As the voltage goes up, the
minimum capacity goes down. The spark from Barney's finger to your neck, Matilda,
was at least ¼ inch long, which indicates his body had a charge of around 20,000
volts. At that voltage, a capacitance of only 1¼ pF., or about that of a 20-penny
nail, could store enough energy to ignite gasoline fumes. Incidentally, remember
when we used to see gasoline trucks dragging metal chains to ground static buildup?
We don't see that any longer because it didn't work well. Now they bond the metal
tank of the delivery truck to the ground tank before gasoline is pumped into or
out of the truck tank.
"Electrostatic voltages far below the 1500 -volt level can do expensive damage
to the equipment we use here. Some MOSFET's will be permanently damaged if as little
as 30 volts is applied across the thin oxide region from gate to source. If your
body is charged, just touching a lead can destroy such a device. Safe handling of
many IC's and transistors requires that everything coming in contact with them be
devoid of static charges, which is another way of saying everything should be grounded."
Special Products
"Custom Materials, Inc., Chelmsford, Mass., manufactures a special electrically
conductive plastic product called VELOSTATR just for this purpose. The conductivity
is inherent and not dependent on moisture attraction, as is the case with many other
'antistatic' materials. When workbench, stool seat, and floor mat are covered with
Velostat; when the worker is wearing Velostat boots, apron, or gloves; and when
all these Velostat items are bonded to each other and to ground with Velostat ribbon;
there's no opportunity for damaging electrostatic charges to accumulate.
"Now, let's consider some other ways of eliminating static charges.
"The shape of a conductor has much to do with the distribution of a charge on
its surface. The mutually repelling action of like charges concentrates the charge
on the portion with the least radius of curvature. On a needle point, where the
radius is near zero, the charge can be so concentrated that it ionizes the air at
the point and is absorbed in the process. The discharge in such a case starts at
a much lower voltage than would be the case if the conductor did not have a sharp
point. Static-discharging 'wicks' bolted to the trailing edges of airplane wings
are often made of frayed copper braid to take advantage of this ability of small
diameter points to dissipate a charge.
"If a series of needle-pointed conductors is fastened to a metal bar so as to
form a 'comb' and this is mounted close to a fabric belt being electrostatically
charged by running over a metal pulley, the belt charge will induce an opposite
charge in the comb. An ionized path between the belt and the needle points then
dissipates the charge as rapidly as it forms. This works equally well with paper
passing over rollers.
"A battery of such combs can be made to ionize the air by placing a high voltage
on them. Then a fan blowing this conducting ionized air across any area where troublesome
static charges may form will bleed off the charge to some nearby grounded object
it can accumulate."
"What's this picture of a handheld device that looks like a ray gun ?" Barney
asked.
"That's a picture of Custom Material's instrument for measuring the potential
of static electric charges from a safe distance. Here's a diagram of how it works.
The meter, which is essentially at ground potential, is handheld at distance, d,
from the charged object. A potential difference V, develops between the object and
the gun tip setting up an electric field, E, directly proportional to V and inversely
proportional to d. An opening in the gun barrel is covered with a wire screen; and
directly behind the screen and to the side is a small sample of radioactive tritium
emitting beta rays or electrons toward the screen. Electron- bombarded air molecules
are ionized to form a thin layer on which the electric field of the static charge
impinges. Since a field exerts a force on a charge, the ionized air molecules move
forward or backward, depending on the polarity of the charge, the amount of movement
being related to the intensity of the charge.
"A metal electrode behind the layer of ionized air is connected to a sensitive
FET amplifier and the current produced by the movement of the charged air molecules
is displayed on a meter on the back of the gun. By holding the gun tip at distances
of 2", 6", and 12" from the charged object and using a 3-position mechanical range
selector, nine full-scale values covering a range of 500 to 200,000 volts are achieved.
"Now, we've got to knock it off and start earning a buck, although we've barely
scratched the surface of the fascinating subject. We've talked before about lightning,
the really big show of static electricity. Sometime, maybe we will discuss precautionary
measures used in operating rooms, around explosives, in dusty locations, on helicopters,
etc.
Mac's Radio Service Shop Episodes on RF Cafe
This series of instructive
technodrama™
stories was the brainchild of none other than John T. Frye, creator of the
Carl and Jerry series that ran in
Popular Electronics for many years. "Mac's Radio Service Shop" began life
in April 1948 in Radio News
magazine (which later became Radio & Television News, then
Electronics
World), and changed its name to simply "Mac's Service Shop" until the final
episode was published in a 1977
Popular Electronics magazine. "Mac" is electronics repair shop owner Mac
McGregor, and Barney Jameson his his eager, if not somewhat naive, technician assistant.
"Lessons" are taught in story format with dialogs between Mac and Barney. There
are 131 stories as of January 2026.
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