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April 1973 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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Long before any one was
overly concerned with relatively paltry electrostatic discharge (ESD) current causing
damage to semiconductor components, there was a need to model the human body's resistance
to current flow due to electric shock concerns. Even with a huge number of people
being severely shocked and/or killed due to exposure to potentially lethal voltage
levels, it was not until the late 1960s that OSHA and the National Electric Code
began requiring exposed metal components (chassis, switches, etc.) to have a safety
ground connection. Popular Electronics magazine ran an article titled "Shocking
But True" in the August 1959 issue dealing with the subject. Many older radio
and TV chassis would be "hot" if the 2-pronged plug was inserted the wrong way into
the wall receptacle, so touching any metal component (even an exposed tuning knob
or volume control shaft) would light you up. The situation was even worse in the
early days of AC electric service because in many cases there was no earth ground
established at the service entrance, so both conductors were floating and could
be at any voltage between ±170 volts (120 * √2) wrt ground.
Today's formal human body model (HBM, latest version are
JS-001-2017,
IEC 61000-4-2, and
MIL-STD-883K) ESD test source consists of a100 pF capacitor
in series with a 1,500 Ω resistor. The capacitor is charged to the specified
voltage and then it is discharged through the resistor into the device under test
(DUT).
Mac's Service Shop: Leakage Current Testing and Using Square Waves
By John T. Frye, W9EGV, KHD4167
 April was never more welcome. The winter had
been long and cold and bitter, filled with natural and man-made disasters; but now
it was over and spring had returned just as, mercifully, it always does.
Barney came skipping through the front door of the service shop and gently laid
a long-stemmed yellow tulip atop Matilda's typewriter; then without a word to the
astonished office girl, he went on back to the service department, where he found
Mac, his employer, examining two new instruments resting on the service bench.
The larger, at first glance, looked like a conventional Bakelite-cased VOM, but
lettering on the front revealed it was a "Simpson Model 229 Leakage Current Tester."
The range switch had positions designated: OFF, BATT TEST, 150 VAC, SHORT TEST,
10mA, 3mA, 1mA and 0.3mA. There were matching scales on the meter face.
"What do you do with that odd-ball meter?" Barney asked.
"You measure the amount of 60-Hz current that flows from an electrical line conductor
to the metal exterior of an electrical device and thence through an electrical simulation
of a human body to an earth ground. Such a dangerous current flows when there is
a conductive path, resistive or capacitive, between the hot wire of the device and
the case and when the person using the electric drill, hedge-clipper, sander, or
what have you simultaneously touches the case and a grounded object.
"This current flows because one side of the 117-volt, single-phase, two-wire
line is grounded at the pole transformer and again at the electrical entrance to
the building. That means you do not have to touch both wires simultaneously to be
shocked. All you have to do is to establish a return circuit between the 'hot' wire
and the earth. You can prove this to yourself by connecting a 117-volt bulb between
the hot wire and a water pipe. The bulb will burn almost as brightly as if it were
connected across both wires of the line. It will also burn, although usually less
brightly, if a poor ground such as a metal stake driven into the earth is used.
"The American National Standards Institute suggests the circuit I've drawn there
on the blackboard to test for leakage current. As you can see, one lead of a meter
whose characteristics correspond to their specified standards - and this one does
- is connected to the grounded neutral conductor. The other meter lead is used to
probe the case or any exposed metal parts of the appliance being tested. The ac
line is connected to the appliance through switches S1 and S2. All switches on the
appliance are turned on.
"With S1 open, only the hot wire is connected to the electrical circuit, and
the return path from the case to the grounded lead is through the meter. With S1
closed, power is applied normally to the device being tested, but leakage current
reaching the case still returns to ground through the meter because the ground pin
socket of the 2-pole, 3-wire grounding type socket is left open for these tests;
otherwise any leakage current would bypass the meter. S2 reverses the hot and ground
wire connections to the appliance. Motor operated appliances are tested under 'no
load' conditions. Heating appliances are tested at maximum heat setting of controls."
Circuit suggested by American National Standards to check leakage
current.
"Why reverse the line cord connections?"
"Because, if the leakage path happened to be between the grounded lead and the
case, no leakage current would be present; but when the line plug was reversed,
the path would be between the hot lead and the case and leakage current would appear.
Take that signal tracer of ours. With S2 in one position, no leakage current is
seen; but in the other position, 2.25 mA of current passes from the case through
the meter."
"A partial short from one end of the transformer primary to the core, huh?"
"No, although that could happen. The leakage current path is through an 0.05
capacitor from one side of the line to the chassis, which is bolted to the case.
The reactance of this capacitor at 60 Hz is about 53,000 ohms, which will pass almost
precisely 2.25 mA of current when subjected to 117 volts, as happens when the side
of the line to which the capacitor is connected is hot."
Resistance of Body. "You say that meter simulates the resistance
of the human body to electrical current. How does it do that?"
"By presenting a terminal impedance of 1500 ohms noninductive resistance shunted
by 0.15 μF of capacitance. This is the experimentally determined 50 percentile
threshold-of-perception-curve value determined by Charles F. Dalziel and others
for an average human being in an average environment of temperature, pressure, humidity,
etc., when subjected to small ac or dc currents. Remember that name of
C.F. Dalziel. You're
going to hear much more about him when we talk about the biological effects of electric
shock in the near future. But for now, remember the impedance of the human body
to electrical current can vary tremendously from this value under non-average conditions.
Also health and other conditions can affect the individual's tolerance to electric
shock. That's why the maximum leakage current for appliances is set by the ANSI
at only 0.5 mA, although most human beings cannot feel even the faintest tingle
of electricity until the current is twice this value. The Underwriters' Laboratories
feel that this 0.5-mA value of leakage is not likely to produce such adverse effects
as ventricular fibrillation, inability to let go of a current-carrying device, or
an involuntary reaction which can result in in-jury from secondary causes (e. g.
fall from a ladder, spill hot liquids, etc.)"
"Then why does that meter have all those current ranges?"
"So it can be used to detect everything from a direct conductor-to-case short
circuit to very small leakage currents produced by damp insulation or carbon dust
paths. Note this instrument is designed to perform one specific job: to measure
leakage currents of appliances in accordance with American National Standard specifications.
We're going to use it to test all our instruments and electrically operated tools
for leakage on a routine basis and to check every set we work on to make sure it
is safe for the customer to use. I suggest you take it home with you and check all
your tools and all the appliances in your mother's kitchen: refrigerator, dishwasher,
toaster, mixer, blender, etc. After you do you'll probably conclude as I have that
only a careless idiot employs a cheater plug so he can use his three-wire hedge
clipper ungrounded with a two-wire extension cord. He's usually asking for it."
Pocket Pipper - February 1971 Electronics World
www.American
RadioHistory.com
What's a Pipper? Barney didn't answer. He had picked up the other smaller instrument
from the bench and was examining it curiously. It was a little tan-colored metal
box about 2 3/4" X 2" X 1 1/2" with a BNC connector sticking out one end and a control
knob and a slide switch on top. One switch position was marked FAST and the other
SLOW.
"Who is 'TFE' and what is a 'PP-1A'?" Barney read from the little case.
"TFE is the manufacturer of that little gem. The letters stand for 'Tools for
Electronics,' P.O. Box 2232, Denver, Colorado 80201. The PP-1A tells you that is
a 1A Model of the Pocket Pipper" Mac explained with an anticipatory grin.
"A pocket what?" Barney asked incredulously.
"Pipper. Actually it's a miniaturized, sophisticated, battery-powered, fast rise-time
step generator that puts out pulses of stepped voltage with repetition rates of
either 2 kHz or 200 kHz. Open circuit output voltage is about 500 millivolts, which
falls to about 2/3 this value when working into the output impedance of 50 ohms.
But listen to this: the rise time of that stepped voltage is less than 2 ns when
working into a 50-ohm load, the following flattop is free of overshoot and ringing,
and the fall time is about 5 ns. When working into an open circuit, the rise time
only increases to about 3 ns while the fall time increases to about 20 ns. The open
circuit rise and fall times cannot be measured exactly because scopes with sufficient
bandwidth to measure them have 50-ohm inputs."
"What on earth is inside that little box?" "A few ordinary components and some
extraordinary ingenuity. Two transistors are used to form a free-running multivibrator.
The square wave output at the collector of one of these drives another transistor
as an emitter follower. The emitter load of this transistor is a tunnel diode. Now
you will recall that a tunnel diode can be biased so that only a very small change
in applied voltage will switch it with great rapidity from a high-voltage to a low-voltage
state, and vice versa. The control on top of the Pocket Pipper is used to adjust
the bias to that condition. Then the square wave from the multivibrator feeding
through the emitter follower triggers the tunnel diode back and forth between the
high and low voltage states and produces the square-wave-like waveform. But let
me show you."
Mac slipped a BNC connector carrying about three inches of coax into the Pocket
Pipper connector and clipped the leads of the bench scope to the conductor and shield
of the coax. He adjusted the scope sensitivity for 100 millivolts/cm and the sweep
for 100 microseconds/cm. With the slide switch of the PP-1A in the SLOW position,
he turned the control knob clockwise from the OFF position. A couple of cycles of
a squarewave-like trace, about 1/2 cm high, appeared on the scope. As Mac advanced
the control, the trace suddenly jumped up to a height of more than 5 cm. But now
only the horizontal lines marking the tops and bottoms of the square waves could
be seen clearly. The bottom lines had curious little up-and-to-the-right hooks on
their right ends. Only by advancing the brightness away beyond normal could the
vertical rise and fall lines be made out dimly.
"Those little hooks on the bottom lines show the increase in the triggering voltage
on the tunnel diode just before it switches, "Mac explained. They're not involved
in the rise time that's defined as the time it takes a stepped voltage to increase
from 10% to 90% of the final value. Now let's look at the 200-kHz output." He readjusted
the scope sweep to 1μs/cm and switched the Pocket Pipper to FAST.
"Oh, oh!" Barney exclaimed. "The waveform's not so good on that speed. Look at
the overshoot and ringing." Sure enough, there was about 7 or 8% overshoot of the
leading edges of the square waves and a definite wrinkling of the first part of
the horizontal lines.
Scope Makes Difference. "That's what I thought until I got suspicious
and had a friend take a look at the output of the PP-1A on his Tektronix 547/1A1
scope," Mac said. "Not a trace of overshoot or ringing appeared on it at the FAST
speed. The vertical amplifier of our scope simply isn't up to handling a 200-kHz
stepped voltage with that fast a rise time without distorting it."
"Why not? Our scope has a bandwidth out to 5 MHz."
"The trouble lies in how the vertical amplifier response tapers off on the high
end. Our vertical amplifier falls off too abruptly because the high end has been
overcompensated by propping it up with peaking circuits. Ideally, the response of
an amplifier on the high end should follow a Gaussian curve in which the response
at twice the 3-dB down frequency is only down 12 dB. It's a smoothly tapering curve
like this," Mac said, illustrating with a hand wave.
"Why, Mac, I didn't think you'd noticed my figure!" Matilda said from the doorway
where she was striking her best starlet pose, leaning hack against the jamb, her
back-flung head cradled in her hand, lips parted, and holding the stem of the yellow
tulip between her teeth.
"Back to your typewriter, wench!" Mac said, getting red in the face. "It's getting
so a man can't talk sense around here. Anyway, Barney, I've long wanted a really
fast rise time generator we can use to test, for example, video amplifiers for bandwidth,
rise time, and transient response. Our old square wave generator is perfectly adequate
for most of our requirements, but its rise time of 2 microseconds is too slow to
trigger transients in extended-range amplifiers. Still I can't afford to put $500
or so into a fast rise time pulse generator we need only occasionally. But this
little gem costs only $12.95 in kit form and will serve our needs handily. Why are
you grinning like a Chessy cat?" he broke off.
"I was just thinking that now you're not going to be happy until you get a scope
that can keep pace with that Pipper."
Mac tried to scowl but couldn't quite manage it. "Some people around here are
getting just a little too smart," he growled.
Posted May 2, 2024
(updated from original post
on 11/24/2017)
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.
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