|
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.
|
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 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 November 24, 2017
Mac's Radio Service Shop Episodes on RF Cafe
This series of instructive 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.
|
