Note: Descriptions are shown in the official language in which they were submitted.
5~
SHOC~-PROOF MAINS ~OLTAGE SUPPLY OUTL~T AND MET~OD
The presènt invention relates to mains voltage supply
outlets used to connect movable appliances to the source of
power. This includes those outlets of the type used ln
residences, offices, businesses, hotels and in public
places, to power household and other appliances ranglng from
lamps to vacuum cleaners, heaters, toasters, hair dryers and
similar devices; and is more particularly directed to
insuring the safety of such outlet from electrical shock by
the inadvertent or misguided purposeful touching of the
outlet terminals by children, animals or adults, as may be
encountered through insertion of paper clips or nails or
even small fingers into outlet apertures and into contact
wlth the metal terminals of the outlet.
Numerous devices have been evolved over the years for
mitigating against such dangers including the current use of
fi.Yed or rnovable plastic lnserts to cover the outlet
apertures and mechanical on-off switches -- both requiring
human operation or control. Illustrative of prior
approaches or atternpts at improved plugs and the like are
2~ s~
U.S. Letters Patent Nos. 3,169,239; 3,368,110; 3,441,799;
3,706,008; 3,864,~81; 3,909,566; 4,o80,640; 4,175,255;
4,306,374; 4,484,1~5; 4,584,430; and 4,722,021. Most of
these devices show methods of disconnecting power from the
appliance ln case of overload or describe mechanical devices
to prevent inadvertent contact with the source of power.
Underlying the present invention, however, is the
concept of employing appropriate electronic circuits
cormected between the mains supply lines and the outlet
terminals that~ in effect, respond automatically to the
lmpedance presented between the outlet terminals
unambiguously to distinguish between a condition where the
human body is connected thereto and the condition where an
electrical appliance that is to be powered ls so connected
-- insuring automatically that no or only a trivial amount
of voltage or power is available in the former case, and
~ubstantlally full power is connected to the appliance in
the latter instance.
An obJect of the inventlon, accordingly, is to provide
a new and improved mains outlet and method of operating the
same that obviate the above-described problems and
automatically and ~afely distinguish between the condition~
2~ 5~
of human or animal contact with the outlet and appliance
contact therewlth to prevent any substantial voltage or
po~er being drawrl in the former case and to apply
substantially full mains voltage ln the latter condition.
Other and further obJects will be explained hereinaEter
and are more fully dellneated ln the appended claims.
In summary, from one of its broader aspects, the
invention embraces a method of rendering an outlet connected
to a mains power supply safe from shock upon human touching
of the outlet terminals, that comprises, sensing the
impedance presented between said outlet terminals;
responding to said sensing to apply no or very small and
"6afe" amounts of voltage and power from said supply for
sensed impedance values corresponding to the relatively high
impedance presented by the human body, wet or dry; and
responding to said sensing to apply substantially full
supply voltage for sensed impedance values corresponding to
the relatively low impedance presented by appliances.
Preferred and best mode appara~us and details are
hereinafter presented.
The inventlon will now be described with reference to
the accompanying dra~ings~ Fig. 1 of whlch is a simpllfied
s~
circuit diagram of a part of a circuit useful for the
purposes of the lnvention and which is exemplary to explain
the principles underlying the invention;
Figs. 2 and 4 are similar circuit diagrams of
modifications; and
Flg. 3 ls a preferred implementation Or the inven-tlon.
Referrlng to Fi6. 1, some Or the principles underlying
the invention will be e~plalned in simplifled fashion, even
though the circuit of ~ig. 1 is not commercially adapted for
operation without additional refinements as later explained.
In thls Fig. 1, there is shown a transistor 10 which has its
base biased to, for example, +5 volts by resistor 4, which
may, for example, be of the order of 250,000 ohms and a
resistor 2 whlch may have a much lesser value of the order
of 12,500 ohms, thus reducing the voltage at the base of the
transistor to about the said ~5 volts. The emitter of this
transistor is biased by resist,or 6, which may also have a
value of the order of 250,000 ohms, and resistor 8, which
may be of the order of 250,000 ohms~ resulting in an emitter
voltage of about +10 volts. Thus the transistor 10 cannot
conduct and the voltage at the terminals 7 and 5, which
represent the outlet, will remaln at a triv~al value of
s~
ab~5 l~/ v~ts. 5~ long as the trans1stor lO does not
conduct, the ma~imum current from the malns supply lA and
lB, such as the 110-115 volt or 220 volt supplies used
~7
throughout the world, which can flow to terminals is of the !,
order of a totally safe 0.40 milliamperes. When the
external resistance applled to termlnals 5 and 7 o~ the
outlet ls reduced far below the resistance values of the
human body, becomes less than about, say, 500 ohms, the
emltter voltage falls below the base voltage and the
transistor 10 wlll conduct. There ls then the classlcal
case of an emitter follower operation, wherein the voltage
across the outlet terminals 5 and 7 will be equal to the
5-volt base voltage appearing in transistor lO.
The above explains the basic operation of the
electronic swltching system of the circuit interposed
between the mains supply llnes lA and lB and the outlet
terminals 7 and 5, though in actual practice, non-llnear
devices must be incorporated to produce full power at the
termlnals 5 and 7 when the impedance presented across the
outlet terminals 5 and 7 is less than a relatively low
value, of at most a few hundred ohms as prescribed by an
appliance.
35~
In accordance with the invention, the outlet 5-7 ls
rendered safe from shock upon human touchlng of the outlet
terminals 5 and 7 through the interposition of the
electronlc impedance or resistance sensing and switching
circuit such a~ that in Fig. 1 lnterposed bètween the outlet
terminals 5 and 7 and the power supply llnes lA and lB. The
electronic switching device 10 of the circuit, a~ above
indicated, is biased to permit no or a trivial and safe
amount of voltage and thu~ power to be passed through the
circuit from the power supply lines lA and lB to the outlet
terminals 5 and 7 when the impedance between the terminals 5
and 7 is relatively high, say of the order of the impedance
of the human body when fingers, wet or dry, are touched to
ths terminals 5 and 7, thereby to prevent any shock. This
is somewhat analogous to the trivial voltage applied to toy
electric railroad tracks and trains which has long been
recognized as a totally safe condition, even for children
touching the terminals. As prevlously indlcated, these
relatively hlgh impedances have been measured to be of the
order of from hundreds of thousands of ohms to several
megohm~, depending upon the wet or dry condition of the
fingers or the portion of the body that becomes connected
s~
between the outlet plug terminals 5 and 7. When, however~ a
load impedance is sensed across the terminals 5 and 7 that
is relatlvely low compared to the above, say, in practice,
for lamps and similar electrlcal devices o~ the order of a
few hundred ohms and less, the above-mentioned condition of
conduction of the transistor 10 takes place and the mains
voltage is then applied with substantially full power
available to energize the appliance that has been plugged
into the outlet as previously described.
The clrcult of Fig. 1 has been described in simplifled
form ln connection with the positive cycles of the supply
voltage applied at the lines lA and lB. In order not to
lose the energy of the negative half-cycles, thls circuit
may be combined with a complementary clrcuit~as shown in
Fig. 2 to accommodate for the negatlve half-cycles as well.
If a circuit which is symmetrical to ground is desired, the
circuit of Fig. 4 containing transistors 30A, 30B, 30C and
30D and corresponding resistor networks 32A through D, 34A
through D, 36A and 36B and 38A and 38B may be employed.
In preferred ~orm, however, the ~witching devices can
a~sume the form o~ trlacs and diac~ whlch are not polarity
sensitive and therefore there is no need to provide multlple
9~
clrcuits such as shown in Fig. 2 to take care of the
condition of both the positive and negative polarity
half-cycles. Furthermore, mains plug outlets are normally
wired with a ground and a hot conductor, at least in the
United States~ and there is no need to provide a balanced
output in such instances.
The precedlng has descrlbed the translstor clrcuits in
which the resistor 6 of Fig. 1, for example, wlll dellver
very small current lnto the hlgh impedance load of the human
_ _ _ , _
body touching the outlet and in which the transistor 10 in
switching to conduction will deliver ~dditional current of
the type necessary to enable powering of a low output
impedance device, ~uch as appliance, when it ls applied -to
the outlet terminals 5 and 7. The addltlonal current thus
applied to the low impedance device is limited because, in
the emitter-follower circuit of Fig. 1, the current in the
transistor cannot rise beyond the point where the voltage
acros3 the emitter circuit equals the voltage at the base.
Thus, ~n a commercially ùseful system, to develop effective
power in the output clrcult for po~ering the deslred
appliance when plugged into the outlet termlnals 5 and 7,
the transistors are supplemented with or preferably replaced
35~
by non-linear devices such as the triac and diac type
electronic switching illustrated in Fig. 3. Referring to
that figure, the triac 110 replaces the transistor 10 of
Fig. 1. The triac has terminals 112, 114 and 116. In the
data sheets these are generally referred to respectively as
"Main Terminal-l" (112)~ "~ain Terminal-2" (114) and "Gate"
(116). Critical voltages are the voltages occurring between
terminals 112 and 116. The triac, of course, is a device
which in its idle condition has a high lmpedance between lts
main terminals 112 and 114, and which can be switched to a
low impedance device by applying a voltage pulse of a
magnitude which exceeds the voltage at the terminal 112 and
causes the switching of the triac. This conversion will
take place with supply voltage of either polarlty.
A voltage divider consisting of resistors 102 and 104
is installed at the gate side and another voltage dlvider
consisting of resistors 106 and 108 is installed in the
"mains terminal" side of the circuit. The Junc-tion of
resistors 106 and 108 is connected to the "Maln Terminal-l"
(112), and to the output terminal 12~ of the outlet shown at
122. The ~unction of resistors 102 and 104 is connected to
the gate 116 of the triac serially through a diac 120,
having the property that lt presents a very hlgh lmpedance
to currents of either polarity until the voltage across the
same has reached a certain threshhold; and then, when that
voltage is exceeded, the dlac becomes ~ a very low
impedance device. When the voltage across the diac is
reduced to elther zero or to a very low value, it reconverts
into a high impedance device As,a result of these
properties, the diac prevents any significant amount of
current fro~ entering the triac gate 116 untll the time that
the voltage appearing at the ~unction o~ resistors 102 and
104 is sensed to be sufficiently high to trigger the triac
to its switched conducting mode. A condenser 118 is
connected across resistor 104 and the energy stored in the
condenser at the time of triggering is applied through diac
120 to the gate 116 of the triac and helps to insure
positive triggering o~ the triac. Because of the external
capacities, and specifically because of the capaclty between
the terminal~ and the gate, a very ~hort duration spike on
the power supply can, in some instances, cause a spike to
appear on the gate terminal 116 and such could cause a
spurious triggering o~ the triac. Insertlon of resistor
130, whlch is a low re~istance value~ prevents such spikes
2~ S~
from a~fectlng triac operation. If desired~ a protectlve
fuse 128, as shown, may be inserted in the power supply.
The operatlon of the clrcult shown in Flg~ 3 is as
follows: so long as a hlgh impedance which may conslst o~
parts of the human body is sensed at the output terminals
124 and 126 of the outlet, or when there ls inflnite
impeda~ce or the human body or some portlon thereof is
.~ . .. . ~
connected across those terminals, the voltages appearing at
the gate 116 are equal to or lower than the voltage which
appears on the "Termlnal-l" (112) and the triac cannot
conduct. Suitable numerical examples of voltages are shown
in the flgure. The resistance between the two hands of an
adult person measure to be, say, about 500,000 ohms when
test leads of a conventional analyzer are squeezed between
salt water the moistened thumb and index finger of each
hand, resulting in an equivalent resistance between the
terminals 124 and 126 of about 43,000 ohms. As a result of
the presence of resistance 130 there occurs a voltage of
about llc4 volts at "Terminal-l" (112) and at the gate. In
thi~ case the triac will not conduct. In contrast with
thi~, when a 25-watt ligh-t bulb is plugged into the outlet
termlnal~, a resistance to ground at terminal 112 and the
5a~
gate 116 of about 400 ohms is sensed. ~t this point the
voltage across resistor 108 is momentarily reduced to a
fraction of a volt. This triggers the diac which in turn
triggers the triac which thereafter shows a voltage drop
of a fraction of a volt.
The resisLance-capacitance values and voltage appear-
ing in the figure are approximate vaLues to those used and
observed in experimental apparatus.
Referring to the before-described Fig. 4, a furt11er
transistor version embodying a "symmetrical to ground"
output is shown. The transistors in this circuit can be
replaced by non-linear devices such as those described in
connection ~ith Fig. 3.
In practice, it is to be understood that these cir-
cuits may be formed on chips or in very small packages and
made integral with the outlet plug itself or can be made
in the form of kits to be added to existing mains outlets
terminals, extension cords, etc.
Current sensing may also be used to determine the
impedance presented at the outlet terminals instead oE
voltge sensing (dividing) as herein lllustrated and which
is par~icularly useful for the use oE motors and other
inductive loads introducing phase shif~s.
12a
The saee outLet circuit Oe the invention may also be
interposed between conventional ground fault inLerruptor-
ground neutral detector systems (for example, the type in
"Industrial Blocks," p. 9-100 of the current National
Semi-Conductor Application ~landbook) and the appliance or
load ouLlet, usLng the outlet Oe the invention for the
appliance, to render such sysLems safe from leakage
current and o~her unsaFe conditions that may occ-lr
therein.
~ urLher moclifications will occur to Lhose skilled in
this art and such are considered to fall within the spiriL
and scope of Lhe lnvent:lon as defined in the appended
claims.
