Note: Descriptions are shown in the official language in which they were submitted.
CA 02168709 2004-03-05
TBREE WIRE AIR GAP OFF
POWER SUPPLY CIRCUIT
Field of the Inveatioa
The invention relates to three wire electrical power supply
circuits for connecting a load to an alternating current (AC)
power source and supplying power to a load switching element when
the load is disconnected from the power source. The invention
also relates to power supply circuits which limit line to load
current and line to ground current.
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Background of the Invention
A number of electrical power supply circuits such as
wall switch units for lighting fixtures are potentially
hazardous to individuals (e. g., repairmen). They comprise
an ON/OFF switch or other identified or implied OFF
function which most users assume isolates the circuit from
the power source when the switch is off. In other words,
a user may assume during servicing and maintenance that
there are no live parts on the load side of the power
supply circuit while the power supply circuit is in the
identified OFF mode.
Until recently, safety requirements under Underwriters
Laboratories (UL) standard 773 for nonindustrial
photoelectric switches for lighting control have not been
as stringent as requirements for other electric control
circuits in different environments, and most ON/OFF
switches and OFF mode identifying functions have been in
compliance with UL 773. New safety standards have been
devised, however, under the newly proposed UL 773A standard
which requires an air gap switch in these types of
electrical circuits. The newly proposed UL 773A standard
requires that a power supply circuit incorporate either an
air gap switch, or a solid-state switching device which
restricts leakag8 currents to 0.5 milliamperes or less back
to the load.
U.S. Patent No. 4,713,598 discloses a power supply
circuit 36 which comprises a current transformer XFR to
derive operating current, as shown in Figs. lA and 1B. The
primary winding W1 of the transformer XFR is in series with
a switching mechanism SW (e.g., a relay). When the
switching mechanism SW is closed, current flows through the
primary winding W1 and is induced in the secondary winding
W2. Voltage across the secondary winding W2 provides
operating power via a power supply 42 (i.e., diode CR1 and
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capacitor C1) for the control circuitry 44 (i.e., sensor S
and amplifier AMP). When the switching mechanism is open,
the voltage differential for deriving operating current is
across the secondary winding W2 to operate a power supply
42.
One of the drawbacks of this design is possible
noncompliance with the newly proposed UL 773A safety
standard. When the relay SW is open, the device 36 is
still electrically connected to the AC source via the
capacitor C2 and the secondary winding W2. When analyzed
with electronic~test equipment, it can be found on some
devices that a 2.5 milliamp current flows through the
secondary winding W2 of the transformer XFR even though the
switching mechanism SW is in the OFF or open position and
the load (e. g., a lamp) is no longer energized by the power
source. Further, the device 36 does not appear to comprise
energy or memory storage means for interrupting the full
line to load current path when the load has been opened
prior to the device 36 being put in an OFF position by, for
example, a slide switch (not shown) or other identified or
implied OFF switch. Thus, if the switch SW is a latching
relay, and the lamp has burned open, it appears that a
repairman could be exposed to full AC line current (e. g.,
15 amperes). This is because the power supply circuit in
Figs. lA and 18 does not provide means for changing the
state of the switch SW, that is, no identified or implied
OFF switch is provided to either directly or indirectly
open the current path to the load. The lamp, therefore, is
actually powered on until the relay SW is opened,
regardless of whether the slide switch is placed in the OFF
position. In addition, current transformers also have a
minimum load requirement. Thus, a need exists for a power
supply circuit which complies with the newly proposed UL
773A standard.
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Summarv of the Invention
The disadvantages and deficiencies of existing power
supply circuits are overcome by the present invention. In
accordance with an aspect of the invention, a three wire
power supply circuit is provided for selectively connecting
and disconnecting a load from an alternating current power
source having neutral, line and ground conductors, the load
being connected to the neutral conductor, which comprises
a relay connected at one terminal to the load and at
another terminal to the line conductor, a control circuit
connected to the relay and operable to open and close the
relay, a first rectifier circuit connected in parallel with
the relay and operable to supply as much as full line power
to the control circuit when the relay is open, and a second
rectifier circuit connected at one terminal to the line
conductor and connected at another terminal to the ground
conductor, the second rectifier circuit being operable to
supply power to the control circuit.
In accordance with another embodiment of the
invention, the power supply circuit is provided with an air
gap off switch.
In accordance with yet another embodiment of the
invention, the power supply circuit can provide circuit
components with, steady state power or with selectively
pulsed power to'both the load and ground.
Brief Description of the Drawinas
These and other features and advantages of the present
invention will be more readily apprehended from the
following detailed description when read in connection with
the appended drawings, which form a part of this original
disclosure, and wherein:
Figs. lA and 18 are schematic block diagrams of a
prior art power supply circuit; and
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Figs . 2 through 14 are sdhematics of three wire air
gap off power supply circuits constructed in accordance
with respective embodiments of the present invention.
Detailed Description of the Preferred Embodiments
Fig. 2 depicts a power supply circuit 76 constructed
in accordance with an embodiment of the present invention.
The power supply circuit 76 connects a load such as a
lighting fixture to an AC power source. The load is
connected to the neutral conductor 56 of an AC power
source. The power supply circuit 76 is connected to the
load via a load conductor 60, and is connected to the AC
power source via the AC power or hot line conductor 54.
With continued reference to Fig. 2, the power supply
circuit 76 comprises a switch mechanism K1 for controllably
completing or interrupting the current path between the
line or power conductor 54 and the return path to the AC
power source, i.e., the load conductor 60, the load 52 and
the neutral conductor 56. The switch mechanism K1 can be,
but is not limited to, a slide switch, a press switch, a
relay, a semiconductor switch, an optocoupler, a thyristor,
or any other mechanical, electromechanical or electronic
device for opening and closing a circuit. The switching
mechanism can be, controlled manually (e. g., a press button
or slide switch] , or by an electronic control circuit which
can include, but does not require, a microcontroller. For
example, the relay K1 of the power supply circuit 76 can be
switched to the ON position by a microcontroller 74 to
provide power to the load, and to the OFF position to power
down the load. A rectifier circuit D5 through D8 and
34 resistors R1 through R4 are connected in series with the
line and ground conductors 54 and 58, respectively. The
rectifier circuit D5 through D8 supplies power to relay
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control circuitry, which is described below, when the relay
is open.
With continued reference to Fig. 2, full line voltage
appears across the rectifier circuit D1 through D4 and
capacitor C16 when the relay K1 is open and the load (e. g.,
a lighting fixture) is off. Full line voltage appears
across the other rectifier circuit D5 through D8 and
resistors R1, R2, R3 and R4 and the ground conductor when
the load is on (i.e, relay K1 is closed), as well as when
the load is off. Thus, the lamp can be open (e. g., burned
out) or absent, and the power supply circuit can still
operate (e. g., activate the relay K1). The rectifier
circuit D5 through D8 and resistors R1, R2, R3 and R4 limit
the unpulsed line to ground current to a predetermined
limit such as 0.5 milliamperes as defined by the newly
proposed UL 773A standard. This line to ground current is
preferably only interrupted when an air gap switch SW1 is
open and the AC power source is disconnected from the power
supply circuit 76. This alternate line to ground circuit
allows for the device to derive power when the relay K1 is
ON (i.e., closed) or a lamp load is open. The air gap
switch SW1 can be, but is not limited to, a slide switch,
a press switch, a relay, a semiconductor switch, an
optocoupler, a, thyristor, or any other mechanical,
electromechanical or electronic device for opening and
closing a circuit. The air gap switch SW1 can be
controlled manually (e. g., a press button or slide switch),
or by an electronic control circuit which can include, but
does not require, a microcontroller.
The power supply circuit 76 in Fig. 2 is in effect a
hybrid two and three wire power supply circuit. The
rectifier circuit D1 through D4 and the capacitor C16
provide a parallel supply path to the 0.5 milliampere
current from the line to ground current path (i.e.,
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resistors R1, R2, R3 and R4 and rectifier circuit D5
through D8) when the load is off via an open relay K1. The
circuit 76 is therefore operating as a three wire power
supply circuit. When the load is on, rectifier circuit D1
through D4 and capacitor C16 supply virtually no power, and
the line to ground current is fixed at, for example, 0.5
milliamperes. The load or lighting fixture, therefore,
receives virtually full input power (e. g., 120 volts) when
the relay K1 is closed. The circuit 76 in this case is
operating as a two wire power supply circuit, and it
complies with .the newly proposed UL 773A standard.
Nevertheless, an air gap switch SW1 can be provided at the
AC main to operate as a true, mechanical open circuit when
the relay is in the OFF position.
The circuit 76 is advantageous because it can provide
a low input impedance and therefore low voltage drop across
the AC mains and the switch K1 when the load is on (i.e.,
the switch is closed). The switch Kl also operates in a
high impedance state and therefore creates a high voltage
drop across the AC mains when the load is off (i.e., the
switch K1 is open). A rectifier circuit (e. g., bridge
recti~f ier D1 through D4 ) is provided to rectify the voltage
in this case. The circuit comprises an air gap off circuit
(i.e., the switch SW1, which can be, for example, a form C
relay) for rectifying the secondary (i.e., the line to load
current path) of the circuit 76 when the load is present
but off. A rectifier circuit (e.g., bridge rectifier D5
through D8) is provided to rectify the voltage in the line
to ground current path when the load is open or the switch
K1 is closed.
The circuit in Fig. 2 is advantageous because it
complies with safety standards without consuming voltage
between the line and the load. The power supply circuit 76
depicted in Fig. 2 is also advantageous because it can also
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provide pulsed power versus steady state power to circuit
components requiring more power than a 5 volt regulated
supply (e. g. the relay K1, a buzzer or a light emitting
diode (LED)) in accordance with a signal generated by a
microcontroller 74. The bridge rectifier circuits D5
through D8 and D1 through D4 therefore are not required to
provide high, continuous current on their own. The
resistors R43 and R44, the diode D9 and the capacitor C27
regulate the output of the rectifier circuit D5 through D8
to provide a regulated DC voltage (e.g., 5 volts to the
microcontroller)~. Pulses, however, are generated as needed
by the microcontroller after the relay Kl, or, for example,
an LED or a buzzer, are energized so that a capacitor C17
can be recharged. They can also be controllably derived or
programmed to happen at fixed or varying intervals or duty
cycles. When the microcontroller asserts a pulsed signal
(e.g., a 5 volt signal, or a low signal if transistor Q8 is
a PNP-type transistor) to the transistor Q8, the
transistors Q7 and Q8 conduct and therefore shunt higher
current around the resistor R45 to the capacitor C17 for a
fast charge for discharging at a later time when, for
example, an 8.2 volt supply is needed to energize a
component such as the relay K1. The diode D18 shunt
regulates 8.2 voits to limit the voltage within operational
ratings of the capacitor C17 and other loads. The power
supply circuit allows increased line side or lighting
fixture load, while decreasing the current drawn from the
rectifier circuits. The capacitor C16 is preferably
selected to limit line to load voltage to 120V and current
to 2 or 3 milliamperes or other desired current level.
Further, the resistor R45 can be a high or low impedance,
depending on the trickle charge needs of the device being
energized.
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Fig. 3 depicts a power supply circuit 78 constructed
in accordance with another embodiment of the present
invention wherein the air gap switch SW1 (e. g., a slide
switch or a press button) is located on the load conductor,
as opposed to the power or hot line conductor (Figs. 4, 5
and 6). The relay K1 can be driven open via the
microcontroller when the microcontroller 74 receives a
signal indicating that switch SW1 has been activated.
Unlike the circuit depicted in Fig. 2, the power
supply circuit 80 in Fig. 4 comprises a voltage regulator
VR1 for regulating the voltage output to 5 volts, for
example, for the microcontroller 74. As shown in Figs. 2
and 4, resistors and/or capacitors can be placed on either
side of or on both sides of the bridge rectifier D5 through
D8 to regulate its output voltage. In addition, resistors
and capacitors can be placed on either side of or on both
sides of bridge rectifier D5 through D8, bridge rectifier
D1 through D4, bridge rectifier D3 though D6 and bridge
rectifier D21 through D24, which are depicted in different
ones of Figs. 2 through 14, to regulate output voltage.
Although the bridge rectifiers depicted in the various
views are illustrated as full-wave rectifiers, it is to be
understood that half wave-rectifiers can be used.
In accordance with another embodiment of the present
invention, the power supply circuit 80 can comprise another
bridge rectifier D21 through D24 and an air gap switch SW2
between the line and load conductors, as depicted in Fig.
4. Alternatively, an air gap slide or relay switch SW1 can
be used as described above. If the air gap slide switch or
relay SW1 is on, or the air gap switch SW2 is on, the power
supply circuit operates in substantially the same manner as
the circuit depicted in Fig. 2. The bridge rectifier
circuit D1 through D4 provides a power path for the relay
control circuitry when the load is off but present, and the
1
bridge rectifier circuit D5 through D8 limits the line to
ground current when the load is on or open.
With continued reference to Fig. 4, if the air gap
slide switch or relay SW1 is open, the power supply circuit
receives no power from the AC power source. Alternatively,
if the switch SW2 is in the OFF position, the bridge
rectifier circuit D21 through D24 is energized.
Accordingly, the microcontroller 74 can detect that the
switch SW2 is off and open the relay K1.
In accordance with another embodiment of the present
invention, a power supply circuit 82 can be provided with
an air gap slide or relay switch SW1, or a double-pole,
double-throw switch SW3 in series with both bridge
rectifiers D1 through D4 and D5 through D8, as shown in
Fig. 5, in lieu of the switch SW2 in Fig. 4. If the switch
SW3 is switched to an OFF position, the power supply
circuit is powered down. A low voltage drop out detector
84 in turn detects a drop in voltage and switches the relay
K1 and/or switch SW1 to an open position. Thus, a
repairman who switches the power supply circuit to OFF is
ensured that the load is not energized because the voltage
detector circuit 84 and microcontroller switch the relay K1
and/or switch SW1 to open when a drop in voltage is
detected. Switch SW3 can also be a single-pole, double-
throw switch.
The power supply circuit 86 in Fig. 6 is substantially
the same as the power supply circuit 82 in Fig. 5 except
that a high current-type switch is used. In contrast, a
low current-type switch is used with the circuit of Fig. 5.
The switch SW4a in Fig. 6 interrupts the line to load
current path when the load is powered on. The switch in
Fig. 5 can be rated for lower current because it is located
along a current path with a higher impedance when the load
is powered on.
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Similarly, the power supply circuit 88 in Fig. 7 is
substantially the same as the power supply circuit in Fig.
4 except that a high current-type switch SW5 is used,
whereas a low current-type switch SW2 is used with the
circuit of Fig. 4. The switch SW2 in Fig. 4 can be rated
for lower current because it is located along a current
path with higher impedance, when the load is powered on,
than the switch SW5 in Fig. 7.
Fig. 8 illustrates a power supply circuit 90
constructed in accordance with an embodiment of the
invention wherein at least one variable resistor is used in
conjunction with the microcontroller 74 to limit current to
an acceptable level (e. g., not more than 0.5 milliamperes
as specified in the newly proposed UL 773A standard)
electronically, as opposed to using a mechanical air gap
off switch. A variable resistor 92 can be connected to
rectifier circuit D1 through D4 to limit line to load
current. Alternatively, a variable resistor 94 can be
connected to rectifier circuit D5 through D8 to limit line
to ground current. As a further alternative, both variable
resistors 92 and 94 can be provided in the power circuit to
limit current in both the line to load and line to ground
current paths, respectively. The variable resistors 92 and
94 can be provided on either side of the associated bridge
rectifier. A variable resistor or varying impedance used
in con junction with a power supply circuit in accordance
with the present invention can be a potentiometer, a
varying impedance of an electronic device in which
impedance is variable and controllable, or an electronic
device which is operates in a variable impedance range,
regardless of whether the varying impedance is linear or
nonlinear.
With continued reference to Fig. 8, each variable
resistor 92 and 94 can be a potentiometer which is adjusted
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without microcontroller control. Alternatively, each
variable resistor can be separate from and adjusted by a
mechanically adjustable circuit element such as a
potentiometer 96. As a further alternative, the impedance
provided by each variable resistor can be changed to
accommodate various different and dynamic situations by a
microcontroller 74, depending on the type of feedback
signals the microcontroller is receiving. For example, a
single power supply circuit can be configured for use with
either of two different input voltages such as 120 volts
and 277 volts. The microcontroller 74 adjusts the variable
resistor automatically or depending on an input signal
received from a press button (not shown) operated by a user
to select one of the two input voltages.
Fig. 9 illustrates a power supply circuit 96
constructed in accordance with another embodiment of the
invention employing variable resistors 92 and 94. The
variable resistors 94 and 92 are used in conjunction with
the microcontroller 74 to control mechanical or
electromechanical switches SW6 and SW7, respectively. The
power supply circuit 96 comprises resistors Rsl and Rg2, the
values of which are preferably selected to limit current in
the line to ground and line to load current paths,
respectively. Thus, current in the line to ground and line
to load current, paths can be fixed to a maximum of 0.5
milliamperes, for example. In addition, two parallel paths
are provided. The first parallel path comprises a resistor
Rz in series with the variable resistor 94 and the switch
SW6. The second parallel path comprises the capacitor C16
in series with the variable resistor 92 and a switch SW7.
If greater current flow is desired in the line to ground
and/or line to load paths than that allowed by resistors R8z
and R~1, the microcontroller 74 can be programmed to close
one or both of the switches SW6 or SW7 to lower the
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impedance of one or both ~of these parallel paths via the
.
corresponding variable resistor
. Further, the power supply
circuit 96 can be configured with only a single variable
resistor in series with a switch to provide a parallel path
with only one of the line to ground and line to load paths.
Fig. 10 depicts a power supply circuit 98 constructed
in accordance with an embodiment of the invention wherein
a preferably low current switch SW8 (e. g., a slide switch)
is connected to an input of the microcontroller 74 to drive
the relay K1 open. In Fig. 11, a power supply circuit 100
comprises an air. gap switch SW9 connected to the rectifier
circuit D1 through D4 and to the microcontroller 74. The
microcontroller detects when the air gap switch SW9 is
switched off, and, in turn, opens the relay K1. In
addition, the switch SW1 in Figs. 10 and 11 can be a relay,
the reset coil for which can be activated by the
microcontroller 74, as indicated by the phantom line.
With reference to Fig. 12, a power supply circuit 102
is provided which has parallel paths, as described in
connection with Fig. 9. The resistor RL and the capacitor
C16, however, are in series with TRIACs 104 and 108 and
gate drive circuits 106 and 110, respectively. Thus, the
resistors R~2 and RH1 can be selected to limit current in
the line to ground and line to load paths, respectively.
In addition, the microcontroller 74 can regulate line to
ground and line to load currents by gating the TRIACs.
Further, if the microcontroller detects that an air gap
switch SW10 has been activated, the microcontroller can
open the relay K1.
Another embodiment of the invention is depicted in
Fig. 13. If the microcontroller 74 detects that an air gap
switch SW10 has been activated, the microcontroller can
open the relay K1. Alternatively, the air gap switch SW10
can be an electromechanical relay, and microcontroller can
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be programmed to open or close it. The power supply
circuit is also configured to control the flow of current
through resistors R60 and R45 via pulsing to selectively
recharge the capacitor C27, in accordance with an
embodiment of the invention.
With continued reference to Fig. 13, the resistors R68
and R69 are selected to limit current in the line to ground
path to within an acceptable limit such as 0.5
milliamperes, as specified by the newly proposed UL 773A
standard. A transistor Q9 is provided across the resistor
R60 and is connected to a transistor Q10. The
microcontroller is programmed to turn on the transistor
Q10, which then turns on transistor Q9 to short the
resistor R60, whenever the relay K1 is activated. This is
advantageous because a substantial amount of current is
used whenever the microcontroller operates the relay K1,
resulting in a significantly reduced amount of current
available to recharge the capacitor C27. By shorting
resistor R60, an increased amount of current is permitted
to flow to charge the capacitor C27 that is being depleted
when the relay K1 operates.
In accordance with the embodiment of the invention
depicted in Fig. 13, the microcontroller 74 can pulse the
transistors Q9 aid Q10 on a steady state basis, on a random
basis, or as a 'function of the activation of components
which require more current (e.g., LEDs, buzzers, and
electromechanical relays). In addition, line to ground
current can be limited. Thus, the microcontroller can
pulse the transistors and increase current flow through
resistor R60 in a variety of states and for a variety of
reasons. For example, the microprocessor can pulse the
transistors Q9 and Q10 during power up of the power supply
circuit to more quickly obtain an initial charge. This is
particularly useful for three wire power supply circuits
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which require a magnetizing .current or are otherwise
limited by 0.5 milliamperes in the line to ground current
path.
The microcontroller 74 can also control the amount of
current flowing through resistor R45 by pulsing the
transistor Q7. While the transistors Q9 and Q10 are
preferably primarily pulsed for start-up or capacitor
recharging current. The transistor Q7 is pulsed for the
same reasons, as well as for other functions such as
operating the relay K1 and/or the air gap switch SW10. For
example, if the relay K1 is open but the air gap switch is
on, the transistor Q7 and the microprocessor can operate to
close the relay K1 and recharge the capacitor C17
afterward. As another example, an LED can appear to be
driven solid when it is actually being pulsed by pulsing
transistors Q9 and Q10 to be driven on more frequently than
off.
The pulsing operation of the power supply circuit 104
(Fig. 13) permits momentarily higher currents to flow in
the power supply circuit for short periods of time. For
example, most ground fault (GF) circuit interrupters are
configured to trip when a current greater than 6.0
milliamperes flows for more than two cycles at 60 hertz.
To avoid nuisance tripping, these GF circuit interrupters
do not operate For currents less than 4.0 milliamperes or
having duration less than approximately 5.0 seconds. Thus,
the power supply circuit of the present invention can pulse
as high as 3 milliamperes, for example, for less than two
cycles without causing a GF circuit interrupter to trip,
while maintaining current below allowed maximum levels for
safety.
In accordance with another embodiment of the
invention, a battery can be provided in a power supply
circuit constructed in accordance with the present
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invention. A battery 112 ~is depicted, for example, in the
power supply circuit shown in Fig. 13. The other power
supply circuits disclosed herein can be provided with a
battery as well. The battery 112 is useful because, among
other reasons, wall switch units can be shipped from the
manufacturer with the relay K1 open. If the air gap is
switched on, there may not be sufficient energy to also
switch the relay K1 closed without the battery. As another
example, the relay K1 may be closed, the air gap switch
switched from the ON to OFF position, and there may be
insufficient energy to open the relay K1 without the
battery.
Another embodiment of the invention is depicted in
Fig. 14. The power supply circuit 113 comprises a
rectifier circuit D1 through D4 connected in series with a
capacitor C1 across the line and load conductors 54 and 60
and therefore in parallel with the relay K1. A rectifier
circuit D5 through D8 is provided between the line and
ground conductors 54 and 58. The circuit preferably is
powered on with the relay in the relay off or open
position. The microcontroller 74 receives essentially all
of the available current upon initialization via the
rectifier circuit D1 through D4. In accordance with
program code, the microcontroller 74 monitors and controls
the flow of current to the high current load storage
capacitor C2 by controlling transistors Q3 and Q4. The
capacitor C2 is monitored via an analog-to-digital input on
the microcontroller. The microcontroller uses transistors
Q1 and Q2 to control the flow of return current for
recharging the capacitor C2 via the rectifier circuit D1
through D4. The switch states of transistors Q1 and Q2
depend on the relay K1 state. Transistor Q2 is on and
transistor Q1 is off when the relay K1 is open. Transistor
Q1 is on and transistor Q2 is off when the relay K1 is
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closed. The transistors Q1 through Q4 can also be pulsed
appropriately based on relay K1 and/or switch SW1 states
and the unit 113 functional requirements.
With continued reference to Fig. 14, the power supply
circuit 113 can be provided with an air gap switch SW1 on
the line conductor 54, for example, or the load conductor
60. A voltage detection circuit 115 can be connected to
the rectifier circuit D1 through D4 and to an input of the
microcontroller. If relay K1 is open or not present, and
a lamp load, for example, is burned open or there is a loss
of power, the air gap switch SW1 can be opened
automatically by a reset coil 117, which is activated by
the microcontroller 74, to provide an air gap open to the
load. Thus, even if a lamp load burned open prior to
switching relay K1 to the OFF position, a repairman is not
exposed to full AC line current (e. g., 15 amperes).
In Fig, 13, a metal oxide varistor (MOV) is provided
across the line to ground conductors, as it is in Figs. 3
and 10. In Figs. 2, 4-9, 11 and 12, an MOV is provided
across the line and load conductors. It is to be
understood that in each of the embodiments of the
invention, an MOV can be provided across either or both the
line and ground conductors and the line and neutral
conductors, as well as the neutral and ground conductors.
In Figs. 4-9, 11 and 12, an air gap switch SW1 is provided
at the AC mains in addition to another air gap switch or
other means for limiting current to within an acceptable
level. The switch SW1 at the AC mains is provided to
illustrate an alternative embodiment of the present
invention and is not required in each of the power supply
circuits depicted in Figs. 4-9, 11 and 12. Ground is
illustrated throughout the various views by an inverted
triangle (e.g., Fig. 13) to represent a common point in the
circuit; however, separate electronic circuit grounds can
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be used in accordance with another embodiment of the
invention to control return current through the load or
ground current paths. Fig. 13 illustrates separation of
ground return paths.
While certain advantageous embodiments have been
chosen to illustrate the invention, it will be understood
by those skilled in the art that various changes and
modifications can be made herein without departing from the
scope of the invention as defined in the appended claims.
