Note: Descriptions are shown in the official language in which they were submitted.
CA 02168708 2004-03-24
THRE$ WIRE POWER SUPPLY CIRCUIT
Field of the Invention
The invention relates to a three wire electrical power
supply circuit 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.
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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 the
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 the UL 773 standard. 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 leakage currents to
0.5 milliamperes or less to the load. Also, in three wire
systems, the leakage to ground is proposed to be limited to
0.5 milliamperes, as well.
A need exists for a power supply circuit which
complies with the newly proposed UL 773A standard,
particularly in the manner with which it derives operating
current and the maximum allowable return current, either
through the load or ground. Control circuitry for many
power supply circuits requires power or operating current
apart from the power used to operate the load. For
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example, a wall switch unit with an occupancy sensor is
available which controls a switch to automatically connect
and disconnect a lighting fixture to and from a power
source in response to the detection of movement near the
sensor. With reference to Fig. 1, the wall switch unit 10
comprises a switch 12 which has one terminal connected to
a load 14 and another terminal connected to an AC power or
"hot" line 16. The load 14 in turn is connected to the
neutral conductor 18 of the power source. When the wall
switch unit 10 is off, sufficient voltage differential
exists between~the line and load connections to operate a
power supply 20 for energizing the switch control circuitry
22 in the wall switch unit, but not when the load has
burned open. When the switch is closed, i.e., the wall
switch unit is on and the load is energized, there is
insufficient voltage differential to operate the switch
control circuitry.
One solution for providing operating power when the
load is connected to the power source is to provide a
switch mechanism that is controlled to rapidly open and
close during each cycle or half-cycle of the AC waveform.
The duty cycle (i.e., the ratio of the switch open time to
the switch closed time) is sufficiently low, and the
external load, receives full-rated power, yet sufficient
voltage differential exists to derive operating power for
the control circuitry during the off cycles.
This solution is disadvantageous because semiconductor
or other electronic switch mechanisms are generally
required to switch rapidly during each cycle or half-cycle.
Relay switching mechanisms are generally not sufficiently
responsive, yet they are preferred over electronic
switching mechanisms. Electronic switching mechanisms are
more prone to leakage currents and are not as reliable as
relay switching mechanisms. Leakage currents are of
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particular concern because of safety standards for devices
such as photoelectric wall switches as described above.
In applications where relay switching mechanisms are
preferred, a circuit configuration such as that disclosed
in the U.S. Patent No. 4,340,826 to Muchnick may be used.
With reference to Fig. 2, the Muchnick patent discloses an
electrical switching circuit 24 comprising a pilot or
indicator light 26 to indicate the state of energization of
the load 14 controlled by a switch 30. The patent
addresses the problem of connecting the pilot or indicator
light to the switch 30 such that the pilot light is
energized when the switch is closed. The switch makes use
of a third wire 32, the ground conductor, which is
available in most wall boxes. A full line potential exists
between the hot conductor 16 and the ground conductor 32
for driving the pilot light 26 when the switch 30 is
closed.
U.S. Patent No. 4,713,598 discloses a switching
circuit 36 which does not rely on the presence of a ground
conductor 32, but instead uses a current transformer XFR to
derive operating current, as shown in Fig. 3. The primary
winding 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 provides operating
power via a power supply 42 for the control circuitry 44.
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
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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 ) because the power supply circuit in Figs . lA
and 18 does not provide means for changing the state of the
switch SW. 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.
Summary of the Invention
The disadvantages and deficiencies of existing power
supply circuits are overcome by the present invention. In
accordance with an embodiment of the present invention, a
power supply circuit is provided which comprises a
potential transformer connected between a power or hot line
conductor of AC power source and a ground conductor thereof
to supply operating current when the load switch is closed.
CA 02168708 2004-03-24
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More particularly the invention comprehends a power supply
circuit for use in combination with a load and an alternating
current (AC) power source for selectively connecting and
disconnecting the load and the AC power source, the load being
connected between a neutral conductor of the AC power source and
the power supply circuit. The power supply circuit is connected
to a hot line conductor and a ground conductor of the AC power
source, and comprises a relay having one terminal connected to
the load and another terminal connected to the line conductor,
a control circuit for selectively operating the relay, and a
transformer. The primary winding of the transformer is connected
in series with the line conductor and the ground conductor for
supplying power to the control circuit when the load is
disconnected from the AC power source via the relay, the
transformer being configured to step down the voltage between the
line conductor and the ground conductor.
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In accordance with another embodiment of the present
invention, a power supply circuit is provided which
comprises a switch in series with the primary winding of
the potential transformer for interrupting the primary
winding current at frequencies above the AC line frequency.
The switch creates a minimal primary magnetizing current
for minimizing transformer size.
In accordance with yet another embodiment of the
present invention, the power supply circuit is provided
which comprises a slide or air gap switch connected to the
power or hot line conductor to isolate the power supply
circuit from the AC power source when the air gap switch is
in the OFF position. The air gap switch can be configured
to provide an open circuit, or to limit current to an
approved level (e.g., 0.5 milliamperes as required by the
newly proposed UL 773A standard).
In accordance with various embodiments of the present
invention, a number of three wire power supply circuits are
provided which comprise at least one air gap off switch to
interrupt a current path or to limit current to an
acceptable level.
Brief Description of the Drawings
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:
Fig. 1 is a schematic diagram of a conventional wall
switch unit;
Fig. 2 is a schematic diagram of a prior art three
wire wall switch unit comprising a pilot light;
Fig. 3 is a schematic diagram of another prior art
wall switch unit; and
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Figs. 4 and 5 are schematic diagrams of three wire
power supply circuits constructed in accordance with two
preferred embodiments of the present invention.
Detailed Description of the Preferred Embodiments
Fig. 4 depicts a power supply circuit 50 constructed
in accordance with an embodiment of the present invention.
The power supply circuit controls the application of power
from an AC power source to a load 52 (e. g., a lighting
fixture). The power source is represented by line, neutral
and ground conductors 54, 56 and 58, respectively, to the
AC main throughout the various views. The load is
connected to the neutral conductor 56 of the AC power
source . The power supply circuit 50 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 and
the ground conductor 58.
With continued reference to Fig. 4, the power supply
circuit 50 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 (e. g.,
relay 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, but does not
require, operation of a microcontroller. In accordance
with an aspect of the invention, an electronic control
circuit 64 is preferably used. The electronic control
circuit receives operating current from a power supply 66.
The power supply 66 is~~ connected across the secondary
winding of a transformer 68, the primary winding of which
is connected between the power or hot line conductor 54 and
the ground conductor 58.
The transformer 68 is preferably a potential
transformer. In operation, current flows into the primary
winding of the potential transformer, which steps down the
voltage to generate a low voltage output at the secondary
winding with a relatively high current capacity. The
voltage ratio of the transformer can be, for example, 20:1
or 120 volts root mean square (V~s) in, 6 V~, out, with an
output current capacity of approximately 10 milliamperes.
Thus, the newly proposed UL 773A standard recommendation of
0.5 milliamperes maximum leakage current limit is not
exceeded because only 0.5 milliampere current flows in the
primary winding if a 10 milliampere current flows in the
secondary winding due to the ratio. Another advantage of
using a potential transformer in lieu of a current
transformer is the elimination of high current flow in the
primary winding, and therefore restricting the resulting
high wire and core losses and power dissipation that are
associated with current transformers. Thus, the power
supply circuit 50 can provide an operating current above
0.5 milliamperes without relying on a current transformer.
The transformer 68, however, can be a current transformer
with or without an impedance in series therewith.
The potential transformer 68 typically requires a no
load magnetizing current which flows into the primary
winding to produce the operating flux in the core of the
transformer. The operating flux produces an opposing
voltage potential in the primary winding to limit the
current flow therein. The magnetizing current does not
produce useful current output at the secondary winding.
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Further, it is considered with respect to current limits
set forth in safety standards, codes and regulations.
In accordance with another aspect of the invention,
the magnetizing current is minimized. Rather than using a
transformer with a larger number of turns and relatively
large core area factor, which generally necessitates an
undesirable large core, the invention can employ a switch
K2 and a chopper circuit 70 in series with the primary
winding of the transformer, as shown in Fig. 5.
With reference to Fig. 5, the chopper circuit 70
operates the transformer via the switch K2 at frequencies
higher than the nominal frequency of the AC line voltage
(i.e, 60 hertz). The chopper circuit interrupts the
current in the primary winding at rates which are
preferably much greater than the line frequency, such as 20
kilohertz. The chopper circuit 70 is useful to pulse the
current flowing into the primary winding of the transformer
to selectively increase or decrease the current output from
the secondary winding. The transformer size can therefore
be minimized, along with the power temperature rating and
the associated losses and cost.
In accordance with another aspect of the present
invention, the power supply circuit is provided with an air
gap switch 72,, as shown in Figs. 4 and 5. The air gap
switch 72 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 72 can be controlled
manually (e.g., a press button or slide switch), or by an
electronic control circuit which can, but does not reguire,
operation of a microcontroller. In addition, the air gap
switch can be placed on the power or hot line conductor or
the load conductor outside the wall switch enclosure of the
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switching unit 50. The air gap switch in Fig. 4 interrupts
the flow of current to the relay and the transformer when
placed in the OFF position. The air gap switch in Fig. 5
interrupts the flow of current to the relay and the load
but not to the transformer 68. Thus, line to ground
current is available through the transformer. The air gap
switch can also be configured as a logic-controlled relay
K3 placed in series with the transformer between the line
and ground conductors. For example, a microcontroller 74
can be programmed to open and close a relay at a selected
rate to pulse the primary winding current above an approved
level (e. g., 0.5 milliamperes as required by the newly
proposed UL 773A standard). Pulsing the primary winding
current at a constant or varying duty cycle can be useful
to obtain a large input voltage range to charge a
capacitor, for example, for energizing a circuit component
such as a light emitting diode or buzzer. This method also
allows for the steady state current to be controllably
derived or programmable to any value; however, it is
preferable that the value be less than 0.5 milliamperes for
the reasons stated herein.
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.
