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Description  |
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FIELD OF THE INVENTION
This invention relates to protection of electronic equipment during a power
system surge and more particularly to a circuit which prevents damage to
electronic equipment from line voltage transients associated with a
temporary power line interruption.
BACKGROUND OF THE INVENTION
Switched power supplies of the type used in personal computers, fax
machines, laser printers and the like, include a rectifier circuit which
charges a capacitor from the line voltage. In most equipment, the initial
charging current is limited to a nondestructive value by a
series-connected, negative-temperature-coefficient thermistor. When power
is applied, the thermistor is in a high-resistance state. When the
rectifier draws current, the thermistor heats up and its resistance
reduces to a very low value. The thermistor stabilizes in the low
resistance state when at an elevated temperature.
It has been found that such protective devices may not be effective during
power system transients resulting from a fault due to lightning or to
physical damage to the transmission system. Such transients are typically
characterized by rapid on/off cycling brought about by an automatically
reclosing circuit breaker attempting to reclose itself after opening in
response to the fault.
A problem associated particularly with computers is that the computer hard
disk drive will not have time to come to a complete stop if power is
restored very quickly after a shutdown. It is desirable for the drive to
come to a complete stop before power is restored in order to avoid damage
to the drive's heads.
Heretofore, there have been known circuits directed to protecting computers
and other electronic equipment from problems resulting from a power
interruption. Those circuits, however, have left much to be desired. For
example, one such circuit does not actually disconnect the computer from
the line voltage. Consequently, it cannot prevent damage to the computer's
power supply when A.C. power is restored rapidly after an interruption.
Another such circuit turns off the computer, or other equipment, very
rapidly, for example, within one power cycle. Such rapidity results in
unnecessary shutdowns when the power interruption is only momentary.
Moreover, none of the known circuits automatically reconnects the
electronic equipment to the power line when power is restored after a
shutdown.
SUMMARY OF THE INVENTION
The present invention recognizes that if power is interrupted momentarily
and then reapplied, as by an automatically reclosing circuit breaker on
the transmission line, a thermistor will not have time to return to its
high-resistance state and thus cannot limit the inrush of charging current
when power is suddenly reapplied. The current into the rectifier of the
power supply is then limited only by the impedance of the power supply
circuit including the effective resistance of the capacitor. Such current
is dangerously high for the circuit components involved. Often a rectifier
can handle one such episode, but, if there is a second surge while the
rectifier is still hot from an earlier surge, failure of the power supply
is likely.
In view of the foregoing problems associated with known protection
circuits, it is a principal object of this invention to provide a surge
protection circuit which disconnects a computer or other electronic device
from the power source upon a power interruption and delays bringing the
protected device back on line until it is safe to do so. Thus, when the
power has been restored to normal the protected devices can be brought
back on line without being damaged.
Another object of the present invention is to provide such a surge
protection circuit which stores energy to provide a critical time delay
during a power interruption without the use of batteries.
A further object of the present invention is to provide a surge protection
circuit which connects the A.C. power source to the load immediately upon
the initial turn-on or when re-energizing after an extended shutdown
without the delay which the circuit provides during shorter power
interruptions.
A still further object of the present invention is to provide a surge
protection circuit which permits a predetermined grace period or time
delay to pass before disconnecting the power source from the load in order
to avoid unnecessary shutdowns when a power interruption is very short.
In accordance with the present invention, a circuit is provided for
protecting electronic equipment from a line voltage transient resulting
from a temporary power line interruption. Here and throughout this
application the term "transient" is defined to mean a condition resulting
from rapid on/off cycling of the A.C. power. The surge protection circuit
according to the invention includes a power input circuit which receives
the alternating current line voltage and provides first and second signals
which are derived from the line voltage. The surge protection circuit also
includes a power output circuit which is coupled to the input circuit for
providing the line voltage to the protected electronic equipment. The
power output circuit includes a normally open switch responsive to the
first signal for respectively connecting and disconnecting the protected
electronic equipment from the power line.
A control circuit, including a comparator circuit, a gated switch, and an
energy storage device, controls operation of the normally open switch to
provide power to the protected load or to isolate the load from the power
source. The comparator circuit is responsive to the second signal and a
reference signal, and provides a gate signal when the second signal has a
predetermined relationship to the reference signal. The gated switch is
responsive to the first signal and the gate signal, and transmits the
first signal to the power output circuit to close the normally open
switch. The power output circuit is thereby enabled to connect the
protected electronic equipment to the line voltage when the switch is
closed.
Concurrently, the first signal is transmitted to the comparator through the
gated switch such that the first signal, when so provided, constitutes the
reference signal. An energy storage device is coupled to the comparator so
as to receive the first signal. The energy storage device charges up to
the level of the first signal when the first signal is provided to the
comparator means.
When power is momentarily interrupted and then restored before the first
signal has decayed to a minimum limit, the normally open switch remains
closed and no interruption occurs, thereby providing a grace period or
first time delay to prevent an unnecessary interruption.
When a power interruption occurs that is longer in duration than the first
time delay, the first signal decays to zero and the normally open switch
is re-opened, thereby isolating the electronic equipment from the A.C.
power source. The normally open switch remains in the open position for a
second time delay after power is interrupted. The concurrent interruption
of the first signal causes the energy storage device to discharge at a
preselected rate so as to provide the reference signal. The discharge rate
of the storage device is selected to provide the second time delay which
is long enough for a line voltage transient associated with the power line
interruption to substantially abate.
If power is not restored until after the second time delay, the first
signal is applied to the power output circuit and the normally open switch
is closed to provide A.C. power to the load.
BRIEF DESCRIPTION OF THE DRAWINGS
The foregoing summary, as well as the following detailed description of a
preferred embodiment of the invention will be better understood in
conjunction with the appended drawings in which:
FIG. 1 is a block diagram and partial schematic of a preferred embodiment
of the surge protection circuit according to the present invention;
FIG. 2 is a schematic diagram of a preferred embodiment of the present
invention;
FIG. 3 is a timing diagram illustrating the operation of the circuit of
FIG. 2; and
FIG. 4 is a schematic diagram of a further embodiment of the remote on-off
switch 34 of the circuit of FIG. 2.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring now to the drawings wherein like reference numerals indicate the
same or corresponding parts throughout the various figures, and in
particular to FIG. 1, there is shown a surge protection circuit 10
according to the present invention. The surge protection circuit 10 has a
power input circuit 12, a power output circuit 14, and a control circuit
15 including a gated switch 16, a comparator circuit 18, and an energy
storage device 20. An optional signaling circuit 22 can be provided in the
control circuit 15 to indicate to an operator that power is available and
that the surge protection circuit 10 is timing down to reconnect the
electronic equipment after a power interruption.
The power input circuit 12 receives the A.C. line voltage, usually about
120 volts. The line voltage is provided to the power output circuit 14.
Power input circuit 12 contains circuitry for providing a first or line
signal A and a second or threshold signal C which are derived from the
A.C. line voltage as will be described below. The power output circuit 14
is formed to be connected to an external electronic equipment 23 such as a
computer, fax machine, or laser printer. The power output circuit 14
includes switching means for opening and closing at least one leg of the
A.C. line.
The gated switch 16 receives the line signal A from power input circuit 12.
The comparator circuit 18 receives the threshold signal C from power input
circuit 12 and also receives a reference signal R which has a peak level
of about equal magnitude to the line signal A. The comparator circuit 18
compares the threshold signal C and the reference signal R and provides a
gate signal D to gated switch 16 when the threshold signal C and reference
signal R have a predetermined relationship. In the preferred embodiment,
the gate signal D is provided when threshold signal C is greater than or
equal to the reference signal R.
Upon receiving the gate signal D, gated switch 16 transmits the line signal
A as an energizing signal E. Energizing signal E is transmitted to power
output circuit 14 to control the switching means therein. Signal E is also
transmitted through the charging diode 24 and resistor 26 to comparator 18
as reference signal R and to energy storage device 20. Charge signal B
represents the voltage across the energy storage device 20. As will be
described more fully hereinbelow, when the energy storage device 20 is
fully charged, the charge signal B is at the peak level of line signal A.
The signaling circuit 22, receives the line signal A, when activated, and
provides an audible or visual signal to indicate to the operator that the
surge protection circuit has shut down the protected electronic equipment
23 and is timing down to restore power thereto. The signaling circuit 22
also provides a notching signal to the comparator circuit 18 which aids
that circuit in sampling the reference signal R, as will be explained more
fully below.
Referring now to FIG. 2 there is shown a preferred circuit scheme for the
surge protection circuit 10 of FIG. 1. The power input circuit 12 includes
a three conductor connector 28 which can be embodied, for example, as a
three-pronged, male connector. Current limiting inductors 30a and 30b and
current control varistors 32a and 32b are connected in a known
configuration to both the neutral (N) and phase (.phi.) of connector 28 to
protect the power input circuit from the adverse effect of power surges.
The power input circuit 12 includes an on-off switch 34, which is located
in the surge protection circuit 10 proper or remotely therefrom. A current
limiting resistor 36 and capacitor 38 are connected to the circuit-side of
the on-off switch 34. A rectifier circuit including diodes 40a and 40b is
connected in a known configuration across the A.C. line voltage at the
circuit-side terminal of capacitor 38. A zener diode 42 is connected
across the terminals of the rectifier in order to clamp the rectified
voltage to a preselected level, for example 24 volts D.C. A filter
capacitor 44 is connected across the terminals of the zener diode 42 to
filter out ripple effects of the rectified voltage and thereby to provide
an essentially D.C. line signal, A, derived from the A.C. line voltage. A
voltage divider, including resistors 46a and 46b, is connected across the
terminals of filter capacitor 44. The resistance values of resistors 46a
and 46b are selected to provide a threshold signal C between the resistors
which has a high enough level for the comparator circuit 18 to be
responsive.
The comparator circuit 18 includes an electronic comparator 47 having a
noninverting input (+) terminal and an inverting input (-) terminal. The
threshold signal C is provided to the noninverting input of electronic
comparator 47. The reference signal R is provided to the inverting input
of electronic comparator 47. When threshold signal C is greater than the
reference signal R, the output of the electronic comparator 47 is a high
level output or a high state. A capacitor 50 is connected between the
output of electronic comparator 47 and its noninverting input to assure
that the output of the electronic comparator 47 goes to the high state.
When the output of comparator 47 is in the high state, current flows
through resistor 48, which is connected between the comparator output and
the line signal A, and resistor 52, which is connected between the
comparator output and the gated switch 16.
In the embodiment shown in FIG. 2, the gated switch 16 is embodied as a
silicon controlled rectifier (SCR), sometimes known as a unidirectional
thyristor. The anode of SCR 16 is connected to the line signal A. The
cathode of SCR 16 is connected to the power output circuit 14 and to the
energy storage circuit 20.
The energy storage circuit 20 includes a capacitor 58 having a terminal
connected to the charging resistor 26 and the inverting input terminal of
electronic comparator 47. A bleed-off resistor 60 is connected across the
capacitor 58. The capacitance of capacitor 58 and the resistance of
bleed-off resistor 60 are selected to provide a decay time substantially
longer than that of the RC circuit comprising capacitor 44 and the
effective resistance of SCR 16 and relay coil 54'.
The power output circuit 14 has a three conductor connector 56, for
example, a three-conductor, female connector, into which an external
electronic device can be plugged. One terminal of connector 56 is
connected to the neutral terminal of connector 28 through the inductor 30a
and varistor 32a. The common terminal of connector 56 is connected to the
corresponding common terminal of connector 28. The third or phase terminal
of connector 56, however is connected to the contacts 54" of a relay 54.
The coil 54' of relay 54 is connected to the cathode of SCR 16. The
contacts 54" are normally open. A diode 24' is also connected to the
cathode of SCR 16 across the relay coil 54' in order to suppress any
significant reverse emf, often referred to as kick-back voltage, from the
relay coil 54' during initial energizing of the surge protection circuit
10.
In the embodiment shown in FIG. 2, the surge protection circuit 10 includes
a signaling circuit 22 for providing an audible signal to indicate that,
after a short interruption, the surge protector has power available and is
timing down to restore power to the external device. The use of the
signaling circuit 22 is preferred because it permits the use of less
sensitive components, which are also less expensive, in the comparator 18
without sacrificing reliable operation.
The signaling circuit 22 includes a switching transistor 62 having its base
connected to the relay coil 54' through a coupling resistor 64. A pair of
resistors 66a and 66b are connected as a voltage divider across the line
signal A. The values of 66a and 66b are selected to divide the line signal
A so as to provide a threshold level signal from between resistors 66a and
66b to the noninverting input terminal of an electronic comparator 70. A
pair of feedback resistors 68a and 68b are connected between the output
terminal and the noninverting and inverting input terminals respectively
of comparator 70. A capacitor 72 is connected to the inverting input of
comparator 70, the switching transistor 62, and a diode 74. The diode 74
has its cathode connected to a resistor 76 which is connected to each of
the feedback resistors 68a and 68b. An audible device 78, such as a
piezoelectric beeper, is connected between the line signal A and the
output of comparator 70. A coupling resistor 80 connects the output of
comparator 70 to the noninverting input of comparator 47. A filter
capacitor 82 is connected between the line signal A and the output
terminal of comparator 70.
The operation of the surge protection circuit 10 according to the present
invention may be better understood from the following description when
read in connection with FIGS. 2 and 3. The surge protection circuit is
initially energized by plugging the connector 28 into an electrical
receptacle, for example a wall outlet. When the switch 34 is closed, the
circuit becomes energized through the inductor/varistor surge protectors
30a, 32a, and 30b, 32b. The A.C. current in the neutral leg passes through
the switch 34, current limiting resistor 36 and capacitor 38. The A.C.
line voltage is rectified by means of the diodes 40a and 40b and then
clamped by means of the zener diode 42 to a pre-selected D.C. level, for
example 24 volts. The rectified voltage is applied to filter capacitor 44
which becomes charged to the level of the rectified line signal A. This is
represented in FIG. 3 on the timing line marked "A" which shows the
voltage build-up in capacitor 44 to the peak level, vl, of line signal A.
The charging time is relatively short, being on the order of about 400 to
600 milliseconds.
At time T1 the line signal A reaches the peak level V1 and the threshold
signal C reaches the threshold level, V2, of comparator 47 as shown on
timing line "C". Until time T1 capacitor 58 is uncharged and the reference
signal R, as indicated on timing line "B", is substantially zero.
Accordingly, the output of comparator 47 goes high as shown on timing line
"D", thereby triggering the SCR 16. When triggered, the SCR begins to
conduct and transmits the line signal A to the relay coil 54'. When the
relay coil 54' is energized, it closes the contacts 54" completing the
A.C. circuit between input connector 28 and output connector 56. The A.C.
line voltage is thereby provided to the external electronic equipment 23,
such as a computer or the like that is plugged into connector 56.
Concurrently, the timing capacitor 58 begins to charge up to the level vl
as shown on timing line "B" of FIG. 3.
As timing capacitor 58 charges, the voltage at its terminal B increases to
a level that is greater than the threshold signal C. When that level is
reached, the comparator output goes to a low state as shown at time T2 on
line "D", and the gate signal D to SCR 16 is thereby removed. However, as
long as the magnitude of signal A is at least at the latching level of SCR
16, SCR 16 continues to conduct and relay 54 remains closed.
If the A.C. line voltage is temporarily interrupted at time T3, SCR 16
stops conducting after a short time delay as shown on timing line "E" and
does not restart even when the line voltage is restored as at time T4 to
return the line signal A to the level V1. The voltage across capacitor 58,
reference signal R, begins to decay as the charge bleeds off through
resistor 60. See timing line "B".
For a period of time, depending on the decay constant of the energy storage
circuit 20, the reference signal R remains above the level V2 of the
threshold signal C. The relay 54, which has become de-energized, remains
open. As shown in FIG. 3, line "B", the reference signal R continues to
decay with time and at time T5, when it falls slightly below the level V2
of the threshold signal C, the output of comparator 47 goes high,
retriggering the SCR 16 and re-energizing the relay 54. Power is thereby
restored to the external electronic equipment 23. The time interval from
T3 to T5 constitute delay that is long enough for a line voltage transient
associated with a power line interruption to abate. This time delay has a
duration selected, for example, to permit the varistor in a power supply
to return to its high impedance state or to permit a hard disk drive to
spin down.
It is important to note that the decay constant of the RC circuit
consisting of capacitor 44 and the effective resistance of SCR 16, is
selected to provide a short "grace" period after T3 which constitutes a
time delay, on the order of several milliseconds, within which, if A.C.
power is restored, the SCR 16 continues to conduct, relay 54 remains
closed, and the protected load is not isolated from the line voltage.
When relay 54 is not energized, but A.C. line voltage is available at
connector 28, the signaling circuit 22 becomes operational. Signaling
circuit 22 is configured as a low frequency, short-duty cycle oscillator
which drives an audible or visible device. A.C. line voltage being
available, line signal A is present across resistors 66a and 66b. The
oscillator's cycle begins with the threshold level signal being present at
the noninverting input terminal of comparator 70. The output of the
comparator 70 goes to a high state. The level of the threshold signal is
pulled up by a feedback signal through resistor 68a. The signal level at
the noninverting input terminal is then higher than the signal level at
the inverting input terminal and resistor 68b begins to conduct.
The base of switching transistor 62 is "on" when the relay coil 54' is
energized. In that state the switching transistor 62 conducts current and
is effectively a shunt around capacitor 72. When the relay coil 54' is not
energized, the base of switching transistor 62 is "off" and transistor 62
stops conducting, becoming effectively an open-circuit. Capacitor 72 can
then be charged up.
When the voltage level across capacitor 72 becomes greater than the level
of the signal at the noninverting input terminal of comparator 70, the
output of comparator 70 goes to a low state. Capacitor 72 then discharges
through diode 74, resistor 76 and resistor 68a. In this manner the signal
level at the noninverting input terminal of comparator 70 is pulled up
higher than that at the inverting input terminal. When the signal level at
the noninverting input is greater than that at the inverting input, the
comparator output goes to a high state again, restarting the cycle.
Each time the output of comparator 70 goes to the low state a voltage drop
develops across piezoelectric beeper 78. Piezoelectric beeper 78 emits an
audible signal at each such instance. Furthermore, when the output of
comparator 70 goes to the low state, the level of the threshold signal C
is pulled down or notched for a short interval. The notch in the threshold
signal C permits the reference signal R to drop below the threshold level
for comparator 47. When the threshold signal C returns to its normal level
there is a sufficient offset in the levels of the threshold signal C and
reference signal R to cause comparator 47 to go to its high state. As
previously described, the SCR 16 is thereby triggered and power is
restored to the output connector 56 and in turn to the electronic
equipment 23 connected thereto.
Referring now to FIG. 4, there is shown an alternate embodiment of the
on-off switch 34 of surge protection circuit 10. It is often desirable to
have an indication of the status, i.e., on or off, of the switch 34. Such
objective is easily accomplished by connecting a lamp 342 in series with
the switch. However, the problem arises that when the lamp 342 burns out
or its filament is broken, an open-circuit results disconnecting the surge
protection circuit from the line voltage. The surge protection circuit 10
cannot be used in that case until the lamp 342 is replaced.
A pair of zener diodes 343a and 343b are connected across the lamp 342. The
anodes of the zener diodes are connected to respective terminals 344a and
344b of the lamp and the cathodes are connected together. This arrangement
permits the lamp 342 to operate normally, but, if the lamp should burn out
or break, a circuit is provided to maintain continuity of operation.
Some of the many novel features and advantages of the surge protection
circuit according to the present invention are now apparent in view of the
foregoing description. For example, a novel surge protection circuit has
been described which disconnects a protected electronic device from the
A.C. power source upon a power line interruption, and which, when power is
restored, maintains the device disconnected for a short time delay until
transients in the line voltage have substantially abated. The surge
protection circuit stores energy to keep track of time during a shutdown,
but does not need batteries to do so. The surge protection circuit is
designed to permit a short grace period to elapse before disconnecting the
protected electronic device in order to avoid unnecessary shutdowns when
the power line interruption is very short. The circuit design provides
power to the load without a time delay during initial turn-on or when
re-energizing after an extended power interruption.
It will be recognized by those skilled in the art that changes or
modifications may be made to the above described embodiment without
departing from the broad inventive concepts of the invention. It is
understood, therefore, that the invention is not limited to the particular
embodiments which are disclosed but are intended to cover all
modifications and changes which are within the scope and spirit of the
invention as defined in the appended claims.
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