OVERVOLTAGE CIRCUIT, AND
MOTOR STARTER, OVERLOAD RELAY AND LOW-POWER SYSTEM
INCLUDING THE SAME
BACKGROUND
Field
The disclosed concept pertains generally to overvoltage circuits and
more particularly, to voltage clamping circuits for direct current power supplies, such
as, for example, power supplies for processors. The disclosed concept also pertains to
motor starters, overload relays or low-power systems employing such circuits.
Background Information
It is known to employ a Zener diode as a relatively low cost voltage
clamp circuit However, a Zener diode exhibits non-ideal characteristics, such as
conducting some amount of current prior to reaching the rated Zener voltage. This
current draw under nominal operating conditions may be unacceptably high for very
low power systems.
U.S. Patent No. 5.436.552 discloses a clamping circuit for clamping a
reference voltage at a predetermined level. The clamping circuit includes a constant
current circuit having a constant current source and a current mirror circuit. A
trimmable resistance receives a constant current from the constant current circuit, and
a clamping MOS transistor receives a voltage generated by the trimmable resistance at
its gate to regulate a current flowing through a clamping node. It is possible to make
a rapid current-voltage characteristic and to set an arbitrary clamping potential of the
clamping circuit. The clamping voltage threshold provided by this circuit is
determined by the gate voltage (VG) applied to the clamping MOS transistor. The
accuracy of the clamping voltage is initially provided by precision trimming of a
resistor (R), which receives a constant current (lo), such that the voltage (VG=lo*R)
developed across the resistor during operation allows the clamping MOS transistor to
turn on at a clamping voltage (VC) whenever |VG-VC| > |Vthp|. A problem.
however, is that the clamping MOS transistor's threshold voltage. Vthp. varies over
temperature. Hence, a suitably consistent clamping voltage is not maintained under
varying operating conditions.
For example, it is believed that typical MOSFETs do a relatively poor
job of maintaining a consistent gate-source threshold voltage across varying operating
conditions (i.e., varying load currents: varying temperatures). For example, this could
either conduct excessive current at a nominal input voltage (if the threshold is too
low), or fail to conduct (if the threshold is too high).
It is known to provide a low current voltage clamp using very
voltage, precision enhancement-mode MOSFETs in place of a Zener diode This
employs a precision gate-source voltage provided by programmable MOSFETs. since
conventional MOSFETs do not have sufficient gate-source voltage precision.
However, such programmable MOSFETs are believed to have a relatively high cost.
There is room for improvement in overvoltage circuits.
There is also room for improvement in motor starters, overload relays
or low-power systems employing such circuits.
SUMMARY
These needs and others are met by embodiments of the disclosed
concept, which provides an overvoltage circuit that conducts negligible current below
a threshold voltage, but conducts sufficient current (thereby limiting voltage) above
the threshold voltage.
In accordance with one aspect of the disclosed concept, a motor starter
comprises: a contactor, and an overload relay comprising: a power supply having a
voltage, a processor powered by the voltage of the power supply and being structured
to control the contactor, and an overvoltage circuit comprising: a voltage reference
having a voltage, a comparator comprising: a first input for the voltage of the power
supply, the first input cooperating with the voltage of the voltage reference to
determine a threshold voltage, a second input for the voltage of the voltage reference,
and an output, a load, and a switch controlled by the output, the switch being
structured to electrically connect the voltage of the power supply to ground through
the load whenever the voltage of the power supply exceeds the threshold voltage.
The power supply of the overload relay may be structured to be
parasitically-powered from a number of power lines to a motor.
The overload relay may further comprise a number of current
transformers structured to sense current flowing to a motor and to supply power to the
power supply.
As another aspect of the disclosed concept, an overload relay
comprises: a power supply having a voltage: a processor powered by the voltage of
the power supply; and an overvoltage circuit comprising: a voltage reference having a
voltage, a comparator comprising: u first input for the voltage of the power supply. the
first input cooperating with the voltage of the voltage reference to determine a
threshold voltage, a second input lor the voltage of the voltage reference, and an
output, a load, and a switch controlled by the output, the switch being structured to
electrically connect the voltage of the power supply to ground through the load
whenever the voltage of the power supply exceeds the threshold voltage.
As another aspect of the disclosed concept, a low-power system
comprises: a power supply having a voltage; a processor powered by the voltage of
the power supply; a voltage reference having a voltage; a comparator comprising: a
first input for the voltage of the power supply, the first input cooperating with the
voltage of the voltage reference to determine a threshold voltage, a second input for
the voltage of the voltage reference, and an output; a load; and switch controlled by
the output, the switch being structured to electrically connect the voltage of the power
supply to ground through the load whenever the voltage of the power supply exceeds
the threshold voltage, wherein the processor is structured to dynamically adjust the
voltage of the power supply, and wherein the processor is further structured to
dynamically adjust the voltage of the voltage reference.
As another aspect of the disclosed concept, an overvoltage circuit is for
a power supply having a voltage. The overvoltage circuit comprises: a voltage
reference having a voltage: a comparator comprising: a first input for the voltage of
the power supply, the first input cooperating with the voltage of the voltage reference
to determine a threshold voltage, a second input for the voltage of the voltage
reference, and an output; a load: and a switch controlled by the output, the switch
being structured to electrically connect the voltage of the power supply to ground
through the load whenever the voltage of the power supply exceeds the threshold
voltage.
The voltage reference, the comparator and the switch may be part of a
voltage supervisor integrated circuit.
The voltage of the power supply may be less than the threshold
voltage, and current consumed by the voltage supervisor integrated circuit may be less
than about 1 uA.
The load may be an impedance of the switch.
The voltage of the voltage reference may be one of a predetermined,
fixed direct current voltage and a variable voltage.
The voltage reference, the comparator and the switch may be part of a
processor.
The comparator may he structured to provide hysteresis for the first
and the second inputs to avoid rapid switching of the output when the voltage of the
power supply is almost equal to the threshold voltage.
BRIEF DESCRIPTION OF THE DRAWINGS
A full understanding of the disclosed concept can be gained from the
following description of the preferred embodiments when read in conjunction with the
accompanying drawings in which:
Figure 1 is a block diagram in schematic form of a motor starter system
in accordance with embodiments of the disclosed concept
Figure 2 is a block diagram in schematic form of an overload relay in
accordance with another embodiment of the disclosed concept.
Figure 3 is a block diagram in schematic form of an overvoltage circuit in
accordance with another embodiment of the disclosed concept.
Figure 4 is a block diagram in schematic form of a low-power system
including an overvoltage protection circuit in accordance with another embodiment of the disclosed concept.
Figures 5 and 6 are block diagrams in schematic form of overvoltage
circuits in accordance with other embodiments of the disclosed concept
DESCRIPTION OF THE PREFERRED EMBODIMENTS
As employed herein, the term "number" shall mean one or an integer
greater than one (i.e., a plurality).
As emplowed herein, the term "processor" means a programmable
analog and/or digital device that can store, retrieve, and process data: a computer; a
workstation; a personal computer, a microprocessor a microcontroller; a
microcomputer, a central processing unit: a mainframe computer; a mini-computer; a
server; a networked processor; or any suitable processing device or apparatus.
As employed herein, the term "low-power processor" means a
processor structured to operate with relatively low power, such as, for example, less
than about 10 mW.
The disclosed concept is described in association with motor starters
and overload relays, although the disclosed concept is applicable to a wide range of
low-power systems and direct current power circuits.
Referring to Figure 1. a motor starter system 2 includes a motor starter 4
formed by a contactor 6 and an overload relay 8. The overload relay 8 includes a
power supply 10 having a voltage 12. a processor 14 powered by the power supply
voltage 12 and being structured to control the contactor 6. and an overvoltage circuit
16, which will be discussed, below, in connection with, for example, overvoltage
circuit 50 of Figure 3.
Example 1
The power supply 10 of the overload relay 8 is preferably structured to
be parasitically-powered from a number of power lines 18 to a motor 20 (shown in
phantom line drawing). In that instance, the overload relay 8 further includes a
number of current transformers 22 structured to sense current flowing to the motor 20
and to supply power to the power supply 10.
The example motor starter system 2 further includes a power source 24
(shown in phantom line drawing) and a main disconnect 26 (shown in phantom line
drawing), which supplies power to the overload relay 8.
The example processor 14 controls a solenoid 28 that, in turn, controls
normally closed contacts 30 and normally open contacts 32. The example normally
closed contacts 30 control a solenoid 34 of the contactor 6. The example normally
open contacts 32 control an indicator 36 that indicates the status of separable contacts
38 of the contactor 6. The example processor 14 can also input a reset signal 39.
Example 2
Figure 2 shows an overload relay 8' that can he the same as or similar to
the overload relax 8 of figure 1. The overload relay 8' similarly includes a power supply
10' having a voltage 12' (e.g.. without limitation. +2.5V). a microcontroller 14' (e.g..
without limitation, that can operate from about 1.8V to about 3.3V). an overvoltage
protection circuit 16' and a solenoid 28'.
The disclosed solid state overload relay 8' is a parasitically-powered
motor protection device. Current transformers 22' are employed to transform
electromagnetic fields generated by alternating current (AC) flowing to a motor (not
shown, but see the motor 20 of Figure 1) into power for the solid state overload relay
8'. As such, the corresponding control circuit 40 can operate with relatively very little
power (e.g.. without limitation, about 1.5 mW). By significantly reducing the amount
of power needed to operate the control circuit 40, the sue of the current transformers
22' can be decreased, thereby reducing product cost and size.
Although the example overload control circuit 40 can operate with
relatively very little power, increased motor currents can provide periods of more than
ample power. When insufficient power is available, a voltage supervisor circuit 42
provides a reset signal 44 to the microcontroller 14'.
Example 3
Figure 3 shows an overvokage circuit 50, which can be the same as or
similar to the overvottage circuits 16,16' of Figures 1 and 2. The overvoltage circuit
50 includes a voltage reference 52 having a voltage 54, a comparator 56, a switch 58
and a load 60. The comparator 56 includes a first input 62 for a power supply voltage
64 to be protected, a second input 65 (-) for the voltage reference voltage 54. and an
output 66. In this example, the input 62 includes a divider 63 cooperating with the
voltage 54 of the voltage reference 52 to determine a thereshold voltage. Also, in this
example, the voltage 54 is less than the threshold voltage. The switch 58 is controlled
by the comparator output 66 and is structured to electrically connect the power supply
voltage 64 to ground 68 through the loud 60 whenever the power supply voltage 64
exceeds the threshold voltage
Example 4
The voltage reference 52, the comparator 56 and the switch 58 can be
part of a voltage supervisor integrated circuit 70.
Example 5
When the power supply voltage 64 is less than the threshold voltage,
current consumed by the over voltage circuit 50 is less than about 1 uA.
Example 6
Referring again to Figure 2. during periods of ample power, excess
energy is provided to the power supply rail 12' of the microcontroller 14'. for
example, by way of the microcontroller's I/O protection diodes (not shown). For
example, a pair of diodes (not shown) limits the voltage at an I/O pin (not shown) to a
level less than a positive supply voltage (e.g., Vdd) and greater than the lowest
nominal voltage for that circuit (e.g.. ground 114; a negative supply voltage, Vss (not
shown)). This prevents damage to the transistors (not shown) of the I/O circuit (not
shown). Normally, both of these diodes are reverse-biased and do not conduct.
However, if the voltage at an I/O pin rises above Vdd or below Vss, then the
corresponding protection diode will be forward-biased and conduct, thereby limiting
the voltage at that I/O pin. but potentially providing excess energy to the power
supply rail 12'.
Since the control circuit 40 operates with relatively very little available
power, this surplus energy could cause the voltage level on the power supply rail 12'
to increase to a level that could damage the microcontroller 14' or other circuitry. The
disclosed overvoltage protection circuit 16' consumes negligible current when the
power supply voltage 12' is nominal, and draws a sufficient amount of current to keep
the power supply voltage 12' in check during periods of excess energy. This behavior
is advantageously contrasted with that of a Zener diode, which is not suited for a
relatively low power application since it conducts relatively significant current below
its Zener voltage. For example, a nominal voltage on the power supply rail 12' would
cause a Zener diode (not shown) to conduct an unacceptably high amount of current.
As such, the disclosed concept employs a low power (and low cost) solution.
Example 7
Although an overload relay 8' is disclosed in Figure 2, the disclosed
concept is applicable to any low-power circuit in which excess energy can accumulate
faster than is dissipated by normal operation of that circuit. This could include, for
example and without limitation, circuits that are powered via energy harvesting (e.g.,
without limitation, harnessing solar, kinetic or electromagnetic energy) which may
experience periods of excessive input power, or input circuits (e.g.. without limitation,
for wireless sensor networks) winch may experience periods in which excessive
sensor input voltage is generated.
Example 8
The disclosed overload relay 8' of Figure 2 employs the overvoltage
protection circuit 16' such as the example overvoltage circuit SO of Figure 3 including
the example voltage supervisor integrated circuit 70 and the example load 60 to
provide a low-power voltage clamp. The voltage supervisor integrated circuit 70
monitors the voltage level of the power supply voltage 64, such as the power supply
rail 12' of Figure 2. When the power supply rail 12' is below a suitable threshold
voltage level (e.g., without limitation, above the nominal power supply voltage, but
lower than the maximum voltage rating of the microcontroller 14' of Figure 2), then
no current flows through the external load 60 (Figure 3) and the only current
consumed by this circuit is from the supply current of the voltage supervisor
integrated circuit 70 (e.g.. without limitation, typically less than about 1 u/\). This is
a significant improvement over a Zener diode alone which may draw 50 times more
current at nominal voltage. When the power supply rail 12' rises and exceeds the
threshold voltage level, then the switch 38 is turned on. which allows current to flow
through the load 60, thereby clamping the power supply rail 12' to a safe voltage
level.
Example 9
Although the overvoltage circuit SO of Figure 3 can employ a
commercially available voltage supervisor integrated circuit 70, a wide range of other
suitable circuits can be employed. For example and without limitation, the voltage
reference 52. two-input comparator 56 and output FET switch 58 features of the
voltage supervisor integrated circuit 70 could be constructed using discrete
components. Alternatively, the loud 60 could be included with a voltage supervisor to
form an overvoltage protection integrated circuit (not shown). Also, constant or
variable voltage references can be employed.
Example 10
The load 60 can be a Zener diode.
Example 11
Also, alternative loads can be employed, such as for example and
without limitation, resistors, transistors and/or diodes (including Zener diodes and
LEDs), or any combination thereof. Furthermore, the function of the voltage
supervisor integrated circuit 70 can be performed by any suitable programmable
device such as. for example and without limitation, a processor such as a low-power
microcontroller.
Example 12
An example voltage supervisor integrated circuit 70 is a Voltage
Detector Series NCP300 or NCP301 marketed by On Semiconductor of Phoenix,
Arizona.
Example 13
The power supply voltage 64 to be protected in Figure 3 can be a
nominal voltage (e.g.. without limitation, a nominal voltage of about 2.5 VDC in a
range of about 1.8 VDC to about 3.3 VDC). The threshold voltage (e.g., without
limitation, about 3.0 VDC). which is determined by the divider 63 and the voltage
reference voltage 54. is greater than the nominal voltage but less than a maximum
voltage (e.g.. without limitation. 3.3 VDC). which is greater than the nominal voltage.
Example 14
The voltage reference voltage 54 can be one of a predetermined, fixed
direct current voltage and a vuriable voltage, as will be discussed below in connection
with Figure 4.
Example 15
The load 60 can be selected from the group consisting of a resistor, a
transistor, a diode, a Zener diode and a light emitting diode.
Example 16
The comparator 56 can be structured to provide hysteresis for the first
and the second inputs 62.65, in order to avoid rapid on/off switching of the
comparator output 66 when the power supply voltage 64 is almost equal to the
threshold voltage as determined by the voltage reference voltage 54 and the divider
63. This could avoid rapid switching that would otherwise be generated due to
relatively small signal "noise" on the power supply voltage 64 or the example power
supply rail 12' of Figure 2.
It will be appreciated that the divider 63 is not required. See, for
example and without limitation. Example 19, below.
Example 17
Referring to Figure 4. a low-power system 100 includes a power
supply 102 having a voltage 104. a processor 106 powered by the power supply
voltage 104. and an overvoltage circuit 108. which can be similar to the overvoltage
circuits 16.16' of Figures 1 and 2. Here, however, the processor 106 is structured to
dynamically adjust the power supply voltage 104 using line 105. and is further
structured to dynamically adjust the threshold voltage 110 of the overvoltage circuit
106 using line 107.
For example, line 107 can control • variable voltage reference 112 in
an intelligent way. The low-power system 100, for instance, can dynamically adjust
its power supply voltage 104 (e.g., dynamic voltage scaling) by an output from the
processor line 105 to the power supply 102. Decreasing the power supply voltage 104
results in decreased power consumption, but also a corresponding decrease in
maximum clock rate. Hence, such a system 100 could lower its power supply voltage
104 during periods of little activity to conserve power, but then temporarily raise its
power supply voltage 104 when it needs to quickly complete some task. Such a
system 100 could, in turn, employ the overvoltage circuit 108 with the adjustable
threshold voltage 110 and variable voltage reference 112 to suitably "follow" the
dynamic voltage sealing
Example 18
Figure 5 shows another overvoltage circuit 50', which is similar to the
overvoltage circuit 50 of Figure 3. Here, the impedance 60' of switch 58' (e.g.. FET) of
voltage supervisor integrated circuit 70' is the load. When the switch 58' is on or
closed, this briefly electrically connects power supply voltage 64' to ground 114. as
limited by the ON impedance (resistance) of the switch 58'. until the power supply
voltage 64' dips below the predetermined voltage threshold determined bv reference
voltage 54 and divider 63. It is believed that this could function by relatively rapidly
switching the switch 58' on and off.
Example 19
Figure 6 shows another overvoltage circuit 120 in which a voltage
reference 122, a comparator 124 and a switch 126 are part of a processor 128. The
example processor 128 includes an analog-to-digital converter (ADC) 130, the
comparator 124 and a routine 132 structured to periodically measure power supply
voltage 134 from the analog-to-digital converter 130, compare measured power
supply voltage 136 to a predetermined value 138, and change output 140 to enable a
load 141 if the measured power supply voltage 136 exceeds the predetermined value
138. For example, a memory 142 stores the predetermined value 138. and the routine
132 periodically executes an arithmetic operation (comparator 124) to compare the
measured power supply voltage 136 of the predetermined value 138. The switch 126
drives the processor output 140 responsive to the comparator 124 of the routine 132 to
enable the load 141 and cause it to conduct current from the power supply voltage 134
to ground 144. In this example, there is no divider 63 (Figures 3 and 5) and the
threshold voltage is the same as the voltage reference 122 and predetermined value
138, all of which represent voltages using corresponding digital voltage values.
Example 20
As an alternative to Example 19, the processor 128 can employ an
external analog-to-digital converter (not shown) and/or an internal comparator circuit
(not shown). For example, such a comparator circuit can be employed to determine if
the power supply voltage 134 (e.g., input to one of the two inputs of the comparator)
exceeds a predetermined voltage reference (e.g.. input to the other one of the two
inputs of the comparator) If so. then the processor employs output 140 to enable the
load 141. If these functions are already available in a particular processor, then this
can provide a relatively lower cost solution since a separate voltage superv isor is not
needed.
While specific embodiments of the disclosed concept have been
described in detail, it will be appreciated by those skilled in the art that various
modifications and alternatives to those details could be developed in light of the
overall teachings of the disclosure. Accordingly, the particular arrangements
disclosed are meant to be illustrative only and not limiting as to the scope of the
disclosed concept which is to he given the full breadth of the claims appended and
any and all equivalents thereof.
REFERENCE NUMERICAL LIST
we claim:
1. An overvoltage circuit (16; 16';50;50'; 108; 120) for a power
supply having a voltage (64:64': 100:104). said overvoltage circuit comprising:
a voltage reference (52) having a voltage (54):
a comparator (56) comprising:
a first input (62) for the voltage of said power supply,
said first input cooperating with the voltage of the voltage reference to determine a
threshold voltage.
a second input (65) for the voltage of said voltage
reference, and
an output (66):
a load (60); and
a switch (58) controlled by the output, said switch being
structured to electrically connect the voltage of said power supply to ground (68)
through said load whenever the voltage of said power supply exceeds said threshold
voltage.
2. The overvoltage circuit (50;50') of Claim 1 wherein said
voltage reference, said comparator and said switch are part of a voltage supervisor
integrated circuit (42).
3. The overvoltage circuit (50;50') of Claim 2 wherein when the
voltage of said power supply is less than the threshold voltage, current consumed by
said voltage supervisor integrated circuit is less than about 1 uA.
4. The overvoltage circuit (50') of Claim 1 wherein said load is an
impedance of said switch.
5. The overvoltage circuit (50;50') of Claim 1 wherein the voltage
of said power supply is a nominal voltage; and wherein said threshold voltage is
greater than the nominal voltage but less than a maximum voltage, which is greater
than the nominal voltage.
6. The overvoltage circuit (50;50'; 108) of Claim 1 wherein the
voltage of said voltage reference is one of a predetermined, fixed direct current
voltage (64;64') and a variable voltage (104).
7. The overvoltage circuit (50;50') of Claim 1 wherein said load
(60;60') is selected from the group consisting of a resistor, a transistor, a diode, a
Zener diode and a light emitting diode.
8. The overvoltage circuit (120) of Claim 7 wherein said
processor comprises an analog-to-digital converter (130), said comparator and a
routine (132) structured to periodically measure the voltage (134) of said power
supply from the analog-to-digital converter, compare (124) the measured voltage
(136) of said power supply to a predetermined value (138), and change said output
(140) to enable the load (141) if the voltage of said power supply exceeds the
predetermined value.
9. The overvoltage circuit (120) of Claim 8 wherein said
processor further comprises a memory (142) storing the predetermined value (138);
and wherein the routine comprises an arithmetic operation (124) to compare the
measured voltage of said power supply to the predetermined value.
10. The overvoltage circuit (120) of Claim 1 wherein said voltage
reference (138), said comparator (124) and said switch (126) are part of a processor
(128); and wherein said processor comprises an output (140), said comparator, and a
routine (132) structured to change the output of said processor responsive to the
output of said comparator to enable the load if the voltage of said power supply
exceeds said threshold voltage.
11. The overvoltage circuit (50;50') of Claim 1 wherein said
comparator is structured to provide hysteresis for the first and the second inputs to
avoid rapid switching of said output when the voltage of said power supply is almost
equal to said threshold voltage.
12. A motor starter (4) comprising:
a contactor (6): and
an overload relay (8) comprising:
a power supply (10) having a voltage (12;64).
a processor (14) powered by the voltage of said power
supply and being structured to control said contactor, and
the overvoltage circuit (16;50) of Claim 1.
13. The motor starter (4) of Claim 12 wherein the power supply of
said overload relay is structured to he parasitically-powered from a number of power
lines (18) to a motor (20).
14. An overload relay (8;8') comprising:
a power supply (10; 10') having a voltage (12;12';64);
a processor (14;14') powered by the voltage of said power
supply; and
the overvoltage circuit (16;16';50) of Claim 1.
15. A low-power system (100) comprising:
a power supply (102) having a voltage (104);
a processor (106) powered by the voltage of said power supply;
and
the overvoltage circuit of Claim 1.
wherein said processor is structured (105) to dynamically adjust
the voltage of said power supply, and
wherein said processor is further structured (107) to
dynamically adjust the voltage of said voltage reference.

A motor starter (4) includes a contactor (6) and an overload relay (8).
The overload relay includes a power supply 110) having a voltage (12;64), a processor
(14) powered by the power supply voltage and being structured to control the
contactor, and an overvoltage circuit (16;50). The overvoltage circuit includes a
voltage reference (52) having a voltage (54), a comparator (56), a load (60) and a
switch (58). The comparator includes a first input (62) for the power supply voltage,
the first input cooperating with the voltage of the voltage reference to determine a
threshold voltage, a second input (65) for voltage reference voltage, and an output
(60). The switch is controlled by the comparator output and is structured to
electrically connect the power supply voltage to ground (68) through the load
whenever the power supply voltage exceeds the threshold voltage.