Patent Application
20120002332
1. 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.
2. 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. Pat. 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 (Io), such that the voltage (VG=Io*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 low 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.
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: 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.
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 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, 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 be 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.
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:
As employed herein, the term “number” shall mean one or an integer greater than one (i.e., a plurality).
As employed 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
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.
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
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′.
The voltage reference 52, the comparator 56 and the switch 58 can be part of a voltage supervisor integrated circuit 70.
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.
Referring again to
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.
Although an overload relay 8′ is disclosed in
The disclosed overload relay 8′ of
Although the overvoltage circuit 50 of
The load 60 can be a Zener diode.
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.
An example voltage supervisor integrated circuit 70 is a Voltage Detector Series NCP300 or NCP301 marketed by On Semiconductor of Phoenix, Ariz.
The power supply voltage 64 to be protected in
The voltage reference voltage 54 can be one of a predetermined, fixed direct current voltage and a variable voltage, as will be discussed below in connection with
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.
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
It will be appreciated that the divider 63 is not required. See, for example and without limitation, Example 19, below.
Referring to
For example, line 107 can control a 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 scaling.
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 supervisor 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 be given the full breadth of the claims appended and any and all equivalents thereof.
