Abstract:
A circuit for supplying power to an intrinsically safe circuit having a power supply and output terminals includes an integrated current source feedback and current limiting element. Voltage limiting circuitry between the power supply and the output terminals limits the voltage across a load. Current limiting circuitry includes barrier resistors which convert current to voltage for input to an operational amplifier comparing a reference voltage with the voltage level representing the load current and providing a control input signal to a transistor for limiting a current applied to the load.

Description:
FIELD OF THE INVENTION 
     This invention relates to a power supply for an intrinsically safe loads. More particularly, this invention relates to circuitry that limits the current supplied to the load to meet intrinsically safe standards. 
     Problem 
     Electronic devices are often used in hazardous environments containing volatile material. It is often a problem that a spark or heat from the electronic devices can cause the volatile material to ignite. Therefore, makers of electronic devices for use in these hazardous environments must provide some protection to ensure that the electronic devices do not ignite the volatile material. 
     One such form of protection is to make a circuit intrinsically safe. Intrinsically safe standards are set by regulating authorities such as the UL in the United States, CENELEC in Europe, CSA in Canada, and TIIS in Japan. In order to be intrinsically safe the current, power, and voltage through the circuit are limited to levels that prevent ignition of the volatile material from a spark or heat generated by the circuit. 
     It is a problem to deliver electricity to an intrinsically safe device. The power, voltage and current of the electricity are limited to the levels insufficient to ignite the volatile material. Therefore, components are needed in the power supply to limit the power, voltage, and current delivered to the intrinsically safe device. 
     In a conventional power supply, voltage is limited by connecting one or more zener diodes between supply line connecting a power supply to output terminals. The zener diodes limit voltage to V. Current limiting is provided by connecting a resistor having a resistance of R in series with a high potential output terminal. The resistor limits current to V/R. Power is controlled by the limiting of current and voltage. 
     The limiting components, i.e. the diodes and resistors, must be protected to prevent the components from exceeding published rating in the event of a fault. A fuse is typically added to the circuit to limit the amount of current that can be delivered to the components. A fuse is chosen that has a rating that ensures power dissipation ratings are not exceeded. 
     Although not required for intrinsical safety standards, a current limiting circuit is often added to the power supply circuit to prevent the fuse from blowing. There many current limiting topologies that may be employed on either a high potential or low potential side of a power supply. Most current limiting topologies include a resistor to convert current to voltage in order to provide feedback that is proportional to current. A comparison to a reference voltage is performed. The impedance of a series element is adjusted in response to the comparison. A problem with adding a current limiting circuit is that the conversion of current to voltage adds to the total output resistance and causes additional voltage loss beyond the voltage loss required to meet intrinsic safety standards. 
     Solution 
     The above and other problems are solved and an advance in the art is made by power supply having an integrated current source feedback and current limiting element in accordance with this invention. One advantage of an integrated current source feedback and current limiting element is the voltage loss may be minimized to the voltage that must be limited for intrinsic safety standards. A second advantage is that the number of components of a power supply circuit are reduced which lowers the cost of producing a power supply. 
     In accordance with the present invention, the function of a current conversion resistor in a power limiting circuit is combined with the function of barrier output resistance. This allows output resistance to be no more that the resistance required to prevent ignition of a hazardous material. The combination of function is provided by moving parts of the current limiting circuit to a point after the barrier resistance. In particular, a variable impedance device is moved to a point after the barrier resistance. One example of a variable impedance device is a MOFSET transistor. 
     When a MOFSET transistor is moved, there are two new paths to the output terminals. A first is a an op-amp control output and a feedback from a feedback from the barrier resistance. An input to an operational amplifier and the gate for the MOFSET transistor are of high impedance and relatively large value resistors compared to a barrier resistor placed in each of these paths. The total barrier resistance in a combination of resistor placed in a path with inputs into the MOFSET gate and operational amp input. This limits the power supply to an output resistance which is negligibly lower than the barrier resistance alone. 
     An aspect of the invention comprises an intrinsically safe circuit configured for supplying power to a load connected to a first output terminal and a second output terminal; 
     said circuit comprising; 
     a power supply source; 
     a voltage limiter connected in parallel with said power supply source to limit the maximum voltage generated by said power supply source; 
     a connection between a first side of said voltage limiter and said first terminal; 
     a current limiter comprising; 
     a reference voltage; 
     a variable impedance; 
     a barrier resistor; 
     a second side of said voltage limiter is connected in series with said barrier resistor and said variable impedance to said second output terminal; 
     said current limiter limits a current delivered to said load via said terminals to a maximum current level in response to a comparison of said reference voltage with a voltage across said barrier resistor representative of the current delivered to said load. 
     Preferably the intrinsically safe circuit further comprises a fuse connected between a positive voltage side of the power supply source and the voltage limiting circuit. 
     Preferably the voltage limiter comprises a diode that has a cathode connected to the positive side of the power supply source and further has an anode connected to the negative side of the power supply source. 
     Preferably the diode comprises a zener diode. 
     Preferably the current limiter and variable impedance comprises: 
     said variable impedance comprises a transistor for controlling the current level through the load, said transistor is connected to the second output terminal; 
     said operational amplifier controls the impedance of the transistor and has an output connected to a gate of the transistor, and further has a first input connected to a voltage representing said load current and further has a second input connected to said reference voltage; 
     a voltage divider including said barrier resistor for generating a voltage level that is representative of said load current, said voltage divider is connected to the first input of the operational amplifier, and to the negative side of the power supply source. 
     Preferably the current limiter further comprises a resistor connected between the output of the operational amplifier and the gate of the transistor. 
     Preferably the current limiting circuit has a transistor comprising Metal Oxide Semiconductor Field Effect Transistor (MOSFET). 
     Preferably the current limiter has a voltage divider comprising: 
     a first resistor connected between the first input of the operational amplifier and drain of the transistor; and 
     a second resistor having one end connected to said drain and a second end connected to the negative side of the power supply source. 
     Preferably the intrinsically safe circuit is characterized in that the load comprises electronics of the Coriolis flowmeter having a signal conditioner circuit for generating a drive signal and for receiving pick-off signals, said signal conditioner circuit is connected to the first and second output terminals of the intrinsically safe circuit. 
     Preferably the signal conditioner circuit comprises: 
     a drive circuit for generating said drive signal, said drive circuit is connected to the first and second output terminals of the intrinsically safe circuit; and 
     a conditioning circuit for receiving the pick-off signals, said conditioning circuit has an input connected to an output of the drive circuit. 
     Another aspect comprises a method of operating an intrinsically safe circuit for supplying power to a load; 
     said method comprises the steps of; 
     connecting a voltage limiter connected in parallel with a power supply source to limit the maximum voltage generated by said power supply source; 
     extending a connection between a first side of said voltage limiter and a first terminal; 
     connecting a second side of said voltage limiter in series with a barrier resistor and a variable impedance to a second output terminal; 
     with a barrier resistor and said variable impedance to said second output terminal; 
     limiting a current delivered to said load via said terminals to a maximum current level in response to a comparison of a reference voltage with a voltage across said barrier resistor representative of the current delivered to said load. 
     Preferably the step of limiting the voltage level includes operating a diode to limit said voltage. 
     Preferably the step of operating a diode includes the step of operating a zener diode. 
     Preferably the method further comprises the steps of: 
     applying intrinsically safe power from said terminal to a drive circuit of a Coriolis flowmeter; and 
     generating a drive signal with the drive circuit in response to the reception of said power. 
     Preferably the method further comprises the step of applying pick-off signals to a conditioning circuit of said Coriolis flowmeter in response to the reception of said power. 
     Preferably the method is characterized in that the step of limiting the current comprises the steps of: 
     operating a voltage divider to generate a voltage level that is representative of the load current; 
     operating the operational amplifier to compare a reference voltage with said voltage level representing the load current; and 
     extending a signal from an output of the operational amplifier to a control input of a transistor for limiting a current to a maximum level through the load. 
     Preferably the method is characterized in that the step of operating the transistor includes the step of operating a MOSFET. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     The above and other features in accordance with this invention can be understood from the Detailed Description and the following drawings: 
     FIG. 1 illustrating a prior art power supply for an intrinsically safe load; 
     FIG. 2 illustrating a power supply for an intrinsically safe load in accordance with this invention; and 
     FIG. 3 illustrating a Coriolis flow meter incorporating a power supply in accordance with this invention into meter electronics. 
    
    
     DETAILED DESCRIPTION 
     A power supply in accordance with this invention allows the total output resitance to be no more than is required to prevent ignition of volatile material in a hazardous environment. A typical intrinsically safe power supply  100  is illustrated in FIG. 1 to show the differences between a power supply in accordance with the present invention and a typical intrinsically safe power supply. 
     Prior Art intrinsically safe power supply  100  delivers sufficient power to a load to ensure that the load may operate properly while limiting the worst case voltage, current, and power to levels insufficient to cause ignition of a hazardous material. Voltage limiting circuitry  101  limits the voltage across the load. Voltage limiting circuitry  101  is a zener diode Z 1  connected between paths  110  and  120 . One skilled in the art will recognize that more than one zener diode may be connected between paths  110  and  120  to limit the voltage. For purposes of this discussion, voltage limiting circuitry  101  limits the magnitude of the voltage across a load (Not Shown) to V z . 
     Instantaneous current limiting circuitry  102  limits instantaneous current applied to a load. In power supply  100 , instantaneous current limiting circuitry includes resistor Rb which is connected in series with output terminals T 1  and T 2 . In this embodiment, resistor Rb is connected between positive output terminal T 1  and power source PS along path  110 . This limits the instantaneous current to V z /R b  where R b  is the resistance of resistor Rb. Power delivered to the load (Not Shown) is limited by the limits of voltage and current. 
     Fuse F 1  is connected between power source Ps and positive output terminal T 1  to protect voltage limiting circuitry  101  and instantaneous current limiting circuitry  102  in case of a fault in the circuit. Fuse F 1  prevents voltage limiting circuitry  101  and instantaneous current limiting circuitry  102  from exceeding published rating of components. 
     Average current limiting circuit  103  prevents fuse F 1  from blowing in the event that output terminals T 1  and T 2  are shorted. There are many well known current limiting topologies that can be used to provide average current limiting circuit  103 . In powersupply  100 , average current limiting circuit is provided by the following components. A source of transistor Q 1  is connected to negative output terminal T 2 . A drain of transistor Q 1  is connected to a resistor Rv. Resistor Rv converts the current through it to a voltage to provide a feedback voltage that is proportional to current through it. An output of operational amplifier OA is connected to the gate of transistor Q 1  and a signal is applied to the gate based on a comparison with voltage “REF” to adjust the impedance of transistor Q 1 . The current limit is set to I lim =V ref /Rv where I lim  is the current limit, V ref  is the reference voltage, Ref, and Rv is the resistance of the resistor Rv. 
     One skilled in the art will recognize that values of components in power supply  100  are constrained by tables and formulas that characterize power and energy at which an ignition of a volatile material occurs. In normal operation, the load (not Shown) is connected to output terminals T 1  and T 2 , draws current and voltage is lost across an output resistance. In order to maximize power transfer, it is desired to limit output resistance to that needed to prevent ignition of the volatile material. 
     It is a problem that adding average current limiting circuit  103  results in additional voltage loss beyond the voltage limits for preventing ignition. Voltage is lost because the voltage conversion resistance add to the total output resistance. 
     Power supply  200  illustrated in FIG. 2 solves this problem in accordance with the present invention. Power supply  200  is an intrinsically safe power supply that limits current, power and voltage delivered to a load (Not Shown). Voltage limiting circuitry  201  limits the voltage across the load. In power supply  200 , voltage limiting circuitry  201  is a zener diode Z 1  connected between paths  210  and  220 . One skilled in the art will recognize that more than one zener diode may be connected between paths  210  and  220  to limit the voltage. For purposes of this discussion, voltage limiting circuitry  201  limits the voltage across a load (Not Shown) to V z . 
     In accordance with the present invention, the functions of limiting the instantaneous and average current are combined into one circuit. The use of one circuit for both functions allows the total output resistance to be reduced to the amount of resistance required to prevent ignition of a volatile material. This is accomplished by moving components of the average current limiting circuits to a point after the instantaneous current limiting circuitry. In particular, the variable impedance device, transistor Q 1  is moved to a point after the barrier circuitry. The location of the variable impedance device creates two new paths to output terminals T 1  and T 2 . The new paths are the operational amplifier OA output and the feedback from a voltage divider formed by resistors R 2  and R 3 . The total output resistance is a parallel combination of resistors along the two new paths and path  210  to power source PS. 
     In power supply  200 , the current limiting circuit  202  in accordance with the present invention is provided in the following manner. A source of Metal-Oxide Semiconductor Field-Effect Transistor, or MOSFET, transistor Q 1  is connected to negative output terminal T 2 . A barrier resistor R 3  is connected along path  220  between transistor Q 1  and power source PS. A second resistor R 2  is connected between a drain of transistor Q 1  and an input of operational amplifier OA to generate a comparison voltage. Operational amplifier OA has a lower input connected to resistor R 2  and it receives a reference voltage Ref on its upper input. Operational amplifier performs a comparison of the voltage on the inputs and applies a current to a gate of transistor Q 1  to adjust the impedance of transistor Q 1 . Resistor R 1  is connected between the output of operational amplifier OA and the gate of transistor Q 1 . Those skilled in the art will recognize that the values of Resistors R 1 , R 2 , and R 3  may be large and that the total output resistance is a combination of the three resistors in parallel which is negligibly smaller than the value of the barrier resistance. Thus power transfer is optimized. 
     One device where power supply  200  is needed is a power supply for electronics in a Coriolis flowmeter operating in a hazardous environment. FIG. 3 illustrates a Coriolis flow meter  300  incorporating power supply  200 . Coriolis flowmeter  300  includes a flowmeter assembly  310  and meter electronics  350 . Meter electronics  350  are connected to a meter assembly  310  via leads  320  to provide for example, but not limited to, density, mass-flow-rate, volume-flow-rate, and totalized mass-flow rate information over a path  375 . A Coriolis flowmeter structure is described although it should be apparent to those skilled in the art that the present invention could be practiced in conjunction with any apparatus having intrinsically safe circuits requiring an intrinsically safe power supply. 
     A Coriolis flowmeter structure is described although it should be apparent to those skilled in the art that the present invention could be practiced in conjunction with any apparatus having a vibrating conduit to measure properties of material flowing through the conduit. A second example of such an apparatus is a vibrating tube densitometer which does not have the additional measurement capability provided by a Coriolis mass flowmeters. 
     Meter assembly  310  includes a pair of flanges  301  and  301 ′, manifold  302  and conduits  303 A and  303 B. Driver  304 , pick-off sensors  306  and  306 ′, and temperature sensor  307  are connected to conduits  303 A and  303 B. Brace bars  305  and  305 ′ serve to define the axis W and W′ about which each conduit oscillates. 
     When Coriolis flowmeter  300  is inserted into a pipeline system (not shown) which carries the process material that is being measured, material enters flowmeter assembly  310  through flange  301 , passes through manifold  302  where the material is directed to enter conduits  303 A and  303 B. The material then flows through conduits  303 A and  303 B and back into manifold  302  from where it exits meter assembly  310  through flange  301 ′. 
     Conduits  303 A and  303 B are selected and appropriately mounted to the manifold  302  so as to have substantially the same mass distribution, moments of inertia and elastic modules about bending axes W—W and W′—W′, respectively. The conduits  303 A- 303 B extend outwardly from the manifold in an essentially parallel fashion. 
     Conduits  303 A- 303 B are driven by driver  304  in opposite directions about their respective bending axes W and W′ and at what is termed the first out of phase bending mode of the flowmeter. Driver  304  may comprise any one of many well known arrangements, such as a magnet mounted to conduit  303 A and an opposing coil mounted to conduit  303 B and through which an alternating current is passed for vibrating both conduits. A suitable drive signal is applied by meter electronics  350  to driver  304  via path  312 . 
     Pick-off sensors  306  and  306 ′ are affixed to at least one of conduits  303 A and  303 B on opposing ends of the conduit to measure oscillation of the conduits. As the conduit  303 A- 303 B vibrates, pick-off sensors  306 - 306 ′ generate a first pick-off signal and a second pick-off signal. The first and second pick-off signals are applied to paths  311  and  311 ′. The driver velocity signal is applied to path  310 . 
     Temperature sensor  307  is affixed to at least one conduit  303 A and/or  303 B. Temperature sensor  307  measures the temperature of the conduit in order to modify equations for the temperature of the system. Path  311 ″ carries temperature signals from temperature sensor  307  to meter electronics  350 . 
     Meter electronics  350  receives the first and second pick-off signals appearing on paths  311  and  311 ′, respectively. Meter electronics  350  processes the first and second velocity signals to compute the mass flow rate, the density, or other property of the material passing through flowmeter assembly  10 . This computed information is applied by meter electronics  350  over path  375  to a utilization means (not shown). It is known to those skilled in the art that Coriolis flowmeter  300  is quite similar in structure to a vibrating tube densitometer. Vibrating tube densitometers also utilize a vibrating tube through which fluid flows or, in the case of a sample-type densitometer, within which fluid is held. Vibrating tube densitometers also employ a drive system for exciting the conduit to vibrate. Vibrating tube densitometers typically utilize only single feedback signal since a density measurement requires only the measurement of frequency and a phase measurement is not necessary. The descriptions of the present invention herein apply equally to vibrating tube densitometers. 
     In Coriolis flowmeter  300 , the meter electronics  350  are physically divided into 2 components a host system  370  and a signal conditioner  360 . In conventional meter electronics, these components are housed in one unit. 
     Signal conditioner  360  includes drive circuitry  363  and pick-off conditioning circuitry  361 . One skilled in the art will recognize that in actuality drive circuitry  363  and pick-off conditioning circuitry  361  may be separate analog circuits or may be separate functions provided by a digital signal processor or other digital components. Drive circuitry  363  generates a drive signal and applies the drive signal to driver  304  via path  312  of path  320 . In actuality, path  312  is a first and a second lead. Drive circuitry  363  is communicatively connected to pick-off signal conditioning circuitry  361  via path  362 . Path  362  allows drive circuitry to monitor the incoming pick-off signals to adjust the drive signal. Power to operate drive circuitry  363  and pick-off signal conditioning circuitry  361  is supplied from host system  370  via a first wire  373  and a second wire  374 . First wire  373  and second wire  374  may be a part of a conventional 2-wire, 4-wire cable, or a portion of a multi-pair cable. 
     Pick-off signal conditioning circuitry  361  receives input signals from first pick-off  305 , second pick-off  305 ′, and temperature sensor  307  via paths  311 ,  311 ′ and  311 ″. Pick-off circuitry  361  determines the frequency of the pick-off signals and may also determine properties of a material flowing through conduits  303 A- 303 B. After the frequency of the input signals from pick-off sensors  305 - 305 ′ and properties of the material are determined, parameter signals carrying this information are generated and transmitted to a secondary processing unit  371  in host system  370  via path  376 . In a preferred embodiment, path  376  includes 2 leads. However, one skilled in the art will recognize that path  376  may be carried over first wire  373  and second wire  374  or over any other number of wires. 
     Host system  370  includes a power supply  372  and processing system  371 . Power supply  372  receives electricity from a source and converts the received electricity to the proper power needed by the system. Processing system  371  receives the parameter signals from pick-off signal conditioning circuitry  361  and then may perform processes needed to provide properties of the material flowing through conduits  303 A- 303 B needed by a user. Such properties may include but are not limited to density, mass flow rate, and volumetric flow rate. 
     In this embodiment, power supply  372  includes the circuitry of power supply  200  shown in FIG.  2 . This allows power supply  372  to provide power meeting intrinsically safe limits to signal conditioner  360  which includes circuitry meeting intrinsically safe standards. 
     The above is a description of a power supply circuitry that has an integrated current source feedback and current limiting element in accordance with this invention. It is expected that those skilled in the art can and will design alternative systems that infringe this invention as set forth in the claims below either literally or through the Doctrine of Equivalents.