Abstract:
Methods and apparatus for supplying power for use in metering electrical energy over a wide range of voltages with a single meter are disclosed. The wide ranging meter includes a processing unit for processing divided input voltage and a current component in order to determine electrical energy metering values. The processing unit is operable in response to supply voltages. A power supply, connected to receive the undivided voltage component, generates the supply voltages over the wide dynamic range. It is especially preferred for the power supply to include a transformer having first, second and third windings, wherein the undivided voltage component is provided to the first winding and wherein the second winding defines the output of the power supply. A switching member is connected to the first winding for permitting and preventing the flow of current in response to a control signal. A control member generates the control signal in response to the output of the power supply. It is also preferred for the control signal to disable the witch member. It is further preferred for the power supply to include a voltage blocking clamp, connected to the transformer for blocking the voltage applied to the transformer. It is still further preferred for an oscillator to be used to generate an oscillating signal for switching the switching member ON and OFF so that the switching member is provided a substantially constant OFF time.

Description:
RELATED APPLICATION DATA  
       [0001]    This application is a divisional application of U.S. patent application Ser. No. 09/047,479, filed Mar. 25, 1998, which is a continuation of U.S. patent application Ser. No. 08/478,605, filed Jun. 7, 1995, now U.S. Pat. No. 5,903,145, which is a continuation of U.S. patent application Ser. No. 08/384,398, filed Feb. 3, 1995, now U.S. Pat. No. 5,457,621, which is a continuation of U.S. patent application Ser. No. 08/259,116, filed Jun. 10, 1994, now abandoned, which is a continuation of U.S. patent application Ser. No. 07/839,967, filed Feb. 21, 1992, now abandoned.  
     
    
     
       FIELD OF INVENTION  
         [0002]    The present invention relates generally to the field of electric utility meters.  
           [0003]    More particularly, the present invention relates to electronic utility watthour meters or meters utilized to meter real and reactive energy in both the forward and reverse directions.  
         BACKGROUND OF THE INVENTION  
         [0004]    Electric utility companies and power consuming industries have in the past employed a variety of approaches to metering electrical energy. Typically, a metering system monitors power lines through isolation and scaling components to derive polyphase input representations of voltage and current. These basic inputs are then selectively treated to determine the particular type of electrical energy being metered. Because electrical uses can vary significantly, electric utility companies have requirements for meters configured to analyze several different nominal primary voltages. The most common of these voltages are 120, 208, 240, 277 and 480 volts RMS. Presently, available meters have a different style for each of these applications, both electro-mechanical and electronic. This forces the electric utility companies to inventory, test and maintain many different styles of meters. Consequently, a need exists for reducing the number of meter types a utility need inventory by providing a meter capable of operation over a wide dynamic range.  
           [0005]    The problem of wide amperage dynamic range was addressed in U.S. Pat. No. 3,976,941—Milkovic. It was there recognized that solid state electronic meters were becoming more desirable in metering applications, however, such solid state meters had a critical drawback in their amperage dynamic range. An effort was described to improve the amperage dynamic range of solid state meters so that such meters would be operationally equivalent to prior electro-mechanical meters. The problem with such meters, however, was their failure to address the multiple voltage situation. Utility companies utilizing such meters would still be forced to inventory, test and maintain many different styles of meters in order to service the various voltages provided to customers.  
           [0006]    It has been recognized in various meter proposals that the use of a microprocessor would make metering operations more accurate. It will be understood, however, that the use of a microprocessor requires the provision of one or more supply voltages. Power supplies capable of generating a direct current voltage from the line voltage have been used for this purpose. Since electric utility companies have requirements for various nominal primary voltages, it has been necessary to provide power supplies having individualized components in order to generate the microprocessor supply voltages from the nominal primary voltage.  
           [0007]    Consequently, a need exists for a single meter which is capable of metering electrical energy associated with nominal primary voltages in the range from 96 to 528 volts RMS. Applicants resolve the above problems through the use of a switching power supply and voltage dividers. It will be recognized that switching power supplies are known. However, the use of such a power supply in an electrical energy meter is new. Moreover, the manner of the present invention, the particular power supply construction and its use in an electrical energy meter is novel.  
           [0008]    It will also be noted, in order to solve the inventory problem, designing a wide voltage range meter in the past involved the use of voltage transformers to sense line voltage. A significant problem associated with the use of such transformers was the change in phase shift and the introduction of non-linearities that would occur over a wide voltage range. It was not easy to remove such a widely changing phase shift or to compensate for the non-linearities.  
           [0009]    Consequently, a need still exists for a single meter which is capable of metering electrical energy associated with nominal primary voltages that also minimizes phase shift in the voltage sensors over a wide voltage range.  
         SUMMARY OF THE INVENTION  
         [0010]    The present invention is directed to a power supply for an apparatus for metering at least one type of electrical power over a predetermined range of service voltages supplied by electrical service providers, where the apparatus comprises a voltage input circuit connected to receive a voltage component, and a processing unit. The power supply comprises a surge protection circuit which receives an input voltage, a rectifier circuit which receives an alternating current voltage from the surge protection circuit and outputs a rectified direct current voltage, a transformer which receives the rectified direct current voltage at a first winding so that current flows through the first winding, and a second winding defines an unregulated output voltage of the power supply, a switching device for permitting and preventing the flow of current through the first winding in response to a control signal, and a controller for generating the control signal based on the voltage across the third winding. The control signal output by the controller operates to disable the switching member.  
           [0011]    According to another feature of the present invention, the output of the power supply is input to a linear regulator, which outputs a regulated voltage. The regulated voltage is less than the output voltage, and the regulated voltage is output to a precision voltage reference generator. The unregulated voltage is input to the apparatus to determine the presence of a power fail condition.  
           [0012]    According to yet another feature, the power supply comprises a non-volatile supply, and the regulated voltage is input to the non-volatile supply, such that the apparatus is switched to the non-volatile supply when the regulated voltage is not present.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]    The present invention will be better understood, and its numerous objects and advantages will become apparent to those skilled in the art by reference to the following detailed description of the invention when taken in conjunction with the following drawings, in which:  
         [0014]    [0014]FIG. 1 is a block diagram of an electronic meter constructed in accordance with the present invention;  
         [0015]    [0015]FIG. 2 is a schematic diagram of the resistive dividers shown in FIG. 1;  
         [0016]    [0016]FIG. 3 is a schematic diagram of the linear power supply shown in FIG. 1;  
         [0017]    [0017]FIG. 4 is a block diagram of the power supply shown in FIG. 1;  
         [0018]    [0018]FIG. 5 is a schematic diagram of the control and switching members shown in FIG. 4;  
         [0019]    [0019]FIG. 6 is a schematic diagram of the startup/feedback shown in FIG. 4; and  
         [0020]    [0020]FIG. 7 is a schematic diagram of the voltage clamp shown in FIG. 4.  
     
    
     DETAILED DESCRIPTION  
       [0021]    A new and novel meter for metering electrical energy is shown in FIG. 1 and generally designated  10 . It is noted at the outset that this meter is constructed so that the future implementation of higher level metering functions can be supported.  
         [0022]    Meter  10  is shown to include three resistive voltage divider networks  12 A,  12 B,  12 C; a first processor—an ADC/DSP (analog-to-digital converter/digital signal processor) chip  14 ; a second processor—a microcontroller  16  which in the preferred embodiment is a Mitsubishi Model 50428 microcontroller; three current sensors  18 A,  18 B,  18 C; a 12V switching power supply  20  that is capable of receiving inputs in the range of 96-528V; a 5V linear power supply  22 ; a non-volatile power supply  24  that switches to a battery  26  when 5V supply  22  is inoperative; a 2.5V precision voltage reference  28 ; a liquid crystal display (LCD)  30 ; a 32.768 kHz oscillator  32 ; a 6.2208 MHz oscillator  34  that provides timing signals to chip  14  and whose signal is divided by 1.5 to provide a 4.1472 MHz clock signal to microcontroller  16 ; a 2 kbyte EEPROM  35 ; a serial communications line  36 ; an option connector  38 ; and an optical communications port  40  that may be used to read the meter. The inter-relationship and specific details of each of these components is set out more fully below.  
         [0023]    It will be appreciated that electrical energy has both voltage and current characteristics. In relation to meter  10  voltage signals are provided to resistive dividers  12 A- 12 C and current signals are induced in a current transformer (CT) and shunted. The output of CT/shunt combinations  18 A- 18 C is used to determine electrical energy.  
         [0024]    First processor  14  is connected to receive the voltage and current signals provided by dividers  12 A- 12 C and shunts  18 A- 18 C. As will be explained in greater detail below, processor  14  converts the voltage and current signals to voltage and current digital signals, determines electrical energy from the voltage and current digital signals and generates an energy signal representative of the electrical energy determination. Processor  14  will always generate a watthour delivered (Whr Del) and, watthour received (Whr Rec), depending on the type of energy being metered, will generate either a volt amp reactive hour delivered (Varhr Del)/a volt amp reactive hour received (Varhr Rec) signal or volt amp hour delivered (Vahr Del)/volt amp hour received (Vahr Rec) signal. In the preferred embodiment, each transition on conductors  42 - 48  (each logic transition) is representative of the measurement of a unit of energy. Second processor  16  is connected to first processor  14 . As will be explained in greater detail below, processor  16  receives the energy signal(s) and generates an indication signal representative of said energy signal.  
         [0025]    It will be noted again that meter  10  is a wide range meter capable of metering over a voltage range from 96-528V. The components which enhance such a wide range meter include the divider network  12 A- 12 C, which as previously noted are connected to receive the voltage component. The dividers generate a divided voltage, wherein the divided voltage is substantially linear voltage with minimal phase shift over the wide dynamic range, i.e. 96-528 Volts. A processing unit (processors  14  and  16 ) are connected to receive the divided voltage and the current component. The processing unit processes the divided voltages and the current components to determine electrical energy metering values. It will be appreciated from the following description that processors  14  and  16  require stable supply voltages to be operable. A power supply, connected to receive the voltage component and connected to processors  14  and  16 , generate the necessary supply voltages from the Phase A voltage component over the wide dynamic range. Power supply  20  could also run off of phase B and phase C voltages or a combination of the above. However, a combination embodiment would require additional protection and rectifying components.  
         [0026]    In relation to the preferred embodiment of meter  10 , currents and voltages are sensed using conventional current transformers (CT&#39;s) and resistive voltage dividers, respectively. The appropriate multiplication is accomplished in a new integrated circuit, i.e. processor  14 . Processor  14  is essentially a programmable digital signal processor (DSP) with built in multiple analog to digital (A/D) converters. The converters are capable of sampling multiple input channels simultaneously at 2400 Hz each with a resolution of 21 bits and then the integral DSP performs various calculations on the results. For a more detailed description of Processor  14 , reference is made to U.S. Pat. No. 5,555,508, which is incorporated herein by reference and which is owned by the same assignee as the present application.  
         [0027]    Meter  10  can be operated as either a demand meter or as a time-of-use (TOU) meter. It will be recognized that TOU meters are becoming increasingly popular due to the greater differentiation by which electrical energy is billed. For example, electrical energy metered during peak hours will be billed differently than electrical energy billed during non-peak hours. As will be explained in greater detail below, first processor  14  determines units of electrical energy while processor  16 , in the TOU mode, qualifies such energy units in relation to the time such units were determined, i.e. the season as well as the time of day.  
         [0028]    All indicators and test features are brought out through the face of meter  10 , either on LCD  30  or through optical communications port  40 . Power supply  20  for the electronics is a switching power supply feeding low voltage linear supply  22 . Such an approach allows a wide operating voltage range for meter  10 .  
         [0029]    In the preferred embodiment of the present invention, the so-called standard meter components and register electronics are for the first time all located on a single printed circuit board (not shown) defined as an electronics assembly. This electronics assembly houses power supplies  20 ,  22 ,  24  and  28 , resistive dividers  12 A- 12 C for all three phases, the shunt resistor portion of  18 A- 18 C, oscillator  34 , processor  14 , processor  16 , reset circuitry, EEPROM  35 , oscillator  32 , optical port components  40 , LCD  30 , and an option board interface  38 . When this assembly is used for demand metering, the billing data is stored in EEPROM  35 . This same assembly is used for TOU metering applications by merely utilizing battery  26  and reprogramming the configuration data in EEPROM  35 . The additional time-of use billing data is stored in the internal RAM of processor  16 , which RAM is backed by battery  26 .  
         [0030]    Consider now the various components of meter  10  in greater detail. Primary current being metered may be sensed using conventional current transformers. The shunt resistor portion of devices  18 A- 18 C are located on the electronics assembly.  
         [0031]    The phase voltages are brought directly to the electronic assembly where resistive dividers  12 A- 12 C scale these inputs to processor  14 . In the preferred embodiment, the electronic components are referenced to the vector sum of each line voltage for three wire delta systems and to earth ground for all other services. Resistive division is used to divide the input voltage so that a very linear voltage with minimal phase shift over a wide dynamic range can be obtained. This in combination with a switching power supply allows the wide voltage operating range to be implemented.  
         [0032]    Referring briefly to FIG. 2, each resistive divider consists of two 1 Meg, ½ watt resistors  50 / 52 ,  54 / 56  and  58 / 60 , respectively. Resistors  50 - 60  are used to drop the line voltage at an acceptable watt loss. Each resistor pair feeds a resistor  62 ,  64  and  66 , respectively. Resistors  62 - 66  are metal film resistors having a minimal temperature coefficient. This combination is very inexpensive compared to other voltage sensing techniques. Resistors  50 - 60  have an operating voltage rating of 300 Vrms each. These resistors have been individually tested with the 6 kV IEEE 587 impulse waveforms to assure that the resistance is stable and that the devices are not destroyed. Resistors  62 - 66  scales the input voltage to be less than 1 Volt peak to peak to processor  14 . Resistors  62 - 66  should be in the range of from about 100 ohms to about 1K ohms to assure this maximum voltage and maintain maximum signal.  
         [0033]    On grounded, three wire delta systems, those components of the electronics assembly operating on logic voltage levels (including the battery connector) can be at an elevated voltage. In such situations, the two, 1 Meg resistor combinations ( 50 / 52 ,  54 / 56 ,  58 / 60 ) provide current limiting to the logic level electronics. The worse case current occurs during testing of a 480 V, 3 wire delta meter with single phase excitation.  
         [0034]    It will be appreciated that energy units are calculated in processor  14  primarily from multiplication of voltage and current. The preferred embodiment of processor  14 , referenced above as being described in U.S. Pat. No. 5,555,508, includes three analog to digital converters. The necessity for three converters is primarily due to the absense of voltage transformers, present in prior meters.  
         [0035]    The M37428 microcontroller  16  is a 6502 (a traditional 8 bit microprocessor) derivative with an expanded instruction set for bit test and manipulation. This microcontroller includes substantial functionality including internal LCD drivers (128 quadraplexed segments), 8 kbytes of ROM, 384 bytes of RAM, a full duplex hardware UART, 5 timers, dual clock inputs (32.768 kHz and up to 8 MHz), and a low power operating mode.  
         [0036]    During normal operation, processor  16  receives the 4.1472 MHz clock from processor  14  as described above. Such a clock signal translates to a 1.0368 MHz cycle time. Upon power fail, processor  16  shifts to the 32.768 kHz crystal oscillator  32 . This allows low power operation with a cycle time of 16.384 kHz. During a power failure, processor  16  keeps track of time by counting seconds and rippling the time forward. Once processor  16  has rippled the time forward, a WIT instruction is executed which places the unit in a mode where only the 32.768 kHz oscillator and the timers are operational. While in this mode a timer is setup to “wake up” processor  16  every 32,768 cycles to count a second.  
         [0037]    Consider now the particulars of the power supplies shown in FIG. 1. As indicated previously, the off-line switching supply  20  is designed to operate over a 96-528 VAC input range. It connects directly to the Phase A voltage alternating current (AC) line and requires no line frequency transformer. A flyback converter serves as the basis of the circuit. A flyback converter is a type of switching power supply.  
         [0038]    As used herein, the “AC cycle” refers to the 60 Hz or 50 Hz input to power supply  20 . The “switching cycle” refers to the 50 kHz to 140 kHz frequency at which the switching transformer of power supply  20  operates. It will be noted that other switching cycle frequences can be used.  
         [0039]    Referring now to FIG. 4, power supply  20  for use in electronic meters includes a transformer  300  having primary and secondary windings. The input voltage (Phase A Voltage) is provided to the primary winding so that current may flow therethrough. As will be appreciated from FIG. 5, the secondary winding defines the output of the power supply. Referring back to FIG. 4, a switching member  302  is connected to the primary winding of transformer  300 . Switching member  302  permits and prevents the flow of current through the primary winding. Switch member  302  is operable in response to a control signal, which control signal is generated by control circuit  304 . Controller  304  generates the control signal in response to a limit signal generated by the start/feedback circuit  306  in response to the output of power supply  20 . Voltage clamp  308  serves to limit the voltage applied to transformer  300  and switch  302 . Surge protection circuit  309  is provided at the input to protect against surges appearing in the Phase A voltage.  
         [0040]    Referring now to FIG. 5, transformer  300  and switch  302  are shown in greater detail. It will be appreciated that switch  302  is a transistor. At the beginning of each switching cycle, transistor  302  “turns on”, i.e. becomes conductive, and magnetizes the core of transformer  300  by applying voltage across the primary  310 . At the end of each cycle, transistor  302  turns off and allows the energy stored in the core of transformer  300  to flow to the output of the power supply, which “output” can be generally defined by secondary  312 . Simultaneously, energy flows out of the bootstrap or tertiary winding  314  to power the control circuitry  304 .  
         [0041]    Feedback circuit  306  and controller  304  control the output of power supply  20  by varying the ON time of transistor  302 . Controller  304  will be described in greater detail in relation to FIG. 5. Transistor  302  is connected through inverter  316  to receive the output of an oscillator formed from inverters  318 ,  320  and  322 . It will be recognized that such inverters form a ring oscillator. The oscillator has a free-run frequency of 50 KHz. The ON time of transistor  302  may vary between 200 ns and 10 μs. The OFF time is always between 8 and 10 μs. During operation, the bootstrap winding  314  of transformer  300  (pins  10  and  11 ) powers controller  304 , but this power is not available until the power supply has started. The control circuit is a current-mode regulator.  
         [0042]    At the beginning of a switching cycle, transistor  302  is turned ON by the oscillator output. If left alone, transistor  302  would also be turned OFF by the oscillator output. Transistor  302  remains ON until the current in primary  310  of transformer  300  (pins  8  and  13 ) ramps up to the threshold current level I th  represented as a voltage V th . As will be explained below, V th  is generated by feedback circuit  306 . When the primary current of transformer  300 , represented as a voltage V t  and sensed by resistor  326 , ramps up to the threshold level V th , pin  1  of comparator  324  terminates the ON period of the oscillator by forcing the oscillator output HIGH, which output in turn is inverted by inverter  316 , shutting OFF transistor  302 . Transistor  302  then turns OFF until the next switching cycle. Since the V th  indirectly controls the ON time of transistor  302 , controller  304  regulates the output voltage of the power supply by comparing the sensed current in transformer  300  to this threshold level.  
         [0043]    Transistor  362  and pin  7  of comparator  326  can disable the oscillator. Transistor  362 , described in greater detail in FIG. 7, disables the oscillator when the line voltage exceeds 400 volts. Comparator  328  disables the oscillator when the controller  304  has insufficient voltage to properly drive transistor  302 . The voltage in controller  304 , V c , will be described in relation to FIG. 5.  
         [0044]    Consider now feedback circuit  306 , shown in FIG. 6. When connected to the Phase A Voltage, resistor  330  slowly charges capacitor  332 . The high value of resistor  330  and the  400  volt limit by voltage clamp  308  limit the power dissipation of resistor  330 . After a few seconds, capacitor  332  charges above 13 volts. Transistors  334  and  336  then provide positive feedback to each other and snap ON. Controller  304  can run for tens of milliseconds from the charge stored in capacitor  332 . Normally, power supply  20  will successfully start and begin to power itself in this period. If it fails to start, transistors  334  and  336  turn OFF when the charge across capacitor  332  drops below 8.5 volts and capacitor  332  again charges through resistor  330 . This cycle repeats until the supply starts.  
         [0045]    With high input voltages and without resistor  338  (FIG. 5), the current sourced by resistor  330  can hold the control and start-up circuits in a disabled state that does not recycle. When Capacitor  332  drops below 8.5 volts, resistor  338  places a load on the control circuit supply. This load insures that the start-up circuit recycles properly with high input voltages.  
         [0046]    As indicated above, when the primary current of transformer  300  sensed by resistor  326  ramps up to the threshold level V th , pin  1  of comparator  324  can terminate the ON period of the oscillator. When the voltage on capacitor  332  is less than 13 volts, zener diode  340  provides no voltage feedback. Under these conditions, the base-emitter voltage of transistor  336  sets the current threshold I th  to about 650 mA. This maximum current limit protects transistor  302 , as well as those transistors in voltage clamp  306 , and prevents transformer  300  from saturating.  
         [0047]    As the voltage on capacitor  332 , which is representative of the output voltage of the supply, approaches the proper level, zener diode  340  begins to conduct and effectively reduces the current threshold, i.e. effectively reduces V th . Each switching cycle will then transfers less power to the output, and the supply begins to regulate its output.  
         [0048]    When the regulating circuitry requires ON times of transistor  302  less than about 400 ns, the current sense circuitry does not have time to react to the primary current of transformer  300 . In that case, the regulating circuit operates as a voltage-mode pulse width modulator. Resistor  342  (FIG. 5) generates a negative step at pin  3  of comparator  324  at the beginning of each switching cycle. The regulator feedback voltage at pin  2  of comparator  324 , which contains little current information at the beginning of each switching cycle, translates the step at pin  3  into various input overdrives of comparator  324 , thereby driving the output of comparator  324  to a logic HIGH level. The propagation time of the comparator  324  decreases with increasing overdrive, i.e. as the negative step increases, and the circuit acts as a pulse width modulator. The negative step will increase due to the changing level of V th .  
         [0049]    Any leakage inductance between the bootstrap winding (pins  10  and  11  of transformer  300 ) and the output winding (pins  3  and  4  of transformer  300 ) causes inaccurate tracking between the voltage on capacitor  332  and the output voltage of the supply. This leakage inductance can cause poor load regulation of the supply. The bootstrap and output windings are bifilar wound; they are tightly coupled, have little leakage inductance, and provide acceptable load regulation. Since the two windings are in direct contact, the bootstrap winding requires Teflon insulation to meet the isolation voltage specifications. A 100% hi-pot test during manufacture insures the integrity of the insulation.  
         [0050]    Consider now the details of voltage clamp  308 , shown in FIG. 7. A 528 VAC input corresponds to 750 VDC after rectification. Switching transistors that can directly handle these voltages are extremely expensive. By using the voltage clamp of the present invention, relatively inexpensive switching transistors can be utilized.  
         [0051]    In power supply  20 , the switching member  302  is shut down during parts of the AC cycle that exceed 400 volts. The switching transistor, transistor  302 , in conjunction with two other transistors  344  and  346 , can hold off 750 VDC. During surge conditions, these three transistors can withstand over 1500 volts. In the preferred embodiment, transistors  302 ,  344  and  346  are 600-volt MOSFETs.  
         [0052]    Because high-voltage electrolytic capacitors are expensive and large, this voltage clamp  308  has no bulk filter capacitor after the bridge rectifier  348 . Without a bulk filter capacitor, this switching converter must shut down during parts of the AC cycle. It intentionally shuts down during parts of the AC cycle that exceed 400 volts, and no input power is available when the AC cycle crosses zero. The 2200 μF output capacitor  350  (FIG. 5), provides output current during these periods.  
         [0053]    As discussed above, transistors  344  and  346  act as a voltage clamp and limit the voltage applied to switching member  302 . At a 528 VAC line voltage, the input to the clamping circuit reaches 750 volts. During lightning-strike surges, this voltage may approach 1500 volts. When the voltage at the output of bridge rectifier  348  exceeds 400 volts, zener diodes  352  and  354  begin to conduct. These diodes, along with the 33 KΩ resistors  356 ,  358  and  360 , create bias voltages for transistors  344  and  346 . Transistors  344  and  346  act as source followers and maintain their source voltages a few volts below their gate voltages.  
         [0054]    If, for example, the output of bridge rectifier  348  is at 1000 volts, the gates of transistors  344  and  346  will be at approximately 400 and 700 volts respectively. The source of transistor  344  applies roughly 700 volts to the drain of  346 ; the source of  346  feeds about 400 volts to switching member  302 . Transistors  344  and  346  each drop  300  volts under these conditions and thereby share the drop from the 1000 volt input to the 400 volt output, a level which the switching converter  302  can withstand.  
         [0055]    As zener diodes  352  and  354  begin to conduct and as transistors  344  and  346  begin to clamp, transistor  362  turns ON and shuts down the switching converter. Although transistors  344  and  346  limit the voltage fed to the converter to an acceptable level, they would dissipate an excessive amount of heat if the switching converter  302  consumed power during the clamping period.  
         [0056]    When switching converter  302  shuts down, transistor  302  no longer has to withstand the flyback voltage from transformer  300 . Resistor  364  takes advantage of this by allowing the output voltage of the clamp to approach 500 volts (instead of 400 volts) as the input to the clamp approaches 1500 volts. This removes some of the burden from transistors  344  and  346 .  
         [0057]    Zener diodes  352  and  354  are off and the converter  302  runs when the output of bridge rectifier  348  is below 400 volts. During these parts of the AC cycle, the 33 KΩ resistors  356 ,  358  and  360  directly bias the gates of transistors  344  and  346 . The voltage drop across transistors  344  and  346  is then slightly more than the threshold voltages of those transistors along with any voltage drop generated by the channel resistance of those transistors.  
         [0058]    During the off time of transistor  302 , about 10 μS, the 33 KΩ resistors can no longer bias the gates of transistors  344  and  346 . Diode  366  prevents the gate capacitance of transistors  344  and  346  and the junction capacitance of zeners  368  and  370  from discharging when transistor  302  is off. This keeps transistors  344  and  346  ON and ready to conduct when transistor  302  turns ON at the next switching cycle. If the gates of transistors  344  and  346  had discharged between switching cycles, they would create large voltage drops and power losses during the time required to recharge their gates through the 33 KΩ resistors.  
         [0059]    In the preferred embodiment, two 33 KΩ resistors are used in series to obtain the necessary voltage capability from 966 surface-mount packages.  
         [0060]    This power supply must withstand an 8 KV, 1.2×50 μS short-branch test. Varistor  372 , resistors  374 ,  376  and  378 , and capacitor  380  protect the power supply from lightning strike surges.  
         [0061]    A 550 VAC varistor  372  serves as the basis of the protection circuit. It has the lowest standard voltage that can handle a 528 VAC input. The device has a maximum clamping voltage of 1500 volts at 50 amps.  
         [0062]    A varistor placed directly across an AC line is subject to extremely high surge currents and may not protect the circuit effectively. High surge currents can degrade the varistor and ultimately lead to catastrophic failure of the device. Input resistors  374  and  376  limit the surge currents to 35 amps. This insures that the clamping voltage remains below 1500 volts and extends the life of the varistor to tens of thousands of strikes.  
         [0063]    Resistor  378  and capacitor  380  act as an RC filter. The filter limits the rate of voltage rise at the output of the bridge rectifier. The voltage clamping circuit, transistors  344  and  346 , is able to track this reduced dv/dt. Current forced through diodes  382 ,  384  and capacitor  386  (FIG. 5) is also controlled by the limited rate of voltage rise.  
         [0064]    Resistors  374  and  376  are 1 watt carbon composition resistors. These resistors can withstand the surge energies and voltages. Resistor  378  is a flame-proof resistor that acts as a fuse in the event of a failure in the remainder of the circuit.  
         [0065]    The values of resistors  374 ,  376  and  378  are low enough so that they do not interfere with the operation of the power supply or dissipate excessive amounts of power.  
         [0066]    Finally it is noted that resistors  388  and  390  act to generate the power fail voltage PF.  
         [0067]    By using the wide voltage ranging of the invention, a single meter can be used in both a four wire wye application as well as in a four wire delta application. It will be recognized that a four wire delta application includes 96V sources as well as a 208V source. In the past such an application required a unique meter in order to accomodate the 208V source. Now all sources can be metered using the same meter used in a four wire wye application.  
         [0068]    While the invention has been described and illustrated with reference to specific embodiments, those skilled in the art will recognize that modification and variations may be made without departing from the principles of the invention as described herein above and set forth in the following claims.