Abstract:
A power meter has a measuring circuit composed entirely of analog components that measures the instantaneous current draw of one or more electrical devices, such as home appliances. The power meter may be in the form of a hand-held device that includes elongated connections that allow a homeowner to measure the current draw of an electrical device without significant displacement of the electrical device. In addition to measuring current draw, the power meter may provide an estimated wattage consumed by the measured electrical devices.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
   The present application is a continuation application of U.S. Ser. No. 11/844,813, filed Aug. 24, 2007, which claims the benefit of U.S. Ser. No. 60/823,588, filed Aug. 25, 2006, the disclosure of which incorporated herein by reference. 

   BACKGROUND OF THE INVENTION 
   The present invention relates to power meters and, more particularly, to a power meter to which the collective current draw of multiple loads is measured using a measuring circuit composed entirely of analog components. 
   Whether it be as a matter of general interest or to determine the size needed for an auxiliary power supply, such as an electric generator, homeowners are increasingly interested in the energy use associated with the electric load of the home and, more particularly, of selected electrical loads. The total electrical load of a home may be determined through monitoring the utility power meter typically located exteriorly of the home. However, determining the electrical load, i.e., current draw, of selected electrical loads is more difficult. 
   One approach is to electrically isolate all loads except those to be tested and then visually inspect the utility power meter. A drawback of such an approach is that all loads not to be measured, of which there may be dozens for a single home, must be electrically isolated. Alternately, the circuit breakers for the circuit branches loaded by the electrical devices not to be measured may be thrown OFF. In the case of the latter, the homeowner would still be required to electrically isolate loads of a given circuit branch if other electrical devices that load the given circuit are to be measured. Regardless of which approach is taken, it can be time-consuming to electrically isolate the appropriate electrical devices. Moreover, if the homeowner wanted to measure the energy usage over time, the electrical isolation would be required throughout the measurement interval, which may be undesirable and impractical. 
   As such, a number of in-line power meters have been designed that allow current to be supplied to an electrical device through the power meter itself when interconnected between a wall outlet and the electrical device. These devices typically include rather complex digital circuits that measure various electrical parameters associated with the energy usage of an electrical device, such as an appliance. These parameters include instantaneous values, such as instantaneous current, as well as time-based values, such as average current. Moreover, some power meters may be programmed to include cost information associated with energy usage so that a homeowner can monitor the cost of the energy usage of a given electrical device. 
   These conventional power meters, which can be quite costly to the consumer because of the complexity of the measuring circuitry and the functionality provided, are designed to be wall-connected devices. That is, the power meter will include a back-mounted electrical plug that plugs into a conventional wall outlet. The power meter will also include a single front mounted outlet into which the electrical plug of an electrical device to be measured can be plugged. Outside their cost, these conventional power meters are generally practical if the outlet to which the power meter is connected is accessible and viewable. 
   For example, most homeowners connect the power cord of a refrigerator to a wall outlet that is positioned behind the refrigerator. Thus, to make use of a conventional power meter, the homeowner would be required to wheel the refrigerator away from the wall a sufficient distance for the homeowner to connect the power meter. To be able to read the power meter, the homeowner would also be required to wheel the refrigerator out a sufficient distance so that the display panel of the power meter could be viewed. For a typical refrigerator, this may require that the homeowner wheel the refrigerator several feet away from the wall outlet. To measure the energy usage over time and be able to visually read the display panel, the homeowner would be need to keep the refrigerator wheeled away from the wall outlet or wheel the refrigerator away each time the homeowner desired to read the power meter. 
   Another drawback of conventional power meters is that a single outlet is provided for connecting a single electrical device to the power meter. To measure the electrical usage of multiple electrical devices requires the homeowner to connect each of the electrical devices to a multi-outlet receptacle and then connect multiple electrical devices to the multi-outlet receptacle. The power meter may measure the collective energy usage, but the power meter would have to be positioned in sufficient proximity to all of the electrical devices to be measured. Alternately, multiple extension cords could be used to link the electrical devices to the wall-mounted power meter with the additional cost and disruption associated with multiple extension cords strewn about the home. 
   BRIEF DESCRIPTION OF THE INVENTION 
   The present invention is directed to a power meter having a measuring circuit composed entirely of analog components that measures the instantaneous current draw of one or more electrical devices, such as home appliances. The power meter may be in the form of a hand-held device, which includes elongated connections that allow a homeowner to measure and monitor a value of the current draw of an electrical device without significant displacement of the electrical device. In addition to measuring current draw, the power meter may measure an estimated wattage consumed by the measured electrical devices. Since the measuring circuit lacks any digital circuitry, such as microprocessors, the power meter may be light, small, and highly portable while providing a cost savings relative to predecessor digital-based power meters. 
   In one aspect of the invention, a power meter includes a plug insertable into a conventional outlet and a receptacle adapted to receive one or more plugs extending from various electrical appliances or other loads. Between the plug and the receptacles, the power meter includes an analog measuring circuit that can modify the electrical signal passing through the measuring device to determine the amperage drawn or watts consumed by the electrical device(s) connected to the power meter. This measured or determined value for the particular electrical parameter is then displayed by the power meter in a manner that can be readily viewed by a homeowner or other user. 
   In accordance with a further aspect, the invention contemplates a power meter that includes a housing and a current measuring circuit disposed in the housing. The power meter further includes a first cord extending from the housing and electrically connected to the measuring circuit. The first cord has a plug adapted to engage a conventional electrical outlet. The power meter further includes a receptacle formed in the housing and electrically connected to the current measuring circuit, wherein the receptacle is adapted to receive a conventional three-prong plug of an electrical device. 
   According to another aspect of the invention, the present invention contemplates a power meter that includes a plug adapted to be engaged with a conventional electrical outlet and an outlet adapted to receive a plug of an electrical device. A measuring circuit composed entirely of analog components and electrically interconnected between the plug and the outlet is adapted to measure an instantaneous current draw of the electrical device. 
   Numerous other aspects, features, and advantages of the present invention will be made apparent from the following detailed description together with the drawing figures. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The drawings illustrate the best mode currently contemplated of practicing the present invention. 
     In the drawings: 
       FIG. 1  is a rear perspective view of a plug-in power meter constructed according to one embodiment the present invention; 
       FIG. 2  is a front perspective view of the power meter of  FIG. 1 ; 
       FIG. 3  is a side perspective view of the power meter of  FIG. 1 ; 
       FIG. 4  is a circuit diagram of the power meter of  FIG. 1 ; 
       FIG. 5  is an elevation view of another embodiment of a power meter according to the present invention; and 
       FIG. 6  is an end view of a portion of the power meter shown in  FIG. 5 . 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   With regard now to the drawing figures in which like reference numerals designate like parts throughout the disclosure, a plug-in power meter constructed according to a first embodiment of the present invention is indicated generally at  10  in  FIG. 1 . The power meter  10  includes a plug  12  disposed at one end that may be engaged with a conventional electrical outlet (not shown), and a cord  14  extending outwardly from the plug  12 . The cord  14  contains conductive wires (not shown) that enable electrical power to be supplied from the outlet to which the plug  12  is connected, through the cord  14  to a receptacle  16  attached to the cord  14  opposite the plug  12 . The receptacle  16  includes a number of openings  18  in a configuration similar to that of a conventional outlet, such that a plug (not shown) connected to a suitable electric device or appliance (not shown), such as a furnace, air conditioner, sump pump, microwave oven, refrigerator, freezer, toaster, coffee maker, computer, radio, and the like, can be inserted into the openings  18  in order to connect the device or appliance to the outlet using the power meter  10 , such that the device or appliance can be operated on power supplied through the power meter  10 . 
   The receptacle  16  also includes a housing  20  in which the openings  18  are formed. The housing  20  can include an extension  21  projecting outwardly from the housing  20  and in which the openings  18  are disposed. The openings  18  are electrically connected to a measuring circuit  24  (best shown in  FIG. 4 ) such that, when power is supplied to the device or appliance through engagement of the plug within the openings  18 , the electrical power passes through the measuring circuit  24  for measurement of the desired electrical parameter. 
   As best shown in  FIG. 2 , the housing  20  also includes a display  22  generally opposite the openings  18 . The display  22  is also operably connected to the measuring circuit  24  such that the value for an electrical parameter as measured or determined by the circuit  24  can be represented on the display  22 . As best shown in  FIG. 3 , a switch  26  is located on the housing  20 , preferably on one side of the housing  20  between the openings  18  and the display  22 . The switch  26  is also operably connected to the measuring circuit  24  and selectively changes the mode of operation of the circuit  24 , such that the circuit  24  can measure and display one of two electrical parameters that can be determined by the measuring circuit  24 . For example, in one embodiment, the switch  26  toggles the power meter between measuring current (amperage) draw and power (wattage) consumption of the connected electrical devices. 
   Referring now to  FIG. 4 , an exemplary circuit configuration  24  for the power meter  10  includes an amperage measuring circuit  28  that is connected to the power source or outlet via the plug  12 , and to the electrical device that is to be tested via the openings  18 . The amperage drawn by the appliance when operated is measured in an analog fashion by an amplifier  30  and resistors  32 - 38 . The measured or determined value for the amperage is then directed to an analog-to-digital converter  40  that functions only as a driver for the display  22 . That is, the converter  40  does not form part of the amperage/wattage measuring circuit  28 . The signal received by the converter  40  is converted into a format that can readily be represented on the display  22  to illustrate the amperage or wattage value of the device or appliance plugged into the power meter  10 . When the switch  26  is in the proper position to indicate that the value for the amperage drawn by the appliance is to be represented, this value is illustrated in a readable manner on the display  22 . 
   Alternatively, when the switch  26  is moved to the position indicating that the value for the watts consumed by the electrical device is to be represented on the display  22 , the measuring circuit  24  arrives at this value by altering, or multiplying the measured or determined amperage value by a factor of 120 to provide an estimated value for the watts being drawn by the device or appliance being tested. More particularly, when switch  26  is moved to the “WATT” position, the output of the amplifier  30  is presented across a feedback loop comprised generally of resistors  34 ,  36 . This causes the gain of the amplifier  30  to be changed from a unity gain to a gain factor of 120. In other words, the measured current draw is multiplied by the amplifier by 120 to provide an estimated power consumption for the connected electrical devices. One skilled in the art will appreciate that the gain factor of the amplifier may be varied by selecting a resistor  36  with different impedance. Preferably, the value of resistors  32 ,  34  should be equal so that the amplifier  30  has a unity gain when switch  26  is the “AMP” position. 
   One skilled in the art will appreciate that the exemplary circuit shown in  FIG. 4  includes additional electronic components that provide line buffering and conditioning not specifically described herein. Additionally, one skilled in the art will appreciate that other circuit configurations using electronic components other than those shown in the circuit are possible and considered within the scope of the present invention. 
   In a second embodiment on the present invention as shown in  FIGS. 5-6 , in which like components are designated with primed reference characters, the power meter  10 ′ includes the plug  12 ′, the cord  14 ′, and the housing  20 ′ with the display  22 ′ and the switch  26 ′ as in the previous embodiment. The cord  14 ′ is preferably formed to have a length similar to that for a normal extension cord. However, the power meter  10 ′ also includes a second cord  42 ′ that extends away from the housing  20 ′ opposite the cord  12 ′ to a receptacle  16 ′. It is understood that either of the cords  14 ′ or  42 ′ may have any desired length. The receptacle  16 ′ includes multiple sets of openings  18 ′ in the configuration of a conventional electrical outlet, such that multiple plugs from multiple electric devices or appliances can be simultaneously connected to the receptacle  16 ′ for determining a combined amperage or wattage for all of the appliances when in operation. Additionally, the receptacle  16 ′ can include a selector device (not shown) that enables the power meter  10 ′ to determine the amperage or watt value for any combination of the appliances connected to the receptacle  16 ′. 
   Various additional embodiments of the present invention are contemplated as being within the scope of the following claims particularly pointing out and distinctly claiming the subject matter regarded as the invention.