Replaceable resistors for calibrating a watthour meter

A solid-state watthour meter for registering AC electrical energy consumption by a load connected to a source of AC load current and AC line voltage includes a voltage sensing transducer for producing a signal corresponding to the line voltage and a current sensing transducer for producing a signal corresponding to the load current. A solid-state measurement circuit receives the voltage and current signals as its inputs and produces an output signal corresponding to the AC electrical energy consumption of the load. A power supply, including a regulator circuit having a calibration resistor, provides a DC voltage to the measurement circuit. The DC voltage across the regulator circuit is dependent on the impedance of the calibration resistor. The measurement circuit includes apparatus for relating the registration of the meter to the DC supply voltage. The calibration resistor is readily removably electrically connected in the regulator circuit. Replacement of the calibration resistor causes a change in the supply voltage to the measurement circuit which, in turn, causes a change in the registration of the watthour meter.

BACKGROUND OF THE INVENTION 
This invention relates to the field of solid state watthour meters, and 
more particularly to the calibration of the registration of these meters. 
In solid state watthour meters, the registration of the meter is indicative 
of the product of the meter reading and the watthour constant, 
corresponding to the energy measurement of the meter. Percent registration 
is the ratio of the actual registration of the meter to the true value of 
the quantity measured in a given time, expressed as a percentage. Percent 
error is the difference between its percent registration and one hundred 
percent. A low percent error registration is important to ensure accurate 
measurement of the AC electrical energy consumed by a customer and proper 
calculation of the charge for the electrical service. 
In many prior art meters, changes in registration are accomplished by 
altering the rotational rate of the meter disk. In solid state watthour 
meters, calibration has been achieved using a potentiometer to, in 
essence, vary the watthour constant for the meter. However, potentiometers 
are typically expensive and susceptible to reliability and stability 
problems. 
In view of these difficulties, it is an object of the present invention to 
provide means for calibrating the registration of a watthour meter that is 
inexpensive, yet reliable. Another object is to provide a means for 
calibration that is stable, particularly in the harsh temperature 
environment of the watthour meter. An additional object is to provide a 
means for registration that is readily and easily accessible for the meter 
technician or customer. Other objects and benefits of the present 
invention will be observed in the following disclosure and accompanying 
figures. 
SUMMARY OF THE INVENTION 
A solid-state watthour meter for registering AC electrical energy 
consumption by a load connected to a source of AC load current and AC line 
voltage includes a voltage sensing transducer for producing a signal 
corresponding to the line voltage and a current sensing transducer for 
producing a signal corresponding to the load current. A solid-state 
measurement circuit receives the voltage and current signals as its inputs 
and produces an output signal corresponding to the AC electrical energy 
consumption of the load. A power supply, including a regulator circuit 
having a calibration resistor, provides a DC voltage to the measurement 
circuit. The DC voltage across the regulator circuit is dependent on the 
impedance of the calibration resistor. The measurement circuit includes 
means for relating the registration of the meter to the DC supply voltage. 
The calibration resistor is readily removably electrically connected in 
the regulator circuit. Replacement of the calibration resistor causes a 
change in the supply voltage to the measurement circuit which, in turn, 
causes a change in the registration of the watthour meter.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
For the purposes of promoting an understanding of the principles of the 
invention, reference will now be made to the embodiment illustrated in the 
drawings and specific language will be used to describe the same. It will 
nevertheless be understood that no limitation of the scope of the 
invention is thereby intended, such alterations and further modifications 
in the illustrated device, and such further applications of the principles 
of the invention as illustrated therein being contemplated as would 
normally occur to one skilled in the art to which the invention relates. 
In practicing the present invention, the electrical system in which AC 
energy consumption is being measured may be of any type including, for 
example, a conventional 60 Hz power distribution system. Further, the 
system can be a single phase or a polyphase system. In the preferred 
embodiment of the invention, the watthour meter is capable of measuring 
power consumption in a three-phase Wye or Delta configured system. 
Referring to the block diagram of FIG. 1, the present watthour meter 
includes a voltage transducer 9 and a current transducer 10 which are 
electrically connected across an AC source 5. Line voltage and load 
currents drawn by the load are reduced to a usable level by voltage 
transducer 9 and current transducer 10, respectively. This may be 
accomplished by separate circuitry for each phase. The resulting signals 
are applied to a solid-state measurement circuit 11. The measurement 
circuit 11 may include a number of oscillator and modulator networks for 
calculating and summing the power in each phase. Output signals are 
conveyed from measurement circuit 11 to register 12 which represent the 
energy consumed by the load. The output signals from measurement circuit 
11 are a function of the registration of the watthour meter. Register 12 
records and displays the amount of energy delivered to the load across 
which the metering unit is connected. A power supply network 13 provides 
regulated power to the measurement circuit 11 and the register 12 from a 
potential transformer T1, which derives its power from AC source 5. The 
registration of the meter is a function of the supply voltage to the 
measurement circuit 11. 
Referring now to FIG. 2, there is shown a registration circuit which 
provides DC voltage to the solid-state measurement circuit 11 described 
above. The potential of the AC source 5 is reduced at the secondary coil 
50 of the potential transformer T1. The ends of the secondary coil 50 are 
connected across a pair of terminals 70 and 71 of a full wave rectifier 
51, which is in the form of a conventional diode bridge rectifier. Between 
secondary coil 50 and rectifier 51 are a series pair of overvoltage 
protection capacitors 52, which are connected in parallel with the 
secondary windings 50. Parallel capacitors 53, 54, and 55 are connected 
across rectifier terminals 72 and 73. Rectifier 51 and capacitors 53, 54, 
and 55 convert the AC signal supplied by transformer T1 to a DC signal 
usable by the measurement circuit 11. 
An amplifier network includes transistor 30 coupled at its collector to 
terminal 73 of rectifier 51. Resistor 57 connects from terminal 73 to the 
base of transistor 30 and the collector of transistor 29. The base of 
transistor 29 is coupled to terminal 73 across resistor 56. The amplifier 
network is connected to the output of an operational amplifier 23 through 
a parallel network comprising a capacitor 59 and a zener diode 58. In the 
preferred embodiment, diode 58 is of the type 1N750A, 4.7V. The op-amp 23 
is driven by the difference in the negative line voltage at terminal 72 of 
rectifier 51, referred to as V.sub.ss, and the potential at the emitter of 
transistor 29, referred to as V.sub.dd. This difference (V.sub.ss 
-V.sub.dd) is the supply voltage to measurement circuit 11. 
A series of resistors 25, 26, 27, and 28 are connected between the 
potentials V.sub.dd and V.sub.ss. The non-inverting input of op-amp 23 is 
connected at the junction between resistors 26 and 27. Potential V.sub.dd 
is applied to the inverting input of op-amp 23 through a network of 
parallel resistors 20 and 21. Also connected to the inverting input of 
op-amp 23 is a resistor 24. A zener diode 22 is connected between resistor 
24 and V.sub.dd. In the preferred embodiment, diode 22 is of the type 
LM129, 6.9V. A resistor 60 is coupled between resistor 24 and V.sub.ss. In 
parallel connection with resistor network 25, 26, 27, and 28 is a pair of 
capacitors 61 and 62, and a pair of capacitors 63 and 64. 
The value of the supply voltage and, correspondingly, the registration of 
the meter, can be changed by varying the value of either one or both of 
resistors 20 and 21. In the preferred embodiment of the present invention, 
resistor 20 is considered the coarse calibration resistor and resistor 21 
the fine calibration resistor. For the present watthour meter, the 
relationship between the value of resistor 20 and 21 and the meter 
registration is determined by the following proportionality: 
##EQU1## 
Thus, changing resistors 20 and 21 selected according to the above 
equation will increase or decrease the supply voltage to the measurement 
circuit 11 to achieve a specific registration. The resistors 20 or 21 are 
preferably Mil Type RN55C, having a resistance tolerance of .+-.1% and a 
temperature coefficient of 50 PPM/.degree. C. to ensure long-term 
stability over the life of the meter. These resistors are normally 
available from commercial electronics distributors in resistance 
increments of 2%. 
FIGS. 3 and 4 show the placement of certain components of the watthour 
meter on a printed circuit board 80. Solid state measurement circuit 11, 
power supply op-amp 23, transistor 29, diode 22, and resistors 25, 26, 27, 
and 28 are positioned at the front of the circuit board 80. Potential 
transformer T1 is mounted on the underside of circuit board 80. In another 
aspect of the present invention, calibration resistors 20 and 21 are 
mounted near the edge of circuit board 80 on two pairs of bifurcated posts 
82 which are connected to the front side of the circuit board 80. This 
manner of mounting resistors 20 and 21 makes the resistors readily 
accessible for easy replacement in the event that the meter registration 
drifts. 
The resistors 20 and 21 are very inexpensive, particularly relative to the 
potentiometers employed in prior art devices. Moreover, the resistors are 
more reliable than a potentiometer because there are no moving parts to 
contend with. The thermal coefficient of the resistors of the preferred 
embodiment is sufficiently high to ensure long term stability of the 
resistor impedance, even in the watthour meter thermal environment. 
Finally, the connection of the resistors using bifurcated posts makes 
replacement of the resistors easy and reliable. 
While the invention has been illustrated and described in detail in the 
drawings and foregoing description, the same is to be considered as 
illustrative and not restrictive in character, it being understood that 
only the preferred embodiment has been shown and described and that all 
changes and modifications that come within the spirit of the invention are 
desired to be protected.