design energy meter with pic

AN939Designing Energy Meters with the PIC16F873A

INTRODUCTION

The deployment of electronic energy meters hasgained a great deal of momentum over the past severalyears. This is due to their two main advantages overthe traditional electromechanical designs: improvedaccuracy and an expanded set of features. Currentmicrocontroller technology allows designers to buildmeters that are competitive in price with traditionaldevices, while maintaining the required IEC 1036Class 1 accuracy of ±1% for domestic applications.Microcontrollers also allow the easy incorporation ofadded features, such as rms voltage and current andpeak demand metering, as local electric utilitycompanies desire to implement them.

In this application note, we will discuss the implementa-tion of a basic watthour meter using PICmicro® Flashmicrocontrollers. In the process, we will show how oneADC with a single sample-and-hold circuit can effectivelymeasure both voltage and load current and maintainClass 1 accuracy. The firmware discussed measures anddisplays rms voltage and current, as well as kWh,presented in a clear digital format on an LCD.

Besides basic energy measurement, this design alsoincludes features that many electric utilities are veryinterested in rolling out on a wider basis. Dual-channelmeasurement provides a simple method for monitoringfor tamper conditions. An on-board RTC provides atime source for calculating and tracking current and his-torical peak demand. All metered data is securelystored as it is updated in nonvolatile memory.

The design discussed here uses the PIC16F873A andtwo Current Transformers (CTs) for current sensing. Itcan be implemented just as easily with the pin compati-ble PIC18F2320. Current measurement using a shuntmay also be used in this design, with little or no changeto the current amplifier design.

PRINCIPLES OF MEASUREMENT

Basically, a watthour meter is designed to measureenergy or power consumed over time. In simple terms,electrical power is the product of voltage and current. Ifwe make repeated measurements of both instanta-neous voltage and current, or Vi and Ii, we can keep arunning total of their products over time. By dividing thetotal accumulated energy over the number of samples,we have the average power (the first expression inEquation 1). Multiplying the average power by timegives the total energy consumed.

EQUATION 1: CALCULATING AVERAGE ENERGY AND CONSUMED POWER

For alternating current, such as that from the mains,average power also must account for power factor,which is the phase relationship between voltage andcurrent. In simple terms, average AC power is V I cosθ,where V and I are average rms voltage and current,and θ is the phase angle between the two. Instanta-neous sampling does not directly use power factor; thevalue of the phase angle is essentially embedded in theinstantaneous current measurement. Recovering theactual phase angle for the purpose of calculating anddisplaying the power factor can be done separately andis very calculation intensive. If we are just measuringenergy consumption, it is not necessary.

Author: Sandip ChattopadhyayMicrochip Technology Inc.

Average Power(watts) =

N

Σ Vi • Iik = 1

N

⎛⎜⎝ k k

⎞⎟⎠

Energy Consumed(wattseconds) =

N

Σ Vi • Iik = 1

Fs

⎛⎜⎝ k k

⎞⎟⎠

© 2005 Microchip Technology Inc. DS00939A-page 1

AN939

To get an accurate picture of power consumption for anAC system, we need to make frequent measurements,preferably many times that of the supply frequency. Inthis application, we use a sampling rate of 400 Hz,which provides 8 samples per full cycle of the linefrequency (for AC supply frequency of 50 Hz). For asampling rate Fs, we get N samples in N/Fs seconds. Bymultiplying this expression for time by average power,we obtain an expression for energy consumed in termsof wattseconds (the second expression in Equation 1).From here, we can use simple math to calculatekilowatthours.

Of course, it is difficult for a microcontroller to makedirect measurements when the supply voltage iscoming straight off the mains: say, 230V at up to 50A.This makes it necessary to indirectly measure linevoltage and current at a level consistent with a micro-controller, then rescale these measurements to arriveat the original value. The best way to do this is toreduce the voltage to a level and dynamic range that iscompatible with digital circuitry. (Measuring currenthere is essentially the same as measuring voltage, inthat we will use a transducer that generates a voltageproportional to the load current.) The actual voltageand current readings can then be derived.

For this application, the derived voltage reading, Vd, isrelated to the actual instantaneous line voltage Vi bythe expression, Vd = Vi Kd/Kv or Vi = Vd Kv/Kd, where Kdis the digitization constant for the ADC in this applica-tion and Kv is the voltage proportionality constant forthe circuit design. For this particular application, Kd is204.6, the digital value from the ADC that represents1V. Kv is the factor by which the input line voltage isreduced by a voltage divider; in our design, it is 300.

Similarly, the derived current reading, Id, is related to Iiby the expression, Id = Ii Kd/Ki or Ii = Id Ki/Kd, where Kiis the current proportionality constant specific to thisdesign; it is calculated by dividing the CT turn ratio bythe product of the current amplifier gain and the inputburden resistance. For this application, based on a5000-turn CT, the value of Ki works out to beapproximately 8.7. Kd is the same as before.

By substituting the attenuated values of Vd and Id forthe Vi and Ii in the original power measurementequation, we get an expression that relates the con-sumed power directly to the indirect voltage and currentmeasurements, as shown in Equation 2.

EQUATION 2: CALCULATING CONSUMED ENERGY FROM INDIRECT MEASUREMENTS

We could accumulate a running total indefinitely anddirectly interpret it for energy consumed over time. How-ever, it’s more practical to accumulate up to some fixedamount, then increment a counter to indicate energyconsumption. For our application, we will accumulate10 Wh (0.01 kWh) before incrementing the counter. Thisvalue represents the resolution limit of the meter. It isequivalent to 36,000 wattseconds (10 Wh x 60 x 60); thismeans that we increment the counter every time that theright side of Equation 2 reaches 36,000.

We can also rearrange Equation 2 to define the powerconsumed entirely in terms of Vd and Id. Since we havealready defined Fs, Kv, Ki and Kd in constant terms, wecan give the whole quotient on the right side of theequation a constant value, D (Equation 3).

EQUATION 3: REDEFINING POWER IN TERMS OF Vd AND Id ONLY

In simple terms, any time that the accumulated sum ofthe voltage and current products equals or exceeds D,we increment the kWh counter. We also save anyremainder in excess of D to be used in the next roundof accumulation.

Note that anything which might influence the value ofthe constants may also affect the value of D andrequires changes to the amplifier design. This includesthe use of a shunt instead of a CT, or even changing theCT turn ratio, both of which may change Ki.

Note: The calculation of Ki when using a shunt issomewhat different. The actual circuitdesign for current measurement, and thedesign considerations for using a shunt,are discussed in more detail in “HardwareDesign”, starting on page 10.

Energy Consumed(wattseconds) =

N

Σ Vd • Idk = 1

Fs • K2

⎛⎜⎝ k k

⎞⎟⎠

• (Kv • Ki)

d

When 0.01 kWh is consumed:

=

N

Σ Vd • Idk = 1 Kv • Kik k

3600 • Fs • K2d = D

DS00939A-page 2 © 2005 Microchip Technology Inc.

AN939

Sampling Voltage and Current

Calculating power assumes that the voltage andcurrent are sampled exactly the same time. Obviously,using a single ADC with one sample-and-hold circuitmakes this impossible. We can do the next best thing,however, by using an interpolated voltage value thatvery closely approximates what the voltage would bewhen the current is sampled. The principle is graphi-cally represented in Figure 1. Although current isshown here as a regular sine wave and in phase withvoltage, this is not a requirement; current can have anywaveform and phase relationship.

In this method, we assume that it takes some time t tosample an analog voltage and convert it to a digitalvalue. If t is sufficiently small, we can use linear approx-imation to calculate the value exactly in the middle ofan interval of 2t. We will work on the assumption that asegment of sine curve spanning 2° can be thought of aslinear. For AC power in many countries, the frequencyis 50 Hz; 2° represents an interval of about 111 μs. If weassume a practical conversion time of 35 μs, the timebetween voltage measurements would be 70 μs. Thisis about 1.26°, well within our margin for linearity.

To calculate the voltage for a particular currentmeasurement:

1. Measure the first voltage sample at time t0.

2. After an interval of t, measure the current (time t1).

3. After another interval of t, measure the voltageagain (time t2).

4. Calculate the voltage at t1 as (Vdt0 + Vdt2)/2.

The actual shape of the current waveform does notaffect this calculation.

FIGURE 1: INTERPOLATING VOLTAGE FOR A CURRENT SAMPLE

Calibration

To compensate for errors introduced by passivecomponents, we need to individually calibrate themeter. We will discuss two varieties here:

• Gain calibration, to compensate for gain errors (in Kv and Ki) introduced by normal variations in the values of different resistors, CT ratios and so on.

• Phase calibration, to compensate for extraneous phase shifts introduced by the current measurement technique (from the CT, from the small but unwelcome inductance generated by a shunt and so on).

GAIN ERROR CALIBRATION

In theory, the proportionality and digitization constantsshould adequately calibrate the meter. In practice,individual component variations may cause differencesbetween calculated and actual energy consumption. Toaccount for this, we introduce a gain calibration factor,Cg, to Equation 3. This constant acts to adjust forchanges in both Kv and Ki. The accumulatedvoltage/current sum is then compared to D, alsoadjusted by the calibration constant C (Equation 4). Intheory, C and Cg are the same value. For practicalapplications, the two constants will have differentvalues to reflect the actual calibration. This isdiscussed in more detail in “Firmware” (page 5).

The value of Ki may also be slightly different at theextremes of the current measurement range. Toaccount for this, we need two different gain calibrationconstants: one for the low end of the dynamic rangeand one for the upper end. In practice, this is done foreach current measurement channel, for a total of fourdifferent values of Cg. The meter firmware chooses theappropriate value to use when a measurement istaken.

EQUATION 4: GAIN CALIBRATION CONSTANT

V, I

V

I

Voltage is sampledat these points

Value forvoltage here iscalculatedfor the currentsampling time

Current issampled halfway

Interval of 2° or less

between voltages

90°0°

= C • D

N

Cg • Σ Vd • Idk = 1 k k


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PHASE ERROR CALIBRATION

Ideally, the relative size of measurement error shouldbe fairly constant at various PFs, assuming that thereis no phase error. In reality, the CTs introduce a smallphase error that is generally constant in the amount ofcurrent lead or lag they create. The relative size the ofmeasurement error can vary significantly with thepower factor, however. If we calculate the percentageof error as ((cosθ) – PF)/PF) × 100), where θ is theangular difference between the theoretical phase differ-ence and the error introduced by the CT, we can seethat measurement error from a constant phase error ismuch greater at a 0.5 PF (lag) than at UPF.

To show this, assume that a CT introduces a constantphase error of about 1°, regardless of the power factor.At UPF, the error is extremely small, on the order of-0.015% ((cos(1° – 0) – 1.0)/1.0). At 0.5 PF with a 60°lag, however, the error is significantly larger; as high as+3% ((cos(60° – 1°) – 0.5)/0.5). This is illustrated inFigure 2.

To correct for this type of error, we can individuallymeasure error at UPF and 0.5 PF to come up with atypical error for the entire range; this can be convertedback to a fixed angular phase error and corrected byaltering the time between the current sample and thenext voltage sample.

As an example, assume that a meter is found to havean error of +0.2% at UPF and +0.7% at 0.5 PF (currentlag). The difference between the errors is +0.5%. Fromour previous equation, the angle x of the phase error at0.5 PF would be:

(cos(x) – 0.5)/0.5 = 0.005

cos(x) = 0.5025

or x is approximately 59.83°. This requires an additionallead of 0.17° to get to the desired phase angle of 60°,which translates to approximately 10 μs for a 50 Hzwaveform. In practical terms, this means delaying thevoltage measurement at t2 by 10 μs. If we assume thatthe time between individual conversions is 35 μs, thismeans that t2 is actually sampled at 45 μs.

Using a CT by itself normally introduces a current lag,typically observed at between 2° to 3°. Introducing acapacitor across the CT, as was done in this design,provides a current lead that is correctable with timingadjustment. It is not necessary to quantify the amountof lead; as long as it is present, it can be measuredindirectly through meter error and compensated.

FIGURE 2: COMPARING MEASUREMENT ERRORS FOR A CONSTANT PHASE ERROR AT DIFFERENT POWER FACTORS

Note: If errors at UPF and 0.5 PF are equal inmagnitude, phase error correction is notnecessary.

Phase Error MeasurementError

MeasurementError

At UPF: At 0.5 PF (lag):

V V

Current Lead introducedby Capacitor

Voltage andCurrent

in Phase

Predicted Phase Lag at arccos (0.5)(60°)

Actual Lag from Phase Error

Phase Error

I I


AN939

FIRMWARE

A high-level overview of the energy metering firmwareis shown in Figures 3 through 6.

Before entering the main metering routine, the firmwareinitializes itself. Measurement buffers and Fault flagsare cleared and the timers for the measurementroutines are reset. If a calibration signal is present(done by setting a jumper), the firmware waits forcalibration data on the serial port. If the signal is absentor no data is received, the data EEPROM isinterrogated for a calibration flag. If this flag is set, thecalibration data stored there is loaded for the firmware’suse. If no calibration data is present, default values inthe firmware are used instead.

Initialization also gives users the opportunity to performa calibration routine, which downloads data from anexternal meter calibration utility through a serialconnection. Details are provided in “Calibrating theMeter” (page 11).

The main loop is responsible for updating the kWhcounter and maintaining the visual display shown onthe LCD. The kWh counter is incremented on the basisof a status flag, set in an interrupt driven power mea-surement routine, discussed below. A separate displaytimer is used to determine how long each measuredvalue is displayed before rolling over to the next value.The default value for this application is 6.26s.

An interrupt signal from an external RTC causes thefirmware to compare the total energy consumed sincethe last interrupt to the value stored in data EEPROMand update the value if the consumed power is greater.This provides a record for tracking peak demand for onemonth. The default value for the interval is 30 minutes.

Voltage and current measurement are performed duringan interrupt service routine triggered by the Timer0 inter-rupt. Measurements are performed in a specificsequence, with the entire sequence being repeatedevery 2.5 ms. The measurement routine is an InterruptService Routine triggered by the Timer0 interrupt. Inter-leaving samples of voltage and current are taken, withthe simultaneous voltage values for the current measure-ments being interpolated by the application. Calibrationsfor phase error are included in this calculation by using adelay counter to adjust the voltage readings; in this appli-cation, each increment of the counter delays the voltagereading by 1.4 μs (assuming a 20 MHz clock). The entiresequence of five measurements and their correspondingconversions takes much less time than the intervalbetween sequences, typically less than 300 μs.

To provide an added measure of accuracy, the offsetvoltage is periodically monitored to check for a stablebaseline. This is done much less frequently than othermeasurements, approximately once every 1.25 seconds.Adjustments to the offset value, subtracted from voltageand current measurements, are made as needed.

A single sampling sequence is taken during every inter-rupt routine. The accumulated energy to that point,adjusted by Cg, is compared with the values for D x C,as well as a fraction of the D xC equivalent to1/3200 kWh. Prior to calibration, the application uses avalue of 200 for both C and Cg. After calibration, Cgmay change to compensate for gain inaccuracies; Cremains at 200. An increment or decrement of one inCg can be used to compensate for a deviation of±0.5%. Increasing Cg compensates for negativedeviations, while decreasing it compensates forpositive deviations. In practical terms, the adjustmentrange possible runs from -25 to +100%.

If the accumulated value exceeds 1/3200 kWh, anexternal signal is generated for calibration purposes. Ifthe energy equals or exceeds the adjusted value of D,the kWh flag is set; this will cause the kWh counter tobe incremented by one during the next execution of themain loop. For amounts exceeding D, the differencebetween the energy buffer and D is saved and addedto the next round of accumulation.

The ISR executes a housekeeping routine once every20 interrupts to monitor for other conditions that mayaffect the energy reading. These include currentchannel amplifier gain, ground Faults, negativeaccumulated value and noise.

Because of the large dynamic range for load current, aselectable gain amplifier is used to match the currentrange to the range of the ADC. The gain is controlledby summing the current for 2½ cycles of the supplyfrequency. If the value exceeds an equivalent of 3A, theamplifier’s gain is switched to low. If the value fallsbelow an equivalent of 2.8A, the gain is switched tohigh. The small overlap between switching thresholdsprovides a built-in hysteresis, which prevents constantswitching between gain states.

The same summed values are also compared fordifferences. If the line current exceeds the neutral cur-rent by 8% or more, a ground Fault is indicated. Whenthis happens, the meter switches from metering on theneutral current (the default) to the line current. Anexternal signal is also sent to indicate a possibletamper condition.

Also included in the measurement routine is logic tohandle reverse-current conditions. Although the energycalculations involve signed operations, the result of thecalculation after 2½ cycles should always be positive.This assumes that the current sensors are connected toproduce voltage and current signals in phase (or nearlyso) at Unity Power Factor (UPF). If this is properly done,the accumulated energy will always be positive, evenacross a PF range from 0.5 lag to 0.8 lead. Neverthe-less, a reverse-current condition, where voltage andcurrent are 180° out of phase, will still produce a nega-tive accumulated energy result. If this happens, a flag isset to permit the energy algorithm to produce a correct(positive) result from the next cycle onward.


AN939

The algorithm also examines the contents of theintermediate results buffers and clears them if thevalues fall below a certain value. This prevents line andcircuit noise from being mistakenly interpreted asactual power consumption.

FIGURE 3: ENERGY METER FIRMWARE (INITIALIZATION ROUTINE)

START

InitializeSystem

Calibrate now?

Is systemcalibrated?

A

Load DefaultCalibration from

Program Memory

Set RTC for30 Minute Alarm

Start Timer0

Initialize LCDto Display Power-on

Values from EEPROM

B

Load Calibration Datafrom Data EEPROM

InitializeRAM

InitializeADC and I/O Ports

InitializeInterrupts

Clear SAMPLEand OFFSET_CNT

Initialize GAIN,EARTH, REVERSEand OFFSET Flags

Buffers

Initialize System Detail

(Calibration Routine)

(Main Program Loop)

Y

N

Y

N


AN939

FIGURE 4: ENERGY METER FIRMWARE (MAIN PROGRAM LOOP AND CALIBRATION ROUTINE)

B

Is kWh flag true?

Increment kWhCounter and Store

Has displaycounter

elapsed?

Hasdemand timer

elapsed?

Compute MaximumDemand and Storein Data EEPROM

Display NextParameter on LCD

N

Y

N

Y

N

Y

Main Loop

Initialize USART

Send CalibrationRequest to GUI

Receive CalibrationData from GUI

Store Calibrationin Data EEPROM

All datareceived?

Disable USART

RETURN

A

N

Y

Calibration routine

in EEPROM


AN939

FIGURE 5: ENERGY METER FIRMWARE (ENERGY MEASUREMENT INTERRUPT ROUTINE)

Timer0 Interrupt

Is OFFSET true?

Sample Voltage

and Current

CalculatePower

C

Have20 samplesbeen taken?

Is

Total ≥ 1/3200

IncrementOFFSET_CNT

IsOFFSET_CNT

= 65536?

Return fromInterrupt

Set OFFSETto True

Set kWh Flagto True

Read OFFSET

Set OFFSET toFalse

from ADC,

Toggle CalibrationPulse and LED

Read Voltage atTime 1 (V1)

Read PhaseCurrent (CP)


Read NeutralCurrent (CN)


Calculate VPand VN from

Calculate SignedValues for Voltage

and Current

Adjust for Gain andCalibration Values

Accumulate CN andCP in CACCN

Integrate Energyaccording to EARTHand REVERSE Flags

Sample Voltage and Current

Calculate Power

V1, V2 and V3

(Gain and Fault Handling)

Y

N

Y

N

Y

N

Y

N

Store Energy Valuein DECISION

and TOTAL Buffers

and CACCP Buffers

kWh?

Is

Total ≥ 0.01 kWh?Y

N


AN939

FIGURE 6: ENERGY METER FIRMWARE (GAIN AND FAULT HANDLING ROUTINE)

C

AreCN and CPunequal?

Set EARTH Flagto True

Set EARTH Flagto False

Is signof DECISION

negative?

Doescurrent exceed

high limit?

Iscurrent below

low limit?

Isintegrated energy

very low?

Clear DECISIONand Current

Buffers

Toggle REVERSEFlag

Set Amplifier Gainto Low

Set Amplifier Gainto High

Clear Buffer

Return to ISR

Y

N

Y

N

Y

N

Y

N

Y

N


AN939

HARDWARE DESIGN

The conceptual design for the energy meter is shownin Figure 7; a more detailed schematic is presented inAppendix A: “Schematics”. As previously noted, thisdesign was prototyped using the PIC16F873A. Usersmay also implement this design without modificationusing the pin-compatible PIC18F2320, if desired.

Line voltage and current are sampled sequentially atregular intervals, with voltage and current being pre-sented to different analog input channels. To measurevoltage, the AC line is sampled across a potentialdivider, R19 and R20, which divides the input voltageby about 300. For current measurement, two currenttransformers create voltage signals across burdenresistors (R8 and R9) that are proportional to the loadcurrent. As the core design of the energy meter willaccommodate different types of transducers, the CTsthemselves are not shown on the schematic.

Regardless of the sensor used, current is measuredsequentially on both line and neutral, alternating withvoltage measurements. Measuring both currents isnecessary for detecting ground Faults and errorconditions. An analog switch selects between theappropriate channels and ground.

A fixed offset of approximately 2V is added to both thecurrent and voltage signals. This maintains the signalwell above VSS, which is an operating requirement ofthe microcontroller’s ADC.

By itself, the ADC does not have the dynamic range orresolution to perform the necessary measurements. Forthe current signal, an amplifier with two selectable gainstages follows the analog switches; it is used to compen-sate for the wider dynamic range of the current sample.The application firmware senses if the amplified currentsignal is above or below the ADC’s capabilities andautomatically adjusts the gain accordingly. Single stage

gain is set by the values of R5, R6 and R7; together withthe turn ratio of the CT and the value of the burdenresistors, these determine the value of the currentproportionality constant, Ki. (For reference, the formula isKi = ((CT Ratio)((R5 + R6)/R7)/(R8 or R9).) The micro-controller controls the amplifier gain through an analogswitch.

Energy consumption is calculated as previouslydescribed in the “Firmware” section. Whenever theenergy is incremented by ten counts (i.e., 0.1 kWh), thevalue is also stored in EEPROM as well. In the event ofa power failure and subsequent recovery, thepreviously accumulated energy is retrieved andaccumulation resumes from that value.

An external Real-Time Clock (RTC) is used to generatean interrupt signal every 30 minutes. This is the exter-nal timing source for peak demand period calculationsdiscussed previously. The RTC can also be used toimplement other time/date functions.

Information on energy consumption is sent over a 4-wireinterface to an external LCD with integrated controller.The current version of the application firmware displayscumulative energy use to date, as well as several otherparameters, in a continuous rollover fashion.

Four indicator LEDs are provided to indicate Fault andcalibration states. The kWh LED flashes each time that1/3200 kWh (0.3125 Wh) is consumed, producing anoptical signal that test equipment can use for meter cal-ibration. The other LEDs indicate normal operation,reverse-current operation and ground Fault (tamper)conditions.

The core hardware design also includes a serial(RS-232) interface for calibration. The data lines areelectrically isolated from the rest of the meter circuitryto reduce the risk of damage to external equipment.

FIGURE 7: CONCEPTUAL BLOCK DIAGRAM OF THE ENERGY METER

ADC

PIC16F873APIC18F2320

AC Fail

Voltage

Line CurrentNeutral Current

Attenuation

Selectable GainAmplifier

3

4

LEDs

Numeric LCD

RTC2

with Controller

2 Serial Port

CalibrationJumper

Calibration

Communication/ControlMeasurement

Transistor

Control from Firmware

Switch


AN939

Design Considerations with Shunt Measurement

When substituting a shunt for current measurement,the value of Ki is calculated as the reciprocal of theproduct of the shunt resistance and the amplifier gain.For example, a shunt of 0.005 ohm used with the sameamplifier circuit will also produce approximately thesame value of Ki (around 9.3).

However, IEC specifications call for a power loss ofless than 2W in the measurement circuit. At the designvoltage and current, this would mean using a shuntsmaller than 0.0001 ohm. To compensate, the amplifiergain would have to be increased by reducing R7, whichwould create different gain requirements for the CT andshunt circuits. Since a single amplifier is used for bothcurrent channels, combining shunt and CTmeasurement may not be possible in this design.

Tamper Proofing and Current Measurement

As already noted, the energy meter design measuresboth the phase and neutral currents. Measuring twocurrent channels allows for tamper Fault detection. Iftampering is indicated, the meter will automaticallymeasure and calculate usage from the leg with thehigher load current. The threshold for detection is set inthe firmware at an 8% difference between the twochannels.

It is possible to configure the meter to use only one CTand a single current measurement channel. Doing thiswill disable tamper detection. Provisions are made inthe firmware to bypass the tamper detection feature.

If a shunt is used for current measurement, only onecurrent measurement channel is available. As before,doing this disables tamper protection.

CALIBRATING THE METER

The firmware contains a default set of values for themeter calibration constants. These values assume theuse of CTs to measure current and are based on theprototype design. Using different current sensors(shunts or Hall sensors), in addition to component vari-ations in individual devices, may require individualmeter calibration to account for intolerance variationsand phase error from sensors. To perform calibration, itwill be necessary to use the Meter Calibration Softwareprovided with the firmware. Users must also haveaccess to a watthour meter test stand, or other systemthat can provide power at fixed current levels andpower factors.

The calibration software uses inputs provided by theuser to calculate the gain calibration constant, Ki, forboth current channels, for both low-gain and high-gainsettings of the current amplifier. In addition, it calculatesthe phase error and required timing correction from thesame inputs. The calibration data is then downloadedto the meter through a serial (RS-232) interface.

The software is designed to run on desktop or laptopcomputers under any 32-bit version of Microsoft®

Windows® operating system. The computer must alsohave an available serial port.

FIGURE 8: CALIBRATION SCREEN FOR THE METER CALIBRATION SOFTWARE


AN939

To calibrate the meter:

1. Connect the meter to a test stand or othersystem, following the equipment manufacturer’sdirections.

2. Measure and record the signed error betweenthe meter and the test equipment on both cur-rent channels (CT1 and CT2) for each of theseparameters:• 1A supply at UPF• 10A supply at UPF

• 10A supply at a power factor with 0.5 lag (60 degree current lag)

3. Launch the Meter Calibration Software. At themain screen, click on Calibration. TheCalibration window appears (Figure 8).

4. Enter the six error measurements in the placesindicated. Click on the Process button. Thesoftware will automatically calculate and displaythe proper calibration values. At this point, youshould also set the meter serial number anddate and time to be downloaded, as well as theserial port to be used.

5. Disconnect the meter from the test stand.Connect it to a power source and to the computerusing a standard DB9 serial cable.

6. Place a jumper across JP1 to enable calibration.7. Click on the Connect button. The software

automatically connects to the meter, whichdownloads the calibration data and stores it indata EEPROM. The progress of the dataconnection and download is displayed in thelower right-hand corner of the window. Whencomplete, the software displays “CalibrationComplete”.

8. Remove the calibration jumper. Disconnect theserial cable.

RESOURCE USAGE

Although the energy meter design provides both basicand additional special features, other features may berequired by a particular customer. This implementationprovides additional room, both in terms of memory andhardware resources, to expand the application to thecustomer’s needs. A summary of the resources usedfor the basic PIC16F873A device-based design isprovided in Table 1.

TABLE 1: RESOURCES USED BY ENERGY METER FIRMWARE (PIC16F873A VERSION)

CONCLUSION

Implementing a electronic watthour meter with amicrocontroller should be a straightforward exercise indesign. The PIC16F873A and PIC18F2320 micro-controllers provide a cost-effective way to implement ametering solution with a low part count. Simple currentand voltage sampling techniques yield a design thatmeets Class 1 accuracy for residential meteringrequirements, while delivering additional features, suchas peak demand tracking and tamper detection. All ofthis can be achieved without sacrificing costcompetitiveness.

Note: While the meter’s serial data port isisolated from the rest of the meter circuitry,the meter ground is not. This poses apotential risk to the computer and the user.When performing any calibration, alwaysrun the computer on a power source thatdoes not share a ground with the meter.The best option is to run the computerfrom a battery.

Resource Used by Application

Program Memory (words) 3.5K

RAM (bytes) 190

Data EEPROM (bytes) 78

I/O Pins 22

Timer Resources 1 (Timer0)

ADC Channels 2


AN939

APPENDIX A: SCHEMATICS

FIGURE A-1: ENERGY METER, SCHEMATIC, PART 1 (MICROCONTROLLER, RTC, LCD INTERFACE AND LEDs)

PIC

16F

873A

33 p

F

32 k

HzV

CC

VS

S

VD

D

.01

μF

VC

C

VC

C

VC

C

VS

S

VS

S

33 p

F

VC

CV

CC

VC

C

.01

μF

VC

C

VC

C 10K

33 p

F

C8


AN939

FIGURE A-2: ENERGY METER SCHEMATIC, PART 2 (CURRENT AND VOLTAGE SAMPLING, OFFSET GENERATION, GAIN AMPLIFIER AND SERIAL CALIBRATION INTERFACE)

1A

13C

2B

6 89

10 98

C

5 67

B

11 1210

3 A5 C 4B

12 1314

D

3 21

A

12 3

5 6 8

47

A CB

ACB

VC

C

VC

C

VC

C VC

C

680

pF

VC

C-

VC

C

VC

C

33 p

F

.01

μF


AN939

FIGURE A-3: ENERGY METER SCHEMATIC, PART 3 (POWER SUPPLY)

OP

TIO

NA

L

VC

CV

CC

-

22 n

F

1000

μF

.1 μ

F.1

μF

1000

μF

1 μF

1 μF

230V

, 50

Hz

AC


AN939

APPENDIX B: SOFTWARE DISCUSSED IN THIS APPLICATION NOTE

For the sake of brevity, a complete listing of the energymeter firmware discussed in this document is notprovided here. The complete application, along with theaccompanying Meter Calibration utility software, maybe downloaded from the Microchip corporate web siteat:

www.microchip.com


Note the following details of the code protection feature on Microchip devices:

• Microchip products meet the specification contained in their particular Microchip Data Sheet.

• Microchip believes that its family of products is one of the most secure families of its kind on the market today, when used in the intended manner and under normal conditions.

• There are dishonest and possibly illegal methods used to breach the code protection feature. All of these methods, to our knowledge, require using the Microchip products in a manner outside the operating specifications contained in Microchip’s Data Sheets. Most likely, the person doing so is engaged in theft of intellectual property.

• Microchip is willing to work with the customer who is concerned about the integrity of their code.

• Neither Microchip nor any other semiconductor manufacturer can guarantee the security of their code. Code protection does not mean that we are guaranteeing the product as “unbreakable.”

Code protection is constantly evolving. We at Microchip are committed to continuously improving the code protection features of ourproducts. Attempts to break Microchip’s code protection feature may be a violation of the Digital Millennium Copyright Act. If such actsallow unauthorized access to your software or other copyrighted work, you may have a right to sue for relief under that Act.

Information contained in this publication regarding deviceapplications and the like is provided only for your convenienceand may be superseded by updates. It is your responsibility toensure that your application meets with your specifications.MICROCHIP MAKES NO REPRESENTATIONS OR WAR-RANTIES OF ANY KIND WHETHER EXPRESS OR IMPLIED,WRITTEN OR ORAL, STATUTORY OR OTHERWISE,RELATED TO THE INFORMATION, INCLUDING BUT NOTLIMITED TO ITS CONDITION, QUALITY, PERFORMANCE,MERCHANTABILITY OR FITNESS FOR PURPOSE.Microchip disclaims all liability arising from this information andits use. Use of Microchip’s products as critical components inlife support systems is not authorized except with expresswritten approval by Microchip. No licenses are conveyed,implicitly or otherwise, under any Microchip intellectual propertyrights.

© 2005 Microchip Technology Inc.

Trademarks

The Microchip name and logo, the Microchip logo, Accuron, dsPIC, KEELOQ, microID, MPLAB, PIC, PICmicro, PICSTART, PRO MATE, PowerSmart, rfPIC, and SmartShunt are registered trademarks of Microchip Technology Incorporated in the U.S.A. and other countries.

AmpLab, FilterLab, Migratable Memory, MXDEV, MXLAB, PICMASTER, SEEVAL, SmartSensor and The Embedded Control Solutions Company are registered trademarks of Microchip Technology Incorporated in the U.S.A.

Analog-for-the-Digital Age, Application Maestro, dsPICDEM, dsPICDEM.net, dsPICworks, ECAN, ECONOMONITOR, FanSense, FlexROM, fuzzyLAB, In-Circuit Serial Programming, ICSP, ICEPIC, MPASM, MPLIB, MPLINK, MPSIM, PICkit, PICDEM, PICDEM.net, PICLAB, PICtail, PowerCal, PowerInfo, PowerMate, PowerTool, rfLAB, rfPICDEM, Select Mode, Smart Serial, SmartTel and Total Endurance are trademarks of Microchip Technology Incorporated in the U.S.A. and other countries.

SQTP is a service mark of Microchip Technology Incorporated in the U.S.A.

All other trademarks mentioned herein are property of their respective companies.

© 2005, Microchip Technology Incorporated, Printed in the U.S.A., All Rights Reserved.

Printed on recycled paper.

DS00939A-page 17

Microchip received ISO/TS-16949:2002 quality system certification for its worldwide headquarters, design and wafer fabrication facilities in Chandler and Tempe, Arizona and Mountain View, California in October 2003. The Company’s quality system processes and procedures are for its PICmicro® 8-bit MCUs, KEELOQ® code hopping devices, Serial EEPROMs, microperipherals, nonvolatile memory and analog products. In addition, Microchip’s quality system for the design and manufacture of development systems is ISO 9001:2000 certified.


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10/20/04

design energy meter with pic

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