Current Shunt Meter Interface Using the TERIDIAN 71M6511
Contents
1. Cig G agi Neutral 4 A 4 4 4 ge 000 4 LOY O4 SP c32 L bead Ts 30nF 1000Vdc R2 radial lead style X 30TSSD47 al 8K Tut C 1000pF 01D RV D3 C1 02 8 Dt 4C4 Er C NC i VARISTOR pq m FN2200UF 16V Four SV T E Y FNUF V TT SSe 10nF 1000Vdc INAO 6 8V 1W R4 e R8 zl 25K m L8 C R6 D4 R7 al LIVE van j 1 ri 1 1 1 L 1 GND VA IN 10 2W 100 2W 1N4148 130 amp R118 0 47UF 1000VDC Cm Now Jij ea Labeled V IN on Board 1206 PACKAGE To the Shunt V3P3 connection PS SEL 0 Figure 5 Typical Capacitive Power Supply AN 6511 006 v 2 4 Current Shunt TERIDIAN Application Note 5 18 AN 651X 006 Current Shunt Meter Interface Using the TERIDIAN 71M6511 The output voltage of the above circuit JP16 pin 2 will be 3 3VDC along with the ripple caused by the 60Hz input signal This output voltage can be expressed using the following mathematical equations t Vu f V Nene for the duration of the discharge in a cycle Va t V cos t for all other times in a cycle Based on the output voltage calculated above the DC output can be mathematically represented as x V Vc oe oRC 1 e sin x EQU 1 The Voltage Measurement Circuit The reference for the voltage measurement can pick up some of the noise coming from the analog signal con ditioning circuit This noise may not be as significant as the noise generated by the power supply circuit The Curren2. Error 96 0 1 1 10 100 Current A R24 open 8 R24 10K As can be seen installing the 24kQ resistor brings significant improvements in accuracy at low currents AN 6511 006 v 2 4 Current Shunt TERIDIAN Application Note 16 18 AN 651X 006 Current Shunt Meter Interface Using the TERIDIAN 71M6511 Shunt Shunt Configuration For the purpose of tamper detection two current sensors are sometimes used one in the LIVE connection to the load and the other one in the return connection from the load to NEUTRAL as shown in Figure 2 Connections Required for the Shunt Shunt Configuration If two CTs or one CT and one shunt are used no isolation issues occur If two shunt resistors are used isolation has to be provided by an isolation transformer Figure 10 shows the necessary connections LIVE SHUNT1 O O Isolation Transformer LOAD NEUT SHUNT2 71M6511 Figure 10 Shunt Shunt Configuration The connections from SHUNT 1 to the isolation transformer should be made using a twisted pair cable This cable should be shielded if severe EMI conditions are encountered Since the isolation transformer does not have to carry the primary current as a CT would have to but only the small current caused by the voltage drop at SHUNT 1 it can be significantly smaller than a comparable CT Therefore the isolation transformer will be easier to shield against magnetic fields than a comparable
3. AG GND i3 FERRITE 100 2W 1N4148 130 0 8 18 AN 651X 006 Current Shunt Meter Interface Using the TERIDIAN 71M6511 Current Shunt Input Schematic Figure 7 shows an example for implementing a current shunt input on the TERIDIAN 71M6511 Demo Board pu L1 i lt FERRITE HDR3X1 CURRENT SHUNT CONNECTIONS BAV99DW EE FERRITE c2 C3 1000pF Figure 7 Current Shunts input signal conditioning circuit Key EMI EMC precautions for the input signal conditioning circuit are a L1 and L2 are Ferrite beads that provide 6000 impedance for signals above 100MHz e g TDK MMZ2012S601A b The combination of R3 and C1 provides a low pass filter with cutoff frequency of around 212kHz c The V3P3 trace connecting to the input signal conditioning circuit should be wide d The J3 connector has a three input pin provision to accommodate for the connection of a shield for the shunt cable The shield may be connected to the digital ground to prevent high frequency noise entering through the shunt metal plate e In environments with high electromagnetic radiation R2 can be used at 10kQ to prevent spurious measurements f Unused current inputs should be properly terminated or shorted Software Adjustments The ADC in the 71M6511 6511H chip is operating in 22 bit resolution mode This means that both the control registers CFIR and MUX DIV are set to 1 AN 6511 006 v 2 4 Current Shunt TERIDIAN App
4. AN 6511 006 v 2 4 Current Shunt TERIDIAN Application Note 3 18 AN 651X 006 Current Shunt Meter Interface Using the TERIDIAN 71M6511 Single Shunt Configuration Wiring for Shunt Resistors Figure 3 shows the general topology of the current shunt sensor operation The voltage drop across the shunt resistor is fed into the IA input of the 71M6511 6511H device with V3P3 being the reference The voltage at the shunt is divided by a resistor divider R1 R2 associated with the voltage input and supplied to the VA input of the chip The Demo Board generates the power for the 71M6511 chip by attenuating the voltage between NEUTRAL and LIVE through a network consisting of resistors capacitors and diodes JP17 provides the NEUTRAL reference for the voltage divider used for VA JP4 provides the NEUTRAL return path for the on board power supply and J3 provides both the reference and the signal for the voltage drop across the shunt resistor se Note that a separate path is provided for the power supply to the board and for the references of the i e measured signals NEUTRAL O The three wires must be connected m directly at the shunt resistor Low crosstalk demands that these three current ele Canal paths not share a LOAD BLUE common wire No 71M6511 71M6511H IA V3P3 J9 NEUTRAL 2 5MOhm R15 R18 Power Voltage Divider PLE 71M6511 6511H Demo Board Figure 3 Wiring for Current Shunt Mode Shown f
5. CT Determining IMAX for the Shunt Shunt Configuration For the channel measuring the current via the isolation transformer IMAX is determined using the following formula IMAX 176mV N S Where N winding ratio of the isolation transformer secondary over primary Rs shunt resistance With a winding ratio of 10 and a shunt resistance of 16000 IMAX will be 110A AN 6511 006 v 2 4 Current Shunt TERIDIAN Application Note 17 18 AN 651X 006 Current Shunt Meter Interface Using the TERIDIAN 71M6511 Phase Compensation The isolation transformer will introduce a phase shift which will have to be compensated with the calibration factors of the CE PHADJ A PHADJ B This phase compensation factor is determined just like the com pensation factor for a regular CT using the methods described in the Demo Board User s Manual The various calibration procedures found in the calibration spreadsheets provided by TERIDIAN can also be used to easily determine the phase compensation factors Termination on the Meter PCB Unlike a CT that needs a burden resistor to drop a voltage proportional to the scaled voltage the isolation transformer provides a voltage proportional to the primary voltage i e the voltage drop across its associated shunt resistor This means that no burden resistor is required However just as in the case of a directly connected shunt resistor a high value termination resistor will help to suppress unwanted curren
6. TERIDIAN ud Energy Meter IC SEMICONDUCTOR CORP A Maxim Integrated Products Brand APPLICATION NOTE NOTE AN_651X_006 NOVEMBER 2010 Current Shunt Meter Interface Using the TERIDIAN 71M6511 The 71M6511 6511H Demo Board is designed to use all combinations of sensors i e current transformer CT current shunt CT or single shunt resistor When an isolation transformer is used shunt shunt configurations can be implemented The Demo Code Firmware revision 3 05 is designed to support all operating modes The 4 layer round Demo Boards are normally shipped in CT configuration Some 2 layer rectangular Demo Boards are shipped in shunt configuration and complete with a shunt resistor and wire harness With a few quick modifications a 71M6511 6511H Demo Board configured for CT can be adapted to operation with a current shunt CT or single shunt combination This application note describes how current shunts are connected to the 71M6511 6511H Demo Board and what precautions have to be taken to ensure proper operation Measurement results are presented for the single shunt configuration with various shunt resistance values AN_6511_006_v 2 4 Current Shunt TERIDIAN Application Note 1 18 AN 651X 006 Current Shunt Meter Interface Using the TERIDIAN 71M6511 Safety Precautions In shunt configuration the whole Demo Board will be at line voltage Touching the board or any components must be avoided It is highly recommended to isola
7. ent Shunt Meter Interface Using the TERIDIAN 71M6511 Performance with 18000 Shunt A 180uQ shunt resistor was connected to the 71M6511 Demo Board as per the instructions provided in this application note The following parameters were used e PHADJ_A 1850 e QUANT 1200 CE QUANT variable e WRATE 7933 for kh 1 0Wh pulse e IMAX 10420 set for 1042A The theoretical computation based on 180p provides only 950A However when measuring the shunt resistor the resistance appeared 10 lower than specified which is 16810 The measured data and performance curve are given below step volt amp Phase O0 Phase 60 1 240 125 0 0479 0 0628 2 240 100 0 0232 0 1123 3 240 50 0 0332 0 08 4 240 25 0 0511 0 067 5 240 10 0 0335 0 0857 6 240 3 0 0453 0 1298 7 240 1 0 0466 0 0358 8 240 0 5 0 029 0 0634 9 240 0 25 0 2532 0 4062 180UOHM SHUNT OVERALL PERFORMANCE e k Accuracy 10 100 1000 Current Amp e Phase 0 m Phase 60 AN 6511 006 v 2 4 Current Shunt TERIDIAN Application Note 15 18 AN 651X 006 Current Shunt Meter Interface Using the TERIDIAN 71M6511 Importance of Proper Termination A 6511 Demo Board was tested with a shunt resistor of 180100 Measurements were taken with a 10kQ termination resistor installed at position R24 and with R24 open The results are shown in the table and graph below R24 Open R24 10k 180p Ohm Shunt Performance
8. he instructions provided in this application note The measured data and performance curve are given below step volt amp revs freq No_gain with 8x gain 2 240 100 100 60 0 23 0 2453 3 240 50 50 60 0 1547 0 0949 4 240 25 25 60 0 0917 0 043 5 240 10 10 60 0 0491 0 0066 6 240 3 3 60 0 0459 0 0178 7 240 1 1 60 0 1166 0 0409 8 240 0 5 1 60 0 0355 0 1906 9 240 0 25 1 60 0 1862 0 2799 10 240 0 1 1 60 0 2876 0 3789 400Micro Ohm Shunt_Performance Accuracy Current Amp No Gain 9 8xGain AN 6511 006 v 2 4 Current Shunt TERIDIAN Application Note 13 18 AN 651X 006 Current Shunt Meter Interface Using the TERIDIAN 71M6511 300pQ Shunt Performance TERIDIAN 71M6511 BO3 Demo Board accuracy was tested with a 300pO shunt The shunt resistor was connected to the 71M6511 demo board as per the instructions provided in this application note The measured data and performance curve are given below Step Volt Amp NoGain 8xGain 1 240 60 0 0128 0 0128 2 240 30 0 09 0 0141 3 240 10 0 0617 0 0341 4 240 3 0 0938 0 016 5 240 1 0 0615 0 083 6 240 0 5 0 0303 0 1756 7 240 0 3 0 1086 0 0294 8 240 0 1 0 2453 0 3387 300uOhm Shunt Performance 6 4 02 e EU Accuracy Amperes e Without Gain With Gain AN 6511 006 v 2 4 Current Shunt TERIDIAN Application Note 14 18 AN 651X 006 Curr
9. iew of a two layer 6511 Demo Board connected to a shunt resistor As can be seen in Figure 6 the nets V3P3 V3P3J2 and V3P3_JUMPER are all connected at the shunt resistor AN 6511 006 v 2 4 Current Shunt TERIDIAN Application Note 7 18 AN 651X 006 Current Shunt Meter Interface Using the TERIDIAN 71M6511 I 10 2W JP2 HDR2 Figure 6 Top level View of Shunt Connections DB6511T4A7 AN 6511 006 v 2 4 Current Shunt TERIDIAN Application Note L6 IA TP21 Rm Rid 3 AN 1A lA 6511 FERRITE GND ana 0 750 V3P3 A HDR2 bns E ile cii dg 2 lo co 1000pF c17 crs xH aca az 1000pF 000p uU BAVSSDW 1 vaps FERRITE m IB i mn R104 was BIN a B B GND FERRITE 0 750 6 HDR2 pre Bia e d ij act HX ES o Zici cH 1000pF on cio BY ica 1000pF 000p Ld BAV99DW nan V3P3 FERRITE L3 FERRITE VA ay me vas R2 css C rename nF apie GND pre vaP3 JUMPER n TRA R6 Ris e vaP3 vars eee jh GND ls 3 750 T tooopF VA varas V3P3_JUMPER Ural ca ou aonF 2200uF 16V 1nF e 8 06K TL431 ano RVI D3 ket 22 aca 5s D10 510v tour Br EE UCLAMP3301D m 1N4736A OF 1 6 8v 1W T A4TuF to00dc 25 5K pm Rite C6 R6 ba RT R20
10. lication Note 9 18 AN 651X 006 Current Shunt Meter Interface Using the TERIDIAN 71M6511 from current shunt 71M6511 6511H from isolation transformer star point Bm V3P3A Capacitor Resistor from power supply Resistor from line input Figure 8 PCB Routing of the V3P3 Net AN 6511 006 v 2 4 Current Shunt TERIDIAN Application Note 10 18 AN 651X 006 Current Shunt Meter Interface Using the TERIDIAN 71M6511 Required Modifications to the 71M6511 Demo Board for Current Shunt Sensor Connections The Demo board has provisions to incorporate the changes as pictured in Figure 6 In order to operate the 71M6511 6511H Demo Board D6511BT4A4 or D6511BT4A5 with a current shunt sensor the following measures must be taken a Remove the jumpers on JP4 and JP17 b Remove R24 and R25 if IA IN is the input channel for the current shunt or remove R106 and R107 if IB IN is the input channel for the current shunt c Add a 10kQ resistor at R24 if IA IN is the input channel for the current shunt or add a 10kO resistor at R106 if IB IN is the input channel for the current shunt This resistor will provide noise termination and will suppress unwanted readings d The LIVE line must be connected to the spade terminal J4 bottom of the board e One pair of wires from the shunt resistor must be connected to contacts 1 and 2 on J3 if IA IN is the input channel or contacts 1 and 2 on J16 if IB IN is
11. ns show that for low currents the noise present on the V3P3 Vpc output is comparable to the shunt current input signals The presence of this noise demands compensation via high QUANT values However high values for QUANT are not practical and can create low accuracy drifts with the applied voltage AN 6511 006 v 2 4 Current Shunt TERIDIAN Application Note 6 18 AN 651X 006 Current Shunt Meter Interface Using the TERIDIAN 71M6511 The power computed by the 71M6511 IC is P VA IN IA IN or x P V cos at fona eC io 4 cos at i lorca m snc 2m 2m V I unwanted signals Case 2 Proper Connection The connection is made to the 6511 Demo Board with voltage applied at VA and Neutral and Shunt connected at la in and V3P3 e JP4 pin2 is directly connected to the shunt resistor e JP17 pin 2 is directly connected at the shunt resistor e The shunt resistor has a connection to the IA IN and V3P3 inputs By wiring the connections in the way described above the ripple generated on the DC output voltage has a return path to V3P3 on the shunt Even though the EQU 1 holds good for this circuit also the signal noise on the reference for the analog measurements is reduced because three separate wires are utilized One of the wires carries the current used to power the Demo Board and thus offers a return path for the noise leaving the other wires mostly noise free Figure 6 shows the top level v
12. or the 6511T4A7 2 Layer PCB Figure 4 shows the connections required at the shunt resistor itself AN_6511_006_v 2 4 Current Shunt TERIDIAN Application Note 4 18 AN 651X 006 Current Shunt Meter Interface Using the TERIDIAN 71M6511 to IA IN to VA e to IA reference reference to meter power supply to LOAD from NEUTRAL Figure 4 Recommended Shunt Connections Shunt Connection Theory This chapter is intended to illustrate and mathematically demonstrate the importance of proper shunt wiring The shunt resistors used for electricity metering tend to be in the order of few hundred micro Ohms leading to small voltage outputs across the analog current input of the 71M6511 IC The analog input measurements for the Teridian 71M6511 ICs are referenced to V3P3 Even though the input signal present at the current input is noise free the contamination of the V3P3 reference for measurement can prevent achieving the desired accuracy even with 22 bit ADC resolution The V3P3 signal used for the measurement contains the following sources of contamination e Noise from capacitive power supply e Noise contribution due to the voltage Reference V3P3 e Noise present in the signal conditioning circuit Capacitive Power Supply The capacitive power supply circuit used for on Teridian 651x demo boards is as shown below in Figure 5 did
13. t Measurement Circuit The signals present at the Shunt input are in the order of a few milli volts to micro volts Any noise present at the current measurement input can contaminate the whole power measurement resulting in inaccuracies and repeatability issues The following example will clarify the role of noise in the current measurement circuits and connections Voltage applied 240V 240V sinot where 2 x 60 or 50Hz if the fundamental is 5OHz Current applied 5A for a 71M6511 based meter Current sensor used 400 uQ shunt Case 1 Connection with one wire only The connection is made to the 6511 Demo Board with voltage applied at VA and Neutral and the shunt resistor is connected at la in and V3P3 a JP4 pin 1 and 2 are shorted b JP17 pint and 2 are shorted This means that the V3P3 supply voltage is generated by the capacitive power supply with a modulated output as described by EQU 1 This output voltage is the reference for measurement and the output ripple is present at the analog signals also The analog signal presented at VA IN for voltage measurement is 240 600 176mV 70 4mV Mathematically the signal present at VA IN is 70 4 cos wt Vpc from EQU 1 Please note that rms and peak are intentionally ignored for clarity The current output from the shunt resistor is 5 440 176mV 2mV Mathematically the signal present at IB IN is 2 Cos ot Vpc from EQU 1 The above calculatio
14. t readings This resistor as shown as R2 in Figure 11 H H FERRITE HDR3X1 CURRENT SHUNT CONNECTIONS BAV99DW r a FERRITE C2 C3 1000pF 1000pF Figure 11 Input Circuit for Shunt Shunt Configuration Ferrites L1 L2 capacitors C1 C2 C3 and diodes U1 for EMI suppression are recommended Maxim cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in a Maxim product No circuit patent licenses are implied Maxim reserves the right to change the circuitry and specifications without notice at any time Maxim Integrated Products 120 San Gabriel Drive Sunnyvale CA 94086 408 737 7600 2010 Maxim Integrated Products Maxim is a registered trademark of Maxim Integrated Products
15. te Demo Board and Debug Board when used and to provide separate power supplies for the Demo Board and Debug Board Some 2 layer Demo Boards have the headers JP2 and JP3 for galvanic connection between the Demo and Debug Boards Jumpers on these headers must be removed Emulators or other test equipment should never be connected to a live meter without proper isolation AN 6511 006 v 2 4 Current Shunt TERIDIAN Application Note 2 18 AN 651X 006 Current Shunt Meter Interface Using the TERIDIAN 71M6511 Shunt Configurations The TERIDIAN 71M6511 can be used in CT shunt CT shunt CT CT and shunt shunt configurations as shown in Figure 1 and Figure 2 LIVE LIVE CT oO C _ T LOAD LOAD 71M6511 O 71M6511 Figure 1 Topology for Shunt left and Shunt CT right LIVE CT1 LIVE SHUNT1 O AN m Isolation Transformer LOAD 71M6511 71M6511 Figure 2 Topology for CT CT left and Shunt Shunt right Note that the shunt shunt configuration requires an isolation transformer Without the isolation transformer the LIVE and NEUTRAL wires would be shorted This application note describes the single shunt and the shunt shunt configurations For a description of the shunt CT configuration refer to the Demo Board User s Manual rules laid out in the section Wiring for Shunt Resistors have to be followed et The diagrams above show general topology only For good measurement accuracy the wiring
16. the input channel f Connect Pin1 of JP4 pin closest to the regulator output power supply pin and pin 2 of JP17 resistor divider output together at the shunt V3P3 terminal as shown in Figure 3 g Through the serial terminal command line interface the Demo Program can be set to run in current shunt mode This is done by issuing the commands gt 2A 1 if IA IN is connected to the shunt or gt 2B 1 for IB IN connected to the shunt This will cause the Demo Program to select the proper input channels and to apply the gain of 8 to the shunt input channel from the ADC output before processing for power measurement While connecting Neutral to VA IN J4 care should be taken to prevent shorting between LINE and NEUTRAL Also note that no connection is required at J9 NEUTRAL since the LINE voltage from the shunt resistor becomes the reference for measurement et On older Demo Boards and 4 layer Demo Boards JP4 is labeled JP16 AN 6511 006 v 2 4 Current Shunt TERIDIAN Application Note 11 18 AN 651X 006 Current Shunt Meter Interface Using the TERIDIAN 71M6511 Meter Performance with Shunt Connected A Demo Board was used for testing the meter performance with Shunt resistor using 71M6511 Demo Code revision 3 04 Meter accuracy or performance was verified for several test conditions by varying the input current gain and by using both internal and external power supply Since the signals obtained from the Shunt resistor are small it is be
17. tter to integrate the inputs for a longer time 20 seconds in order to ensure repeatability Results are listed for 8x gain Shunt on 1x gain Shunt off and for operation with the internal power supply of the Demo Board int power The results for internal supply were obtained with 1x gain Shunt off Measurement results are shown in Table 1and Figure 9 Test Results step element volt amp phase angle freq Shunt on Shunt off Int power 1 S 240 10 0 50 0 0939 0 1248 0 1452 2 S 240 5 0 50 0 0894 0 1233 0 1478 3 S 240 3 0 50 0 1054 0 1261 0 1439 4 S 240 1 0 50 0 0627 0 1201 0 1156 5 S 240 0 3 0 50 0 0002 0 0697 0 134 6 S 240 0 1 0 50 0 1944 0 1505 0 0176 7 S 240 0 06 0 50 0 0407 0 1743 0 1042 Table 1 Shunt Measurement Results Shunt Accuracy Shunt Resistance 500uQ 4 0 8 0 6 0 4 0 2 3 0 o int f i 1 1 0 2 ui 0 4 0 6 0 8 LLL 4 4 0 06 0 1 0 3 1 3 5 10 Current A e Shunt Gain on 9 Shunt Gain Off With Internal Power supply Figure 9 Shunt measurement results AN 6511 006 v 2 4 Current Shunt TERIDIAN Application Note 12 18 AN 651X 006 Current Shunt Meter Interface Using the TERIDIAN 71M6511 Measurement Results with Various Shunt Resistors 400pO Shunt Performance TERIDIAN 71M6511 B03 Demo Board accuracy was tested with a 400 uQ Shunt The shunt resistor was connected to the 71M6511 demo board as per t
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