Volume 1 - Issue 8 - ICTACT.IN

(((((Student ColumnT3 15-0-15/500mak15C3 1000 μfR8 1Kv3 7805D1 1N4007 v2 7812++(C5 1000 μfC1 1000 μfT4 5v vccC2 10 μfC4 10 μfmade linear and converted effectively usingIC amplifier of low costIn voltage measurement, PTs are connectedacross two phases of the transformerunder test. The PT is rated at230V/6V.The ac output voltage of the PT isrectified, filtered and converted into puredc 5mV.A1 is an inverting rectifier in therectifier circuit. The output from A1 is addedto the original input signal in A2 (summingmixer). Negative alternations of Einresults in no output at E1 due to the rectification.Ein feeds A2 through 22K ohmsresistor and E1 feeds A2 through a 10Kohms resistorThe signal thus rectified has to be regulatedto obtain a pure signals discussedfor DC power supplies, it is essential thatthey employ a rectifier and a filter circuit.In addition to this some power supplieshave an added circuit called a regulator.The primary purpose of the regulator isto aid the rectifier and filter in providinga more constant DC voltage to load device.+v1 7912Power SupplyD1 LEDcom comcom comT6 12v vccR9 1KQ2 SL 100D2 1N4007Relay Circuit+F1++12v vccR6 1kC5 10 μfR1 1kR5 1kT2-12v vffD3D4D2K1 12v RelayPIC micro controller - PIC16F73 /PIC16F876Microprocessor for processingMicro controllers have built-in peripherals,they are:• Low cost, quality process• Available in commercialPackages-DIP• Technology Support• Easy MP Lab Construction.• In expensive tool costing.Following Features1) Memorya) Program Memory (E.g.PROM, Flash memory)b) Data Memory (E.g. RAM,EEROM)2) I/O Ports3) ADC4) Timers5) USART6) Interrupt ControllersPWM / CapturePotential TransformerIt is used to transform the high voltageof a power line to a lower value suitablefor direct connection to the circuitused as Volt Sensor.Current TransformerThe CT secondary winding is usuallydesigned to deliver a secondary current of0.5 A. A 5A/0.005A bar type current transformerswould have 16 turns on the secondarycoil used as current sensor.LM311 – TI ComparatorOP07 – TI -OpampLM4250 - TI2x16 LCD Display Unit12V Relay – Control CircuitCC2500 RF Transceiver For datacommunicationRegulator (7805,7812,7912)Software ToolsMP LAB IDEVB-Front endMS Access-back end1.5 Hardware ImplementationThe Smart Meter measures both thecurrent and voltage of the test circuit.Using Potential transformers PT, currentTransformer CT are used for V I measurementThe resulting analog signals are fedinto an op amp configured as a summercircuit to scale the signals down to a 0v-5vlogic level. This provides an interface intothe PIC 16F microprocessor, where bothcurrent and voltage signals are input intotwo channels of the on-board 10-bit ADC.A transformer was used to provide electricalisolation from the primary circuit. Thisensured that the sensitive microprocessorson the PCB were not directly exposedto the primary circuit.The configuration of the voltage transformerand the op-amp is shown above.As the voltage from the voltage transformeroscillates from a positive voltage to anegative voltage, it must have an offsetadded to it to allow it to interface to theop-amp, which can only measure voltageswithin its voltage supply range of 0v-5v.To achieve this, the op-amp is configuredas a summer circuit which sums togetherthe voltages present at its + input. One ofthe input voltages is the signal from thevoltage transformer. The voltage emanatesfrom the voltage transformer’s centretap and is halved using a voltage dividercircuit, lowering it by a further factor oftwo. The other input voltage is a referencevoltage provided by another voltage dividercircuit, which remains constant at 2.5v.This offset, or bias voltage, represents 0vat the voltage transformer output. Whenthe test circuit is enabled, the output ofthe op-amp will swing between 0v and 5v,which allows it to interface to the microcontroller ADC. the current measurementcircuit is similarly configured to the voltagemeasurement circuit. It utilizes a ordinarycurrent transformer which is ratedto measure up to 5A in the primary circuit.The current transformer produces a corresponding-2.5v to 2.5v voltage swing at itsoutputs which is summed with a bias voltageto produce a 0v-5v signal range.Wireless linkThe Ti CC2500 chip has been chosento provide the wireless functionalityfor the Smart Metering System. It operateson a low power, 1.8v-3.3v supply.It uses SPI bus as a communications interfacewhich makes it ideal for pairingwith the microprocessor. The chip usesthe IEEE 802.15.4 Wireless Personal AreaNetwork standard for data transmission inthe 2.4Ghz band. It provides full physicalJune 2013 I 20