EFY May 2005

T he low-cost energy meterproject as published in May’05issue of EFY, which made use

of 8-digit, 7-segment displays in con-junction with two counter-cum dis-play driver ICs MM74C926 and a 5Vpower supply, has been modified todisplay the KWH reading on an 8-digit LCD calculator in an identicalmanner, thereby doing away with alarge number of components. Themodified circuit diagram is shownhere in Fig.1.

Difficulty had been experiencedregarding non-availability and highprice of counter-cum-LED displaydriver IC MM74C926 (IC6 and IC7),which led us to present this alternatescheme. An 8-digit LCD calculatorwith character display height of 0.56"for good visibility has been used.

Connection method. Output fromcollector (pin 5) and emitter (pin 4)

MODIFIED LOW-COST ENERGY

METER USING ADE7757

PARTS LISTSemiconductors:IC1 - ADE7757 metering ICIC2 - 7805 5V regulatorIC3, IC4 - MCT2E optocouplerD1 - 1N4007 rectifier diodeZD1 - 15V, 1W zener diodeLED1 - Red LEDResistors (all ¼-watt, ±5% carbon):R1, R3, R7,R8 - 500-ohmR2 - 6.2-kilo-ohmR4 - 470-ohmR5, R6 - 680-ohmR9 - 350-micro-ohm (shunt)R10 - 1.8-mega-ohmR13 - 470-ohm, 1WR14 - 1-kilo-ohmVR1 - 470-kilo-ohm trimpotCapacitors:C1, C3, C7,C8-C10 - 0.1µF ceramic diskC2, C6 - 10µF, 25V electrolyticC4, C5, C11,C12 - 0.068µF ceramic diskC13 - 0.47µF, 630V polyesterC14 - 470µF, 35V electrolyticMiscellaneous:

- Two 6-pin IC baseL1, L2 - 2 turn on 22/24 SWG on

ferrite bead inductorCalculator - 8-digit, LCD based calcu-

lator (with coin cell)J1, J2, J3 - 3-pin SIP connector with

a shorting link each.

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of MCT2E optocoupler (IC3) is ex-tended to ‘=’ terminals on the PCBinside the calculator. To make thingseasy for the customer, M/S Kits ‘N’Spares will supply the calculatorswith the connection from the ‘=’ ter-minals brought out. Thus the cus-tomer has to simply connect the ‘red’and ‘black’ leads from the calculatorto collector pin 5 and emitter pin 4 ofIC3 of Energy meter PCB. A 2-pinconnector is used on the PCB for ter-minating output (100 pulses perKWH unit) from IC3 optocoupler.

Note. Shorting links for J1 and J2and J3 determine output frequencyon CF pin of IC1. In this application,J1 and J2 select logic 1 (short pins 2to3) and J3 selects logic 0 at pins 8, 9and 10 respectively to get 100 pulsesper KWH.

An actual-size PCB layout for thecircuit of Fig. 1 is shown in Fig.2,while its component layout is shownin Fig. 3. The fresh parts list usingidentical parts designation is givenin the box.

Activation procedure. The calcu-lator has its own supply from a coincell, which can last for a long periodwithout replacement. Follow thesimple steps given below for its acti-vation:

(a) Switch on the calculator.(b) Press decimal Key ‘.’, followed

by entry of two zeros ‘00’.(c) Press plus key ‘+’ followed by

entry of decimal, zero and one ‘.01’.

The reading on calculator LCD screenshows ‘0.01’

Now you can switch on your en-ergy meter and its load. On receiv-ing the first pulse via optocoupler(IC3), the reading will continue toshow 0.01. With each successive pulsethe reading will increment by .01 toshow ‘0.02’, ‘0.03’, ———‘.08’‘0.09’,‘0.10’ and so on as was the case whenoriginal circuit/PCB was used. Hencethe reading will be identical.

Cost advantage. M/S Kits ‘N’Spares have decided to pass on thecost advantage of Rs.500/-, as a re-sult of use of this alternate method,to the customers. The redundantitems, as mentioned above, will notbe supplied by M/S Kits ‘N’ Spareswith the kit, while calculator will beincluded as part of the kit.

Circuit

IC ADE7757 (IC1) is at the heart ofthe energy meter. It directly interfaceswith the shunt resistor and operatesoff the AC input. The only analoguecircuitry used in IC ADE7757 is inthe sigmadelta ADCs and referencecircuit. All the other signal process-ing is carried out in digital domain.The power supply for IC ADE7757 isderived directly from mains using thecapacitor divider network compris-ing C13 and C14. Most of the voltageis dropped across C13 (0.47µF poly-ester capacitor rated for 630V), whileresistor R13 (470-ohm, 1W) is used

as a current limiter. The output acrossC14 is limited to 15V DC, whichserves as an input to regulator 7805(IC2). The regulated 5V is fed to IC1at its VDD

Pin 1. In this application, thephase line is connected to AGND (pin6) and DGND (pin 13) and hence tothe common terminal of regulatorIC2.

Tips for soldering ofSMT ICs

Please note that SMD IC ADE7757(IC1) is to be soldered on track-sideafter aligning the pin numbers asprinted on track-side following thetips as given here.

1. Apply flux to the pads wherethe IC is to be soldered.

2. Add a small amount of solderto one of the corner pin pads.

3. Line up the IC with the padson the PCB. Double-check the IC ori-entation.

4. Melt the solder with your ironand move the IC into position withtweezers. Let the solder solidify.

5. Solder the diagonally oppositepin. Check under magnification thatall pins line up with their respectivepads.

6. Solder the rest of the pins andcheck under magnification.

Note. Special techniques may beneeded for some packages.

(For more details refer May’05 is-sue of EFY.)

Fig. 3. Component layoutFig. 2. PCB layout