MODIFIED LOW-COST ENERGY
METER USING ADE7757
The low-cost energy meter project as published in May’05 issue of EFY, which made use of 8-digit, 7-segment displays in conjunction with two counter-cum display driver ICs MM74C926 and a 5V power supply, has been modified to display the KWH reading on an 8- digit LCD calculator in an identical manner, thereby doing away with a large number of components. The modified circuit diagram is shown here in Fig.1. Difficulty had been experienced regarding non-availability and high price of counter-cum-LED display driver IC MM74C926 (IC6 and IC7), which led us to present this alternate scheme. An 8-digit LCD calculator with character display height of 0.56" for good visibility has been used. Connection method. Output from collector (pin 5) and emitter (pin 4) of MCT2E optocoupler (IC3) is extended to ‘=’ terminals on the PCB inside the calculator. To make things easy for the customer, M/S Kits ‘N’ Spares will supply the calculators with the connection from the ‘=’ terminals brought out. Thus the customer has to simply connect the ‘red’ and ‘black’ leads from the calculator to collector pin 5 and emitter pin 4 of IC3 of Energy meter PCB. A 2-pin connector is used on the PCB for terminating output (100 pulses per KWH unit) from IC3 optocoupler. Note. Shorting links for J1 and J2 and J3 determine output frequency on CF pin of IC1. In this application, J1 and J2 select logic 1 (short pins 2 to3) and J3 selects logic 0 at pins 8, 9 and 10 respectively to get 100 pulses per KWH. An actual-size PCB layout for the circuit of Fig. 1 is shown in Fig.2, while its component layout is shown in Fig. 3. The fresh parts list using identical parts designation is given in the box. Activation procedure. The calculator has its own supply from a coin cell, which can last for a long period without replacement. Follow the simple steps given below for its activation: (a) Switch on the calculator. (b) Press decimal Key ‘.’, followed by entry of two zeros ‘00’. © Press plus key ‘+’ followed by entry of decimal, zero and one ‘.01’. The reading on calculator LCD screen shows ‘0.01’ Now you can switch on your energy meter and its load. On receiving the first pulse via optocoupler (IC3), the reading will continue to show 0.01. With each successive pulse the reading will increment by .01 to show ‘0.02’, ‘0.03’, ———‘.08’‘0.09’, ‘0.10’ and so on as was the case when original circuit/PCB was used. Hence the reading will be identical. Cost advantage. M/S Kits ‘N’ Spares have decided to pass on the cost advantage of Rs.500/-, as a result of use of this alternate method, to the customers. The redundant items, as mentioned above, will not be supplied by M/S Kits ‘N’ Spares with the kit, while calculator will be included as part of the kit.
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