The principle of operation of the electronic counter. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Electric meters

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To calculate the electrical energy consumed over a certain period of time, it is necessary to integrate the instantaneous values ​​of active power over time. For a sinusoidal signal, the power is equal to the product of the voltage and current in the network at a given time. Any electric energy meter works on this principle. On fig. 1 shows a block diagram of an electromechanical meter.

Rice. 1. Block diagram of an electromechanical electric energy meter

The implementation of a digital electric energy meter (Fig. 2) requires specialized ICs capable of multiplying signals and providing the resulting value in a form convenient for the microcontroller. For example, an active power converter - to a pulse repetition rate. The total number of incoming pulses, counted by the microcontroller, is directly proportional to the electricity consumed.

Rice. 2. Block diagram of a digital electric energy meter

An equally important role is played by all kinds of service functions, such as remote access to the meter, to information about the accumulated energy, and many others. The presence of a digital display controlled by a microcontroller allows you to programmatically set various modes of information output, for example, display information on the energy consumed for each month, at various tariffs, and so on.

To perform some non-standard functions, such as level matching, additional IS are used. Now they have begun to produce specialized ICs - power-to-frequency converters - and specialized microcontrollers containing such converters on a chip. But, often, they are too expensive for use in household induction meters. Therefore, many global manufacturers of microcontrollers are developing specialized microcircuits designed for this application.

Let's move on to the analysis of building the simplest version of a digital counter on the cheapest (less than a dollar) 8-bit Motorola microcontroller. In the presented solution, all the minimum necessary functions are implemented. It is based on the use of an inexpensive power-to-pulse frequency converter IC KR1095PP1 and an 8-bit microcontroller MC68HC05KJ1 (Fig. 3). With such a structure, the microcontroller needs to sum up the number of pulses, display information on the display and protect it in various emergency modes. The meter under consideration is actually a digital functional analogue of existing mechanical meters, adapted for further improvement.

Rice. 3. The main components of the simplest digital electricity meter

Signals proportional to the voltage and current in the network are taken from the sensors and fed to the input of the converter. The converter IC multiplies the input signals to obtain the instantaneous power consumption. This signal is fed to the input of the microcontroller, which converts it into Wh and, as the signals accumulate, changes the meter readings. Frequent power failures make it necessary to use an EEPROM to save the meter readings. Since power failures are the most typical emergency situation, such protection is necessary in any digital meter.

The program operation algorithm (Fig. 4) for the simplest version of such a counter is quite simple. At power-up, the microcontroller is configured according to the program, reads the last saved value from the EEPROM and displays it on the display. Then the controller enters the mode of counting pulses coming from the converter IC, and, as each Wh is accumulated, it increases the counter reading.

Rice. 4. Algorithm of the program

When writing to EEPROM, the value of the accumulated energy may be lost at the moment of power failure. For these reasons, the value of the accumulated energy is written to the EEPROM cyclically one after another after a certain number of changes in the meter readings, set by the software, depending on the required accuracy. This avoids loss of stored energy data. When voltage appears, the microcontroller analyzes all the values ​​in the EEPROM and selects the last one. For minimal losses, it is sufficient to record values ​​in increments of 100 Wh. This value can be changed in the program.

The scheme of the digital calculator is shown in fig. 5. 1 V power supply and load are connected to connector X220. From the current and voltage sensors, the signals are fed to the KR1095PP1 converter microcircuit with optocoupler isolation of the frequency output. The counter is based on the Motorola microcontroller MC68HC05KJ1, manufactured in a 16-pin package (DIP or SOIC) and having 1,2 KB of ROM and 64 bytes of RAM. To store the accumulated amount of energy during power failures, Microchip's small 24C00 EEPROM (16 bytes) is used. An 8-bit 7-segment LCD is used as a display, controlled by any inexpensive controller, communicating with the central microcontroller via the SPI or I2C protocol and connected to the X2 connector.

The implementation of the algorithm required less than 1 KB of memory and less than half of the I / O ports of the MC68HC05KJ1 microcontroller. Its capabilities are enough to add some service functions, for example, networking meters via the RS-485 interface. This function will allow you to receive information about the accumulated energy in the service center and turn off the electricity in the absence of payment. A network of such meters can equip a residential multi-storey building. All readings via the network will be sent to the dispatch center.

Of particular interest is the family of 8-bit microcontrollers with on-chip FLASH memory. Since it can be programmed directly on the assembled board, the program code is protected and the software can be updated without installation work.

Rice. 5. Digital calculator for digital electricity meter

Even more interesting is the version of the electricity meter without external EEPROM and expensive external non-volatile RAM. In it, in emergency situations, readings and service information can be recorded into the internal FLASH-memory of the microcontroller. This also ensures the confidentiality of information, which cannot be done using an external crystal that is not protected from unauthorized access. Such electricity meters of any complexity can be implemented using Motorola microcontrollers of the HC08 family with on-chip FLASH memory.

The transition to digital automatic systems for accounting and control of electricity is a matter of time. The advantages of such systems are obvious. Their price will keep dropping. And even on the simplest microcontroller, such a digital electricity meter has obvious advantages: reliability due to the complete absence of rubbing elements; compactness; the possibility of manufacturing the case, taking into account the interior of modern residential buildings; increase in the verification period by several times; maintainability and ease of maintenance and operation. With little additional hardware and software costs, even the simplest digital meter may have a number of service functions that are not available in all mechanical meters, for example, the implementation of multi-tariff payment for consumed energy, the possibility of automated metering and control of consumed electricity.

Publication: cxem.net

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