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In many applications, we want to monitor AC electrical parameters like Solar chargers, motor control, Battery Charging stations, or Smart energy meters.

Measurement of AC current and voltage, by the means of the electronics, is a quite tricky part, unlike traditional electromechanical systems. Below is a basic block diagram to know about it.

AC voltage and current signals are first steps down to low voltage and current values.One can not feed High AC electrical voltage and Current signals which can range from 0-1000 units and are even larger than that, in some applications(Power systems, AC motors and etc)  to the electronics and digital world.  So at first by the use of one of the below listed three methods AC signals are brought to low values( 0-10V, 0-5A)

1. Use of Current Transformer,  Potential Transformer, or  Rogowski coil.
2. Use of shunt resistors.
3. Use of Hall effect-based principal for current measurement. 

The signals which we get after processing from these methods are fed into the Digital System where there are signal conditioning circuits, data acquisition circuits, digital signal processing by the use of Digital and Analog Electronics. The Digital System part is designed to embedded into the Integrated chips(ASIC) which are specially designed with the purpose of Energy Metering application.

For the digital computation of key electrical parameters like Power factor, Active power, reactive power, Vrms, Irms and etc. Using these key parameters we can monitor the AC electrical parameters through serial interfaces like SPI/I2C with Host MCU.

ACS71020 Energy Metering IC

Allegro microsystem\’ AC power monitor module ACS71020 is a Single-phase energy monitoring IC that works on the principle of the HALL effect sensing technique (To know about ways to measure current refer to this blog) to measure the AC current and resistor divider network to measure the input AC voltage.

It calculates the key electrical parameters using its  Metrology Engine and digital system from which it sends the data to the host MCU via I2C and SPI interfaces.

The Voltage and current reading that we get from AC voltage and Current measurement blocks via the sense amplifiers are analog in nature. The analog signals from respective Voltage and Current blocks are then fed into the internal ADC\’s(Analog to Digital converter).

ADC samples the current and voltage channels at high frequency and then digitally converts them by filtering and decimating the output signal from sense amplifiers to avoid large anti-aliasing filters. The digital word from the ADC is 16 bits for both the current and voltage, which is fed to the digital system for further calculation of other electrical parameters.

Its Key Features are:

1. Without the need for any Transformer, Rogowski coils, oversized current transformers, or the power loss of shunt resistors one can calculate Vrms and Irms up to 517V and 90A respectively
2. It has an advanced digital system with galvanically isolated current sensing technology which achieves reinforced isolation ratings in a small PCB footprint
3. Apart from the calculation of Electrical Key parameters it also has many extra features too which are essential for monitoring purposes.

ACS71020 IC Pinout Diagram and Pins description

ACS71020 IC has 16 pins,

1. Starting from Pin 1-8 are current channel pins, out of which pins 1-4(Fused internally) are all IP+ and pins 5-8(fused internally) are all IP-. 
2. Pins 16-15 are Voltage measurement pins 

ACS71020 IC Schematic

For using ACS71020 for typical applications its schematic is pretty easy and less complicated in oppose to other metering IC\’s(STMP32 & ADE series).

ACS71020 can be powered directly from the same supply as the system\’s MCU, through its reinforced isolation technology it does need multiple power supplies to power it up. So Vcc and GND pins are connected directly to MCU Vcc and GND pins.

I2C  pins are at a high level(5V or 3v3), before the start of the I2C Serial Communication, thus SDA and SCL lines are connected with a pull-up resistor. When using in I2C mode, pins 9 & 10 act as DIO_1 and DIO_0( Digital Input/Output)  respectively, which are connected directly to MCU Digital Pins( Will get in detail about DIO pins in a later section)

For SPI communication, MOSI, MISO, CE pin are at a high level and CLK is at GRND before the start of SPI serial Communication. When using in SPI mode, pins 9 &10 are used as MOSI and CS pins.

ACS71020 IC  measures the Current and voltage of the input AC signal to calculate all other key parameters. So for inputting the AC voltage & current signals to the ACS71020 IC we will focus on Voltage channel pins(VINP & VINN) and Current channel pins(IP+ & IP-). One thing to recall is that in a single-phase AC supply there are two terminals:

1. Live Wire (Black/Red) carries electricity from the power supply and takes it to the load.
2. Neutral wire(Blue wire) returns the electricity from the load to the power supply to make the circuit complete.

VINN &VINP are terminals from where AC voltage is measured, so resistor network divider of 1mega ohm and shunt resistor is made in b/w the VINP and VINN terminals to fit the input AC voltage within the Range of the differential voltage input buffer of ACS71020( +-275mv) as specified in electrical characteristics of the datasheet.

IP+[1:4] & IP-[5-8] pins are terminals for AC current measurement. IP+ terminals are fused internally and are connected to a neutral wire of load and IP- terminals are also fused internally and are connected to a neutral wire of supply to complete the current loop of the current channel.

EVE ACS71020 Module

For doing the practical demonstration with ACS71020 IC we are going to use the EVE ACS71020 breakout board, which is manufactured by the Evelta.

• The module is cheap and can be used easily with HOST MCU via I2C or SPI communication.
• The module has pull-up resistors of 10k ohm placed with SDA and SCL pins( pins 12 & 11) of ACS71020  and no pull-up resistors are connected with MOSI and CS  pins(pins 10 & 9) means we can use these 2 pins as DIO0 and DIO1 respectively. Making the module to be used in I2C mode by default.
• Voltage levels applied at DIO_0/DIO_1  determine the address of I2C slave. By default When we apply High Voltage signals at DIO_0 and DIO_1 pin the I2C factory reset slave address is configured which is 0d127. ACS71020  has 16 I2C addresses that can be configured by applying different voltage levels at DIO_0 and DIO_1 pins, we will get into that in detail in a later part. Will get into it in detail in a later part. 
• For using the ACS71020 via SPI mode we have to connect the two pull-up resistors of 10k ohm at terminals MOSI and CS(pins 10 & 9).  The module provides the support for that, it does provide the pads for connecting pull-up resistors by the user on the MOSI and CS lines.
• The module has 3 terminals LIVE, LOAD+, and LOAD- for connecting to the input AC signal.

1. Live is connected to Load & Power supply live wire.
2. Load +(IP+) is connected to Load neutral wire.
3. Load- (IP-) is connected to Supply neutral wire.

The schematic of this module follows the official schematic which is given in ACS71020 IC.

References

1. ACS71020 Datasheet: https://drive.google.com/file/d/1AaTVo6FUPfG51TVZ1w3Mf2_LgvXMb9u1/view?usp=sharing
2. EVE ACS71020 module schematic: https://drive.google.com/file/d/1hksQy8g7-i9wMASnlyAZc-gb6dmkRWIZ/view?usp=sharing
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