A Brief Overview of Allegro ACS712 Current Sensor (Part 1) _ Embedded Lab

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A BRIEF OVERVIEW OF ALLEGRO ACS712

CURRENT SENSOR PART 1)

Posted on January 25, 2012 by R-B 10 comments |

Sensing and controlling current flow is a fundamental

requirement in a wide variety of applications including,

over-current protection circuits, battery chargers, switching

mode power supplies, digital watt meters, programmable

current sources, etc. One of the simplest techniques of

sensing current is to place a small value resistance (also

known as Shunt resistor) in between the load and the

ground and measure the voltage drop across it, which in

fact, is proportional to the current flowing through it.

Whereas this technique is easy and straightforward to

implement, it may not be very precise because the value of

the shunt resistor slightly varies with its temperature, which

in fact is not constant because of the Joule heating. Besides,

this simple technique does not provide an isolation

between the load and current sensing unit, which is

desirable in applications involving high voltage loads.Today, we will talk about Allegro ACS712 device which

provides an economical and precise way of sensing AC and

DC currents based on Hall-effect. This discussion is divided

into two parts. The first part will provide a brief overview of

the ACS712 sensor and its characteristics. In the second

part, a test experiment will be carried out to interface the

sensor with a PIC microcontroller to measure a dc current.

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ACS712-05 current sensor module

TheoryThe current sensing technique based on a shunt resistor is

described in How to measure dc current with a

microcontroller? and implemented in the Multi-functional

power supply project. The major disadvantages of this

technique are:

load is lifted from the direct ground connection

non-linearity in the response due to Joule heating

that drifts the resistance value

lack of electrical isolation between the load and thesensing part

The Allergo ACS712 current sensor is based on the principle

of Hall-effect, which was discovered by Dr. Edwin Hall in

1879. According to this principle, when a current carrying

conductor is placed into a magnetic filed, a voltage is

generated across its edges perpendicular to the directions

of both the current and the magnetic field. It is illustrated in

the figure shown below. A thin sheet of semiconductor

material (called Hall element) is carrying a current (I) and is

placed into a magnetic field (B) which is perpendicular to

the direction of current flow. Due to the presence of Lorentz

force, the distribution of current is no more uniform across

the Hall element and therefore a potential difference is

created across its edges perpendicular to the directions of

both the current and the field. This voltage is known Hall

voltage and its typical value is in the order of few

microvolts. The Hall voltage is directly proportional to the

magnitudes of I and B. So if one of them (I and B) is known,

then the observed Hall voltage can be used to estimate the

other.

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Principle of Hall-effect

The ACS712 device is provided in a small, surface mount

SOIC8 package. It consists of a precise, low-offset, linear

Hall sensor circuit with a copper conduction path located

near the surface of the die. When current is applied through

the copper conductor, a magnetic field is generated which

is sensed by the built-in Hall element. The strength of the

magnetic field is proportional to the magnitude of the

current through the conduction path, providing a linear

relationship between the output Hall voltage and input

conduction current. The on-chip signal conditioner and

filter circuit stabilizes and enhances the induced Hall

voltage to an appropriate level so that it could be measured

through an ADC channel of a microcontroller. The pin

diagram of ACS712 device and its typical application circuitis shown below. Pins 1, 2 and 3, 4 forms the copper

conduction path which is used for current sensing. The

internal resistance of this path is around 1.2 m?, thus

providing low power loss. As the terminals of this

conduction path are electrically isolated from the sensor

leads (pins 5 through 8), the ACS712 device eliminates the

risk of damaging the current monitoring circuit due to the

high voltage on the conduction side. The electrical isolation

between the conduction current and the sensor circuit also

minimizes the safety concerns while dealing with high

voltage systems.

Pin diagram and a typical application circuit of ACS712

http://embedded-lab.com/blog/wp-content/uploads/2012/01/PinDiagrams.jpghttp://embedded-lab.com/blog/wp-content/uploads/2012/01/HallEffect.jpg


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In low-frequency applications, it is often desirable to add a

simple RC filter circuit at the output of the device to

improve the signal-to-noise ratio. The ACS712 contains an

internal resistor (R ) connected between the the output of

the on-chip signal amplifier and the input of the output

buffer stage (shown below). The other end of the resistor is

externally accessible through pin 6 (Filter). With this

architecture, users can implement a simple RC filterthrough the addition of an external capacitor (C ) between

the Filter pin and ground. It should be noted that the use of

external capacitor increases the rise time of the sensor

output, and therefore, sets the bandwidth of the input

signal. The maximum bandwidth of the input signal is 80

KHz at zero external filter capacitor. The bandwidth

decreases with increasing C . The datasheet of ACS712

recommends to use 1 nF for C to reduce noise under

nominal conditions.

Functional block diagram of ACS712

Sensitivity and output of ACS712

The output of the device has positive slope when an

increasing current flows through the copper conduction

path (from pins 1 and 2, to pins 3 and 4). The ACS712 device

comes in three variants, providing current range of±5A

(ACS712-05B), ±20A (ACS712-20B), and ±30A (ACS712-30A).

The ACS712-05B can measure current up to ±5A and

provides output sensitivity of 185mV/A (at +5V power

supply), which means for every 1A increase in the current

through the conduction terminals in positive direction, the

output voltage also rises by 185 mV. The sensitivities of 20A

and 30A versions are 100 mV/A and 66 mV/A, respectively.

At zero current, the output voltage is half of the supply

voltage (Vcc/2). It should be noted that the ACS712 provides

ratiometric output, which means the zero current output

and the device sensitivity are both proportional to the

supply voltage, V . This feature is particularly useful when

using the ACS712 with an analog-to-digital converter. The

precision of any A/D conversion depends upon the stability

F

F

F

F

CC

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of the reference voltage used in the ADC operation. In most

microcontroller circuits, the reference voltage for A/D

conversion is the supply voltage itself. So, if the supply

voltage is not stable, the ADC measurements may not be

precise and accurate. However, if the reference voltage of

ADC is same as the supply voltage of ACS712, then the

ratiometric output of ACS712 will compensate for any error

in the A/D conversion due to the fluctuation in thereference voltage.

Let me explain this with an example. Suppose, an ADC chip

uses Vcc = 5.0V as a reference for A/D conversion and the

same supply voltage powers an ACS712 sensor chip. The

analog output of the ACS712 will be digitized through the

ADC chip. When there is zero current through the current

sensor, the output is Vcc/2 = 2.5V. If the ADC chip is 10-bit

(0-1023), it will convert the analog output from the ACS712

sensor into digital value of 512 count. Now, if the supply

voltage drifts and becomes Vcc = 4.5V, then, due to the

ratiometric nature, the new output of the ACS712 sensor

will be 4.5/2 = 2.25V, which will still be digitized to 512 by

the ADC as its reference voltage is also lowered to 4.5V.

Similarly, the sensitivity value will also be lowered by a

factor of 4.5/5 = 0.9, which means if the ACS712-05B is

powered with a 4.5V supply, the sensitivity is reduced to

166.5 mV/A, instead of 185mV, A. This concludes that any

fluctuation in the reference voltage will not be a source of error in the analog-to-digital conversion of the ACS712

output signals.

The curve below shows the nominal sensitivity and

transfer characteristics of the ACS712-05B sensor powered

with a 5.0V supply. The drift in the output is minimum for a

varying operating temperature, which is attributed to an

innovative chopper stabilization technique implemented on

the chip (read ACS712 datasheet for detail).

Output voltage vs sensed current of ACS712-05B at 5.0 V power supply and

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PIC Projects Tips and Tricks

Billy

comsian

varying temperature

Summary

This article briefly described the ACS712 current sensor and

its basic features. Continue reading the second part of this

discussion to see how to use it with a PIC microcontroller tomeasure DC current.

A brief overview of Allegro ACS712

current sensor Part 2)

References

ACS712 datasheet

ACS712 FAQ from Allegro website

Related posts:

1. A brief overview of Allegro ACS712 current sensor

(Part 2)

2. Humidity and temperature measurements with

Sensirion’s SHT1x/SHT7x sensors (Part 2)3. Making a simple clap switch

4. Low-cost thermostat design for cooking appliances

using PIC10F204

tagged with ACS712, current sensor, PIC16F1847

10 COMMENTS

July 18, 2015 5:28 am

The 30A modules you buy from China, are no good, the

solder melts at 8 Amps and open circuits.

Reply

March 7, 2015 2:08 pm

how much power does acs712 consume..??

if i want to check power ratings of my home appliances like

2

37Like

265 17 28

14K

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Zafar

Amit sharma

Ali

Hazem

Hazem

HS

fans ; will acs 712 will be a right option?

Reply

July 17, 2014 9:07 am

i wont calculate current in 230V AC supply with load Current

appox 20-25 amp.so this(ACS712-30A)IC is reliable or not

becos total

POWER = 230*23= 5290watt =5.2KVA

or use CT(Current Transformer)

Reply

March 3, 2014 12:15 pm

Sir,what is the power rating of ACS712-05B current sensor

ic?

Reply

April 19, 2014 7:29 pm

I think 100W

Reply

March 28, 2013 12:59 pm

i will change ACS712-05 by ACS712-20A . plz give me an idea

Reply

March 28, 2013 12:53 pm

plz.you can help me . why maximum current 6.52 ?

thanks for yours answer .

Reply

Pingback: Blog J.Schweiss | AVR Tutorials

August 11, 2012 3:10 am

I am in big trouble, I am on my dead line of project and not

able to measure current. please help me out. My

application is to measure current through 200 ohm to 1000

Mohm resistance, right now I am able to measure 200 ohm

to 20 Mohm but not above than that, help me with this

problem is this IC can help me or any other circuit?

Reply

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