ACS712

Library for the ACS712 Current Sensor - 5A, 20A, 30A.

Description

The ACS712 is a chip to measure current, both AC or DC. The chip has an analogue output that provides a voltage that is linear with the current. The ACS712 library supports only a built in ADC by means of analogRead(). There are 2 core functions:

• int mA_DC()
• int mA_AC(float freq = 50) The frequency can be set to typically 50 or 60 Hz however other values e.g. 50.1 or 40 or 123.456 are possible.

To measure DC current a single analogRead() with conversion maths is sufficient to get a value. To stabilize the signal analogRead() is called twice.

To measure AC current a blocking loop for 20 milliseconds (50 Hz assumed) is run to determine the peak to peak value which is converted to the RMS value. To convert the peak2peak value to RMS one need the so called crest or form factor. This factor depends heavily on the signal form. For a perfect sinus the value is sqrt(2)/2 == 1/sqrt(2). See Form factor below.

Tests

The library is at least confirmed to work with:

Please let me know of other working platforms.

Interface

Base

• ACS712(uint8_t analogPin, float volts = 5.0, uint16_t maxADC = 1023, float mVperAmpere = 100) constructor. It defaults a 20 A type sensor, which is defined by the default value of mVperAmpere. See table below. Volts is the voltage used by the (Arduino) internal ADC. maxADC is the maximum output of the internal ADC. The defaults are based upon an Arduino UNO. These two ADC parameters are needed to calculate the voltage output of the ACS712 sensor.
• int mA_AC(float frequency = 50) blocks ~21 ms (depending on the frequency) to sample a whole 50 or 60 Hz period.
  Since version 0.2.2 frequencies other integer values than 50 and 60 are supported, the lower the frequency, the longer the blocking period. Since version 0.2.3 floating point frequencies are supported to tune even better.
• int mA_DC() blocks < 1 ms (Arduino UNO) as it calls analogRead() twice. A negative value indicates the current flows in the other direction.

Resolution

Midpoint

• void setMidPoint(uint16_t midPoint) sets midpoint for the ADC conversion.
• void autoMidPoint(float frequency = 50) Auto midPoint, assuming zero DC current or any AC current. Note it will block for 2 periods. Since version 0.2.2 frequencies other than 50 and 60 are supported. By setting the frequency to e.g 1, the code will sample for 2 seconds, possibly getting a better average.
• uint16_t getMidPoint() read the value set / determined.
• void incMidPoint() manual increase midpoint, e.g. useful to manually adjust the midPoint in an interactive application.
• void decMidPoint() manual decrease midpoint.

Form factor

Also known as crest factor; affects AC signals only.

• void setFormFactor(float formFactor = ACS712_FF_SINUS) manually sets form factor. Must typical be between 0.0 and 1.0, see constants below.
• float getFormFactor() returns current form factor.

The library has a number of predefined form factors:

It is important to measure the current with a calibrated multimeter and determine / verify the form factor of the signal. This can help to improve the quality of your measurements.

Please let me know if other crest factors need to be added.

Noise

Default = 21 mV.

• void setNoisemV(uint8_t noisemV = 21) set noise level, is used to determine zero level e.g. in AC measurements.
• uint8_t getNoisemV() returns the set value.

mV per Ampere

Used for both for AC and DC measurements. Its value is defined in the constructor and depends on type sensor used. These functions allow to adjust this setting run-time.

• void setmVperAmp(float mVperAmpere) sets the milliVolt per Ampere measured.
• float getmVperAmp() returns the set value.

Typical values see "Resolution" section above, and the "voltage divider" section below.

Frequency detection

Experimental functionality for AC signal only!

• float detectFrequency(float minimalFrequency = 40) Detect the frequency of the AC signal.
• void setMicrosAdjust(float factor = 1.0) adjusts the timing of micros in detectFrequency(). Values are typical around 1.0 ± 1%
• float getMicrosAdjust() returns the set factor.

The minimum frequency of 40 Hz is used to sample for enough time to find the minimum and maximum for 50 and 60 Hz signals. Thereafter the signal is sampled 10 cycles to minimize the variation of the frequency.

The microsAdjust() is to adjust the timing of micros(). This function is only useful if one has a good reference source like a calibrated function generator to find the factor to adjust. Testing with my UNO I got a factor 0.9986.

Current version is experimental and not performance optimized.

Voltage divider

As per issue #15 in which an ACS712 was connected via a voltage divider to the ADC of an ESP32.

Schema

ACS712 ----[ R1 ]----o----[ R2 ]---- GND
                     |
                     |
                ADC of processor

The voltage divider gave an error of about a factor 2 as all voltages were divided, including the "offset" from the zero current level.

By adjusting the mV per Ampere with setmVperAmp(float mva) the readings can be corrected for this "voltage divider effect".

Examples:

For a 20 A type sensor, 100 mV/A would be the normal value. After using a voltage divider one need to adjust the mVperAmp.

Note: setting the midPoint correctly is needed when using a voltage divider.

Operation

With the constructor the parameters volts and maxADC (== steps-1) of the ADC are set together with the milliVolt per Ampere value. The last parameter can be adjusted afterwards, e.g. to calibrate this value runtime. Note this parameter affects both AC and DC measurements.

To calibrate the zero level for DC measurements, 5 functions are available to adjust the midPoint.

To calibrate the RMS value for AC measurements, 2 functions are available to get and set the formFactor.

To calibrate the noise level (used for AC measurements), 2 functions are available to get and set the noise in mV.

The examples show the basic working of the functions.

Future

Should

• return types (0.4.0)
  • float for mA_AC() and mA_DC()
  • actual value for midPoint() functions instead of void.
• investigate blocking calls:
  • mA_AC() blocks for about 20 ms at 50 Hz. This might affect task scheduling on a ESP32. Needs to be investigated. Probably need a separate thread that wakes up when new analogRead is available?
  • detectFrequency(float) blocks pretty long.
• investigate support for micro-Amperes. ACS.uA_DC()

Could

• do we need a int point2point(float frequency) function for AC. Is technically a part of mA_AC() already.
  Needs extra global variables, which are slower than local ones
  Or just cache the last p2p value?

Won't

• external analogue read support? separate class!