Brigadoon - An Occasional Corner on the Internet
Current Sensor: ACS712 - 20A
Designed by Mark Little
Pin Function
VCC Supply Voltage to the Senor.
GND Ground or Earth connection for the supply voltage and the sensor output
OUT Analogue Output - The analogue output proportional to the current.
A Hall effect sensor is a transducer with a varying output voltage in response to a magnetic field.
In a Hall effect sensor a thin strip of metal has a current applied along it, in the presence of a magnetic field the electrons are deflected towards one edge of the metal strip, producing a voltage gradient across the short-side of the strip (perpendicular to the feed current). Hall effect sensors have the advantage that they can detect static (non-changing) magnetic fields.
If the magnetic field is produced by a current flow, the output of the sensor will be proportional to the current flowing through the wire.

For the ACS712 20A bi-directional current sensor (available on eBay), the following test setup was used and a transfer table produced. It can be used to determine what output readings correspond to the input current.
-10.0A 3.550V
-9.0A 3.453V
-8.0A 3.351V
-7.0A 3.251V
-6.0A 3.150V
-5.0A 3.049V
-4.0A 2.948V
-3.0A 2.848V
-2.0A 2.748V
-1.0A 2.650V
0.0A 2.546V
+1.0A 2.446V
+2.0A 2.344V
+3.0A 2.443V
+4.0A 2.141V
+5.0A 2.039V
+6.0A 1.938V
+7.0A 1.835A
+8.0A 1.732A
+9.0A 1.630V
+10.A 1.529V
The +5V supply is used to power the sensor, while the other power supply is used to provide current to the current sensor. In this setup, both power supplies are QW-MS3010D units, The power supply connected to the sensor inputs is set so that its current regulator sets the current to the levels to be measured. As this is a unipolar power supply, the leads must be reversed to get the readings for the opposite polarity current.
The graph show that the output voltage falls as the current flow becomes more positive (+ve), but by  reversing the initial connection of the current being monitored, the output voltage can be made to increase as the current becomes more positive. From the figures above, it can be calculated that the output voltage of the sensor can be approximated by the following formula:
Vout = 2.546V - Iinput * 0.1004
It should be noted that the readings around 0A were not as stable as when the input current was higher, which will introduce some variations between the measured values and the calculated ones.. The reason for this was not investigated, as its impact is not significant for the projects expected to use this sensor. However, it may be that the stray magnetic fields in the test area  influenced the sensor when there was no magnetic field caused by the input current.
Iinput = (2.546V - Vout ) / 0.1004
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