# ACS712 + Operational Amplifier

Hello

I am using Allegro current sensor ACS712 for current measuring. But my current is really low, from 10mA to 50mA. So when i work with the normal resolution of the sensor (185 mV/A) I can’t get good readings at all. To amplify the current I am using a MCP6002 OPAMP.

I am using the following circuit (attached) , available on page #12 of the datasheet of the ACS712. The circuit increases the gain to 610 mV/A from 185 mV/A for a +/-5A sensor (in the datasheet, it’s part number is ACS712ELCTR-05B-T).

The gain of 610 mV/A would mean that when input current is +5A then the output voltage, Vout, is 3.05V, right?

From the circuit, it looks like that the op-amp is operating in inverting configuration therefore I think when the input current is +5A then Vout is going to be -3.05V. Do I have it correct? If I’m correct then another op-amp in inverting configuration with unity gain is required to make the Vout positive. Correct?

The gain can be made more than 610 mV/A by changing the values of R3 and R_F. The given values set the gain at 3.3 so to increase the gain to, say, 5, the values of the resistors can be adjusted. If the gain is 5 then when input current is +5A, then Vout would be 0.925x5=4.63V. Do I have it right?

The circuit uses the op-amp LM321, can I use CMP6002 instead without affecting the gain?

Thank you very much arthuki:
The gain of 610 mV/A would mean that when input current is +5A then the output voltage, Vout, is 3.05V, right?

The circuit uses the op-amp LM321, can I use CMP6002 instead without affecting the gain?

Not quite, OPA is biased about 2.5V on the input 1, so if current =0 OPA has 2.5V at the output. When current flows in one or another direction, voltage would swing up or down depends on its direction. Your math seems right, and with 5A OPA would go into saturation, clipping output at +5V or 0V.

arthuki:
The gain of 610 mV/A would mean that when input current is +5A then the output voltage, Vout, is 3.05V, right?

That circuit outputs 2.5volt VCC/2 with zero current.
5Amp would output 5*0.610 + 2.5 = 5.55volt. Or -55volt with opposite current through the sensor.
Out of range.

10mA resolution is too small for a noisy hall sensor.
What are you trying to measure.
Leo..

Hi,thank you for the answers. it is a simple project for measuring the current through a 100 ohm resistor using a power supply of 5V. By Ohm's law I would have a current of 5/100 = 50mA.

I am using Arduino to read the voltage output and then know the current through the resistor

arthuki:
Hello

I am using Allegro current sensor ACS712 for current measuring. But my current is really low, from 10mA to 50mA. So when i work with the normal resolution of the sensor (185 mV/A) I can’t get good readings at all. To amplify the current I am using a MCP6002 OPAMP.

That’s not the right sensor for low currents, its a Hall sensor, you will be detecting the
earth’s magnetic field and nearby stray magnetic fields as much as the current. Hall sensors like
the ACS712 work with high currents that generate a large magnetic field near the copper conducting strip
inside the device, 50mA is not enough.

For low current use a shunt to measure current.