Interfacing ACS712 Electrical / Current with Arduino Uno

How the ACS712 measures current

The measured current flows through a low-resistance copper conductor inside the IC. Its magnetic field is sensed by a Hall element, which produces a proportional analog voltage on OUT. The high-current conductor and signal pins are galvanically isolated inside the IC, but the safety of a finished module also depends on its PCB layout, terminal spacing, insulation and workmanship.

At zero current, OUT is nominally half the supply voltage (about 2.5 V with a 5 V supply). Current in one direction raises the output; reversing the current lowers it. The output is not a digital ampere value: the microcontroller must sample the voltage and calculate current using the sensitivity of the exact variant.

Calculate current from the output voltage

For a 5 V supply, use the following as a starting model:

Current (A) = (VOUT − VZERO) / sensitivity (V/A)

Variant	Sensitivity for the formula
ACS712-05B	0.185 V/A
ACS712-20A	0.100 V/A
ACS712-30A	0.066 V/A

VZERO should be measured on your actual module with no current flowing. Do not assume it is exactly 2.500 V: offset, supply voltage, ADC reference and noise all affect the result. Averaging several samples at zero current improves the baseline. These values are typical datasheet sensitivities, not a substitute for calibration against a known current.

DC current versus AC current

For DC current, read OUT, convert the ADC reading to voltage, subtract the calibrated zero-current voltage and divide by the variant sensitivity. The sign indicates current direction; a small negative or fluctuating reading at zero current is usually offset and noise.

For AC current, the signal swings around its zero-current midpoint. Sampling only once per cycle or using the DC formula on one sample does not give AC RMS current. Sample fast enough to capture the waveform, remove the measured midpoint, calculate the root-mean-square of the samples and calibrate the result against a known AC current. The ADC sampling rate, analog filtering and waveform frequency determine usable accuracy.

Where it is useful

• DC load monitoring: measure current drawn by a battery-powered load within the selected module's range.
• Motor experiments: observe changes in motor current as load or speed changes.
• AC load measurement: sample the waveform and calculate RMS current in software.
• Overcurrent experiments: compare measured current with a threshold and trigger a warning or shutdown in the controller.
• Energy-monitoring prototypes: pair current measurements with a separately measured voltage if power is required.

The ACS712 measures current, not voltage or energy by itself. Accurate power measurement needs voltage measurement and suitable sampling/calibration as well.

Troubleshooting

Symptom	Likely cause	What to check
Current is not near zero with no load current	Zero-current offset, noise or ADC scaling error	Measure OUT with no current flowing and update ZERO_CURRENT_V; average samples.
Current magnitude is wrong by a consistent factor	Wrong sensitivity constant or ADC reference	Confirm the 5 A / 20 A / 30 A variant and match the code to it.
Reading changes in the wrong direction	Current direction is reversed	Check whether current flows IP+ to IP−; reverse the high-current path if appropriate.
ESP32 readings clip or the board resets	OUT exceeds the ADC's permitted voltage or wiring is noisy	Add a correctly calculated voltage divider, share signal ground and keep wiring short.
AC reading is unstable or too low	Sampling too slowly, wrong RMS calculation or poor calibration	Sample the waveform at a suitable rate, remove the midpoint and calculate RMS.
Output is noisy near a motor	Switching noise and magnetic interference	Separate signal wiring from motor leads, use appropriate filtering and average samples.

Frequently asked questions

Can the ACS712 measure both AC and DC? Yes. Its analog output changes with current in either direction. DC is calculated from the offset; AC current requires waveform sampling and an RMS calculation for an RMS value.

Does OUT read 0 V when no current flows? No. With a 5 V supply, it is nominally near 2.5 V. Calibrate the actual zero-current value on your module.

Can I connect OUT directly to an ESP32? Do not assume it is safe. The ACS712 is powered from 5 V and its output can exceed 3.3 V, depending on variant and current. Use a properly designed divider or suitable signal-conditioning stage so the full possible output remains within the ADC limits.

Which variant should I choose? Choose the lowest range that safely covers the expected peak current. The ±5 A variant has the highest sensitivity; the ±30 A variant covers more current but produces a smaller voltage change per amp.

Does it provide watts or energy directly? No. It measures current. Power requires voltage measurement too, and energy requires integrating power over time.

Is the module safe for mains? Do not judge safety by the IC's isolation rating alone. The complete board, terminals, spacing, enclosure and installation must be appropriately rated.