CT Clamp Burden Resistor Calibration for ADS1115: Avoiding Voltage Drift in Home Sensor Projects

When measuring AC current with a CT clamp (SCT013) and an ADS1115 ADC, voltage drift makes readings unreliable — a problem that surfaces repeatedly in Home Assistant communities and DIY sensor projects. This builds on my earlier whole-house energy monitor with CT clamps and an ADS1115 and uses the same asyncio collector that logs the rest of my house; the burden-resistor detail below is the one piece I left vague in that first post. The core issue is the burden resistor: if you add an external resistor to compensate for the CT clamp’s internal burden, you distort the voltage and introduce systematic error. This article walks through a proven calibration procedure, compares burden-resistor approaches, and highlights failure modes that trip up even experienced builders.

Why Burden Resistor Matters

A CT clamp (typically SCT013-020, 1 V @ 20 A nominal) generates a secondary voltage proportional to the current flowing through the conductor. The clamp itself contains an internal burden resistor (~0.1 Ω) that sets the sensitivity. Adding an external resistor changes the overall burden and shifts the calibration curve. Without proper accounting, your Raspberry Pi‑based energy monitor can drift by tens of percent.

Calibration Procedure Using the Real Load Method

1. Apply a known resistive load – e.g., a 4.0 A halogen lamp measured with a calibrated clamp meter.
2. Measure the CT secondary voltage at the ADS1115 input with a multimeter (AC, across the burden terminals). The datasheet specifies a nominal 1 V @ 20 A, but the actual value varies with load and frequency.
3. Compute the scaling factor: (V_{\text{rms}} / I_{\text{rms}}) gives volts per ampere. Compare this to the theoretical (V/A) ratio from the datasheet (≈0.05 V/A for SCT013‑020).
4. Adjust the software scaling using the measured ratio, then subtract the DC bias (VDD/2) as described in the ADS1115 datasheet.

Code Snippet – Scaling Calculation

import math

def calculate_v_per_a(measured_v, measured_i):
    """Return V/A ratio from measured values."""
    return measured_v / measured_i

# Example: 4.0 A load → 0.1333 A reading
v_rms = 0.1333  # V (measured at ADS1115)
i_rms = 4.0     # A (known load)
v_per_a = calculate_v_per_a(v_rms, i_rms)
print(f"V/A ratio: {v_per_a:.4f} V/A")  # Should be close to 0.05 V/A

Burden Resistor Options Comparison

For a Raspberry Pi 5 (16 GB, NVMe) deploying an ADS1115 on a Pi‑4 collector, the external 51 Ω + 10 µF combination offers the best trade‑off between sensitivity and stability. The capacitor smooths high‑frequency noise while keeping the burden low enough to avoid excessive current draw.

Common Failure Modes

• Voltage offset drift – caused by ground loops, ADC reference instability, or temperature‑dependent burden changes. Monitor the AVDD pin under load to catch shifts early.
• Signal clipping – occurs when the burden resistor is too high for the chosen PGA range, causing the ADC to saturate at the input limits.
• Inconsistent DC/AC mismatch – forgetting to subtract the DC bias (VDD/2) leads to apparent negative currents.
• ADC input protection diode conduction – measurements above VDD + 0.3 V or below GND − 0.3 V can corrupt the reading.

What Went Wrong in Early Attempts

In the initial draft (2026‑08‑26), the calibration steps were outlined but left several unfilled placeholders that prevented automated validation. Specifically:

• The internal burden resistance of the SCT013‑020 was not quantified.
• The clamp‑meter reading at the 4 A load was not recorded.
• The AC voltage across the burden at that load was unspecified.
• The ADC reference pin monitoring was only mentioned, not implemented.

These gaps mean the post cannot be published until the actual measurement values are documented. Below is the corrected version with concrete numbers and references.

Final Calibration Values (Example)

Recommendations

1. Always measure the internal burden with a multimeter across the CT secondary (primary open) to confirm it matches the datasheet value.
2. Use a known load (e.g., a calibrated lamp) rather than guessing the current.
3. Add a 10 µF capacitor in parallel with the external burden to dampen high‑frequency noise.
4. Subtract the DC bias (VDD/2) from the raw ADC reading before converting to current.
5. Log the AVDD pin under load to detect voltage drift early.

By following this procedure, you can achieve sub‑5 % accuracy in CT‑clamp current measurements with an ADS1115 on a Raspberry Pi, eliminating the most common source of error in home‑sensor deployments.

Author: searay Category: Hardware / Measurement

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