Free Current Transformer Burden Resistor Calculator
Size burden resistors (Rburden) for split-core and solid-core CT sensors. Match AC secondary current to micro-controller ADC input ranges and prevent core saturation.
⚡ CT Sensor & ADC Target Parameters
For single-supply microcontrollers (0V to 3.3V), bias the burden resistor to mid-rail (1.65V) using a dual 10kΩ voltage divider and a 10μF decoupling capacitor to allow symmetrical AC swing without negative rail distortion.
Secondary Current at 100A: 50.0 mA RMS (70.7 mA Peak)
• NEVER OPEN-CIRCUIT A LIVE CT: Operating a current transformer without a burden resistor creates lethal multi-kilovolt arc flash pulses across secondary terminals.
• TVS Diode Protection: Place a bidirectional 3.3V or 5.0V TVS diode across $R_{burden}$ to protect ADC from lightning/inrush spikes.
Low-Current Accuracy Tip: If measuring a small 10A appliance with a 100A CT, loop the live conductor 5 times through the CT core. This multiplies measured current by 5× (50A apparent), increasing low-load ADC digital resolution.
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How Current Transformers and Burden Resistors Work
A Current Transformer (CT) behaves as a near-ideal AC current source on its secondary winding. The secondary current is proportional to primary current divided by turns:
Secondary Peak Current: I_sec(pk) = I_sec × √2
Allowed Peak Voltage: V_pk = (V_ADC_range / 2) × Headroom_Factor
Burden Resistor: R_burden = V_pk / I_sec(pk)
Power Dissipation: P_burden = I_sec² × R_burden
Why the Burden Value Must Remain Small
As burden resistance increases, the required secondary voltage rises proportionally ($V = I cdot R$). If $R_{burden}$ is chosen too large, the required terminal voltage exceeds the magnetic core's saturation knee point ($V_k$). The core saturates, severely distorting the waveform, truncating current peaks, and causing phase lag measurement errors.
Frequently Asked Questions
Why do some CTs specify "333 mV" instead of a turns ratio?
333 mV CTs contain an internal factory-installed burden resistor inside the plastic moulding. They output a calibrated 333 mV AC RMS signal at rated current, eliminating the need for an external burden resistor.
What resistor tolerance should I use for a CT burden?
Always use 1% or 0.1% Metal Film resistors with low temperature coefficients (≤ 25 ppm/°C). Standard 5% carbon film resistors drift significantly with ambient cabinet temperature, causing power measurement calibration errors.
How does the microcontroller sample AC current?
Because microcontrollers cannot accept negative voltages, the CT secondary AC sine wave is offset by a DC bias voltage (e.g. +1.65V). The ADC software samples at high speed (e.g. 1000 Hz), calculates the Root Mean Square (RMS) sum of the squared deviations from the DC midpoint, and multiplies by the CT calibration factor.