A donut current transformer is a toroidal instrument transformer that clamps around an AC conductor to step down high primary current into a safe, isolated, and proportional secondary current or voltage for measurement. By passing your main AC line through the center window, this component changes a dangerous, hard-to-measure 100A+ circuit into a safe 5A or millivolt signal without ever breaking the primary wire or exposing your measurement gear to mains voltage. If you are building an energy monitor, wiring a panel meter, or setting up solar production tracking, understanding how to size the core and calculate the burden resistor is the difference between accurate readings and a fried microcontroller.
Spec-Sheet Table: Common Donut CT Ratings
Not all toroidal transformers are built for the same job. The core material, turns ratio, and internal wiring dictate whether a unit is meant for an industrial panel meter or an Arduino breadboard. Below is a reference table of the most common donut current transformers you will encounter in both commercial and DIY applications.
| Model / Type | Primary Rating | Secondary Output | Turns Ratio | Burden Resistor | Primary Application |
|---|---|---|---|---|---|
| YHDC SCT-013-000 | 100A AC | 50mA AC | 2000:1 | External (Required) | ESP32/Arduino DIY Energy Monitors |
| YHDC SCT-013-030 | 30A AC | 1V AC | N/A (Voltage out) | Internal (Built-in) | Plug-and-play appliance logging |
| Standard Panel CT | 100A AC | 5A AC | 20:1 | External (Panel Meter) | Industrial analog ammeters, relays |
| Split-Core 200A (e.g., Emporia) | 200A AC | 333mV AC | N/A (Voltage out) | Internal (Precision) | Smart home panels (Sense, Emporia Vue) |
| PCB Mount Toroid (e.g., Talema) | 15A AC | 15mA AC | 1000:1 | External (PCB trace) | Integrated inverter / UPS control boards |
Assumptions for this table: All primary ratings assume a 50/60Hz sinusoidal AC waveform. Accuracy classes (typically 1% to 3% for these hobbyist/light-commercial grades) degrade if the primary wire is not centered in the donut window or if the primary current is below 10% of the rated maximum.
The Math: Turns Ratios and Burden Resistor Sizing
The most common mistake makers and junior technicians make is wiring a current-output donut transformer directly into a microcontroller's Analog-to-Digital Converter (ADC). Microcontrollers read voltage, not current. To convert the secondary current into a measurable voltage, you must install a burden resistor across the secondary terminals.
Let us walk through a concrete numeric example using the most popular DIY component: the YHDC SCT-013-000 paired with an ESP32 DevKit v1.
Step 1: Define the Parameters
- Primary Current (I_p): 100A RMS (Maximum expected load)
- Secondary Current (I_s): 50mA RMS (at max primary)
- ESP32 ADC Reference Voltage: 3.3V
Step 2: Calculate Peak Values
AC waveforms are sinusoidal. The ESP32 ADC will read the peak voltage, not the RMS voltage. Furthermore, because the ESP32 cannot read negative voltages, we must bias the signal at half the reference voltage (1.65V). This means our maximum allowable peak voltage swing above the bias is 1.65V.
- Peak Secondary Current (I_peak) = I_s × √2
- I_peak = 0.050A × 1.414 = 0.0707A (70.7mA)
Step 3: Calculate the Burden Resistor
Using Ohm's Law (R = V / I):
- R = V_peak / I_peak
- R = 1.65V / 0.0707A = 23.33 Ω
Step 4: The DC Bias Network
To center that 22Ω burden resistor signal at 1.65V, you need a voltage divider. Connect two 470kΩ resistors in series across the ESP32's 3.3V and GND pins. Tap the midpoint to one side of the CT output. Place a 10µF electrolytic capacitor across the 470kΩ resistors to filter out power supply noise. The other side of the CT goes to your ADC pin (e.g., GPIO 34). For complete wiring diagrams and calibration code, the OpenEnergyMonitor project remains the gold standard reference for DIY CT math.
Where You Meet This in Practice
You will rarely see a bare donut transformer on a finished product's exterior, but they are the hidden workhorses of modern power monitoring. Here is where they dictate system behavior in real installations:
- Smart Home Energy Monitors: Systems like the Emporia Vue or Sense use arrays of split-core donut transformers (usually rated for 50A to 200A with internal burden resistors outputting 333mV). They clamp onto individual branch circuits in your breaker panel to provide appliance-level disaggregation.
- Solar Inverters and Grid Export: Hybrid inverters (like those from Sol-Ark or Victron) use CTs clamped to the main utility feeder. The inverter reads this secondary signal to determine if the house is pulling from the grid or pushing back, adjusting its internal relays to maintain zero-export if required by the utility.
- Motor Overload Protection: In industrial VFD (Variable Frequency Drive) setups, 5A secondary donut CTs feed into thermal overload relays. If a 50HP motor jams and pulls 150A, the CT outputs 7.5A to the relay, tripping the contactor in milliseconds to prevent the motor windings from melting.
Common Confusions and Safety Mistakes
When specifying or troubleshooting these components, three specific confusions lead to blown boards or inaccurate data.
CT vs. Shunt Resistor
People often confuse donut current transformers with shunt resistors. A shunt is a low-value precision resistor placed in series with the load. You must break the circuit to install a shunt, and it provides no galvanic isolation—your microcontroller shares a ground with the high-voltage load. A donut CT clamps around the wire, requires no circuit interruption, and provides complete isolation via magnetic coupling.
CT vs. Hall Effect Sensor
Hall effect sensors (like the ACS712) also clamp around or measure current without breaking the wire, but they operate on magnetic field sensing rather than inductive coupling. The critical difference? A donut current transformer only works with AC. If you try to measure a DC battery bank or solar panel string with a standard donut CT, the secondary output will be exactly zero. For DC, you must use a Hall effect sensor or a DC shunt.
A current transformer is essentially a step-up voltage transformer. If the primary conductor is carrying current and the secondary terminals are left open (no burden resistor or meter connected), the core will saturate. This induces a massive voltage spike across the secondary terminals—often reaching several thousand volts. This will arc across the terminals, permanently destroy the core's insulation, and pose a severe lethal shock hazard. Always short the secondary terminals with a jumper wire before removing a meter or burden resistor from a live circuit. For more on instrument transformer safety, refer to Fluke's electrical safety guides.
The 'Orientation and Grouping' Mistake
If you clamp a donut CT around a standard 120V NM-B (Romex) cable containing both the hot and neutral wires, the meter will read zero. The magnetic fields from the outgoing hot and returning neutral perfectly cancel each other out. You must separate the conductors and clamp the CT only around the hot (line) conductor. Similarly, if measuring a 240V split-phase circuit, you can clamp around both hot legs only if they are running in opposite directions through the window (one entering top, one entering bottom); otherwise, pass only one hot leg through the core.
