PCB mount current transformers (CTs) are compact, low-cost current sensors used in power supplies, motor drives, battery management systems and metering. Unlike hall-effect sensors, passive CTs require no external power and provide galvanic isolation between the measured circuit and the control board.
This guide covers how PCB CTs work, key design parameters (turns ratio, burden resistor, accuracy class), selection criteria and common applications.
Shaanxi SiRui Technology Co., Ltd. manufactures PCB mount current transformers and miniaturized current sensors in toroidal and split-core configurations. Contact: sales@sirui-tech.com | Web: www.sirui-tech.com
1. How a PCB Current Transformer Works
A current transformer steps down high AC current on a conductor to a measurable low current. The primary winding is usually a single turn (the PCB trace or a wire through the hole). The secondary has many turns on a high-permeability toroidal core. The current ratio is I_primary / I_secondary = N_secondary / 1.
The secondary current is converted to a voltage by a burden resistor (R_b): V_out = I_secondary × R_b.
- Passive: no supply voltage needed
- Isolation: measured circuit isolated from the ADC
- AC only: typical range 50 Hz – 200 kHz
- No insertion loss: primary is just a trace or wire
2. Key Design Parameters
- Turns ratio: typical 1:100, 1:200, 1:500, 1:1000. Higher ratio = lower secondary current but more noise sensitivity
- Burden resistor: sets output voltage; choose R_b so V_out at max current matches the ADC range. Too high drives CT into saturation
- Accuracy class: 0.1 (precision metering), 0.5 (industrial), 1.0 (protection/monitoring), 3.0 (rough sensing)
- Bandwidth: small PCB CTs work 50 Hz – ~100 kHz
3. Core Materials
Ferrite (Mn-Zn) is the standard for PCB CTs. High permeability (2,000–10,000), low cost, good from 20 kHz to 1 MHz. Toroid cores are small (5–25 mm OD).
Nanocrystalline is used for precision CTs at 50/60 Hz metering. Higher permeability means better accuracy and smaller core for line-frequency currents.
4. Design Steps
- Determine primary current range (max RMS current)
- Choose turns ratio matching desired secondary current
- Calculate burden resistor: R_b = V_max / I_secondary_max
- Check saturation at peak current
- Verify bandwidth covers measured frequency
- Layout: keep CT close to ADC, minimize loop area
5. Application Reference
| Application | Frequency | Current | Accuracy |
|---|---|---|---|
| SMPS current sensing | 50–500 kHz | 1–20 A | Class 1 |
| Motor drive phase current | 50 Hz–20 kHz | 5–50 A | Class 0.5–1 |
| Energy metering | 50/60 Hz | 5–100 A | Class 0.1–0.5 |
| Leakage detection | DC–60 Hz | mA level | High sensitivity |
6. Sirui-Tech PCB CT Capabilities
- PCB mount toroidal CTs: OD 5–25 mm, ratios 1:100 to 1:2000
- Miniature current transformers: for space-constrained applications
- Custom turns ratios: wound to specification
- Nanocrystalline CTs: for precision 50/60 Hz metering
Conclusion
PCB current transformers are simple, passive, isolated current sensors. Choose ferrite for high-frequency power conversion, nanocrystalline for precision line-frequency metering. Size the burden resistor correctly to avoid saturation.
Need a PCB current transformer? Contact Sirui-Tech at sales@sirui-tech.com with your current range, frequency band and required accuracy class.