CT Signal Conversion

Converting CT Current to Voltage

After the CT steps down the current, we still need to convert it to a voltage for our microprocessor. This is done using a precision burden resistor:

Current-to-voltage conversion:
- Burden resistor selection is critical for power efficiency
- Power loss calculation: P = I² × R
- Example with 5A secondary and 1Ω burden:
  - P = 5A² × 1Ω = 25W power loss!
  - This is excessive and wastes energy
- Solutions for different CT types:
  - For 5A secondary CT:
    - Use 10mΩ burden: P = 5A² × 0.01Ω = 0.25W
    - Produces 50mV output (5A × 0.01Ω)
    - Op-amp with gain of 66 amplifies to 3.3V for ADC
    - Low power loss, good signal conditioning
  - For 33.3mA secondary CT:
    - Can use 100Ω burden: P = (0.0333A)² × 100Ω = 0.11W
    - Produces 3.33V output (good for ADC)
    - No additional amplification needed
    - Power loss is acceptable
Burden resistor trade-offs:
- Too large: wastes power, heats up
- Too small: voltage too low to measure (but can be amplified)
- Optimal: matches CT secondary rating and measurement circuit needs

Converting CT output to voltage requires careful consideration: