I’m working on a voltage stabilizer project using a Nuvoton MS51FB9AE microcontroller, and I’m confused about how my input voltage measurement circuit handles a range of 240V AC to 300V AC, 50 Hz. The circuit works fine, but I need help understanding its operation and calibrating it accurately for RMS voltage calculation.
Voltage Scaling
Input Voltage: 300V AC RMS, 50 Hz, with a peak voltage of
300 x √2 = approx 424V
Initial Divider (R1 and R2):
- R1 = 1MΩ and R2 = 100kΩ form a voltage divider before D2.
- Voltage across R2:
100k/1M + 100k x 424V = 38.5V peak.
- D2 (1N4007) rectifies this to half-wave, reducing the peak to ~38.5V (minus ~0.7V forward drop) ≈ 37.8V peak at the cathode.
ADC Input Node (After R2, C1, C2, R3): The ADC input (Pin 20) sees this 37.8V peak, which exceeds the MS51FB9AE’s 5V ADC range.
Questions -
- Why does the circuit correctly read up to 300 V AC without damaging the microcontroller? What design or protection mechanisms are in place that allow the microcontroller to safely measure high-voltage AC signals, such as 230–300 V, without getting damaged?
- How can I accurately measure AC voltage in firmware?
- Should I use the RMS formula for precise measurement? How many samples are enough for accurate voltage measurement ? What should be the logic for reading the AC voltage ?
- What is the most efficient and reliable method to ensure consistent AC voltage readings across all units—such that if one unit displays 240 V, every other unit also shows 240 V with minimal variation? should be same for even 100 units as well.
- To achieve this level of consistency, do I need to implement a calibration routine during production or in firmware?
