Voltage Measurement

The Voltage Challenge

⚠️ Warning

Microprocessors typically operate at low voltages (3.3V or 5V). Connecting high voltages directly (like 230V AC or 325V DC) would instantly destroy them!

Voltage Division

To safely measure high voltages, we use voltage dividers to scale them down to microprocessor-friendly levels:

Voltage Divider Circuit

💡 Key Points

Voltage division ratio determined by resistor values
Higher R1 provides greater voltage reduction
Power dissipation must be considered in resistor selection
Total resistance affects circuit loading

⚙️ Precision Considerations

Resistor tolerances directly affect measurement accuracy:
- 0.1% tolerance on 64kΩ = ±64Ω variation
- Results in proportional output voltage error
- Can lead to significant measurement inaccuracies
Temperature effects:
- Resistor values drift with temperature
- Use matched temperature coefficients
- Consider temperature compensation in software
Recommendations:
- Use precision resistors (0.1% or better)
- Match temperature coefficients
- Consider calibration in software

⚠️ Safety Warning

During grid isolation (open circuit):
- Voltage divider can act as a voltage source
- Can feed voltage back to grid inverter port pins
- May cause false voltage readings or safety issues
- Consider adding protection circuits for floating conditions

🔋 High Resistance & Power Considerations

Low Resistance Example (Dangerous!)

R1 = 64Ω, R2 = 1Ω

V = 325V

I = V/R = 325V/65Ω = 5A

P = V × I = 325V × 5A = 1,625W! 🔥

Extremely high current flow
Dangerous power dissipation
Components would instantly fail
Risk of fire or explosion

High Resistance Example (Safe)

R1 = 8MΩ (cascade of resistors)

R2 = 125kΩ

I = 325V/8.125MΩ = 0.04mA

P = V × I = 325V × 0.04mA = 13mW ✅

Minimal current flow
Safe power levels
R1 split across multiple resistors:
- 2MΩ + 2MΩ + 2MΩ + 2MΩ
- Prevents voltage arcing
- Better heat distribution