FAA Aviation Mechanic General (AMG)Fundamentals of Electricity and ElectronicsMedium
An AC voltmeter, which by design reads RMS values, displays 120 volts across a circuit. What is the approximate peak voltage of this AC waveform?
- A84.8 volts
- B240 volts
- C120 volts
- D169.7 volts
Show answer & explanationAnswer & explanation
Correct answer: D. 169.7 volts
RMS voltage equals 0.707 times peak voltage, so peak voltage equals RMS divided by 0.707. Calculation: 120V / 0.707 = 169.7 volts.
Why the other options are wrong
- A. This value results from multiplying RMS by 0.707 instead of dividing.
- B. 240V would be double the RMS value, not the correct peak conversion.
- C. 120V is the RMS value, not the peak value.
RMS to Peak Voltage Conversion
RMS (root-mean-square) voltage represents the effective DC-equivalent value of an AC waveform and equals 0.707 times the peak voltage.
- Vrms = 0.707 x Vpeak
- Vpeak = Vrms / 0.707 = 1.414 x Vrms
- Standard AC meters display RMS values by default
Memory trick: 'Peak is Prouder' - peak voltage is always larger (1.414x) than RMS.