2nd version with alarm and overload indicator

The 2nd version firmware has been developed featuring the alarm and overload signal input indicators. Figure 9 shows the power on text message.

Figure 9: Power on text message, "Power Line 3rd Harmonic Meter".

As shown below, the first line is the amplitude of the fundamental frequency (50Hz). And the 2nd line is the 3rd harmonic amplitude. The Harmonic Distortion (HD) is displayed as HD=7.3%. When the distortion factor is over 5%, (as shown 7.3%) the symbol X will blink.

Figure 10: The X symbol indicates the 3rd harmonic distortion is over 5%.

If the signal input is overload, resulting the clipped waveform, the ov symbol will be indicated. We can slightly adjust the POT until the ov disappeared.

Figure 11: The ov symbol indicates the signal input is overload.

The signal input for DFT calculation can be only positive cycle (symmetry property), so we can use the bridge diode to provide full wave rectified signal and the voltage divider to provide proper level for ADC input.

Figure 12: The small transformer provides safety isolation for student and the 9VAC input signal.	Figure 13: The front end circuit is built with T1, isolation transformer, D1 bridge diode and R1 variable resistor. R1 is simple voltage divider providing 0-5V full-wave signal.