The instantaneous-trip function was tested using a 50Hz current waveform with a peak current exceeding the configured trip threshold. The following oscilloscope signals were recorded:

• ADC-I: amplified current-sensing signal at the MCU ADC input
• Shunt-P: voltage measured single-ended at the positive terminal of the shunt, referenced to the local circuit ground
• DRV-IN1: relay-driver enable signal

The current source was operated close to the maximum output capability and produced a visibly distorted waveform. In particular, the current waveform became clipped at high amplitudes. The voltage measured directly at the shunt exhibits the same waveform shape as the amplified ADC signal. This confirms that the observed distortion originated from the test source rather than from the current-sensing analog front end.

Figure 4-3 Current Waveform and Upper and Lower Software Instantaneous-Current Thresholds

The configured software instantaneous-current thresholds are 3.165V and 0.135V. With an ADC midpoint of 1.65V, a 1mΩ shunt, and an amplifier gain of 20.6, these values correspond to a peak-current threshold of:

For a sinusoidal current, the corresponding RMS threshold is:

Figure 4-4 Short Positive Threshold Violation Rejected by the Three-Sample Debounce Qualification

During the positive half-cycle of the second mains period, a short current peak exceeded the upper threshold for approximately 228.4µs. Since the ADC samples the current every 125µs and the instantaneous protection requires three consecutive threshold violations, this transient did not satisfy the trip criterion and was rejected.

Figure 4-5 Time From the Sustained Lower-Threshold Crossing to Disabling DRV_IN1_EN

During the subsequent negative half-cycle, the current remained outside the configured window for sufficiently long to satisfy the three-sample qualification. The measured interval from the selected lower-threshold crossing to disabling of DRV_IN1_EN was 768.6µs. This represents the observed electronic response and includes ADC sample alignment, threshold qualification, firmware processing, and GPIO actuation.

Figure 4-6 Total Time from Threshold Crossing to Complete Interruption of the Load Current

After DRV_IN1_EN was disabled, the current continued briefly while the relay contacts moved. The measured interval from the threshold crossing to complete current interruption was 2.7412ms. The resulting approximate mechanical relay-opening interval after the driver command was therefore:

The complete measured response from the selected threshold crossing to current interruption was approximately 2.74ms. During this test, the sustained negative current exceeded the software instantaneous-current threshold but remained within the hardware comparator window. The trip was therefore initiated by the sample-based software protection rather than the ADC-saturation path. The measured response is well below the 100ms maximum operating-time target used for instantaneous protection.