The circuit shown in Figure 1 is designed to detect the correct phase sequence of a three-phase AC supply. When the phase sequence is correct, the circuit generates an output signal that activates a relay, thereby enabling the three-phase power to the connected device. However, if the phase sequence is incorrect, the relay remains inactive, ensuring that the power is not supplied and protecting the device from potential damage.
The core component of the circuit is the CD4013 dual D flip-flop. The three-phase AC input is first stepped down and rectified into a low-voltage pulsating DC signal, which is then fed into the A, B, and C terminals of the circuit. The signals at the A and B terminals are conditioned using resistors and Zener diodes to generate clean digital signals for the two D flip-flops. Meanwhile, the C-terminal signal is processed through a differentiator circuit to produce a sharp pulse, which is applied to the reset (R) inputs of both flip-flops.
When the phase sequence is correct, the positive pulses appear in the order of A, B, and C. As illustrated in Figure 2(a), the rising edge of the A signal triggers Q1 to go high. Then, the rising edge of the B signal causes Q2 to transition to a high state. Finally, the rising edge of the C signal generates a sharp pulse at the R terminal, resetting both flip-flops, causing Q1 and Q2 to return to a low level, thus completing one cycle.
Since the three-phase AC signal is periodic, the frequency of the Q2 output matches that of the AC supply. The DC voltage across the C2 capacitor (22 μF) is used to bias a transistor, which in turn activates the relay to switch on the three-phase power to the load. In contrast, if the phase sequence is incorrect, the Q2 output remains low, keeping the transistor off and preventing the relay from engaging. This ensures that no power is supplied to the device, safeguarding it from improper operation.
The waveforms at various points in the circuit are shown in Figure 2(b), providing a clear visual representation of the signal behavior under correct and incorrect phase sequences.
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