Power Electronics Circuits
When AC is applied to a single-phase bridge rectifier made of four diodes, which of the following correctly describes the path of the current during one half cycle?
- AAll four diodes conduct in parallel and share the current, so each diode carries a quarter of the load current
- BOnly one diode conducts, and the other three carry no current in either half cycle
- CTwo diagonally opposite diodes conduct in series, and the current always flows through the load in the same direction
Show answer and explanation
Answer: C. Two diagonally opposite diodes conduct in series, and the current always flows through the load in the same direction
In a bridge rectifier, the two pairs of diagonally opposite diodes conduct alternately in the positive and negative half cycles of the AC. In both half cycles the current flows through the load in the same direction, so the output has a full-wave rectified waveform. The current always passes through two diodes in series, so silicon diodes drop about 2 × 0.7 V = 1.4 V. A full-wave rectifier with a center-tapped transformer needs only two diodes but requires two secondary windings.
- A. The four diodes never all conduct in parallel. The conducting pair changes between the positive and negative half cycles.
- B. Only one diode conducting is how a half-wave rectifier works, and no current then flows in the negative half cycle.
- C. Correct. Pairs of two conduct alternately in series, so the forward-voltage drop is also that of two diodes.