Question 52 CEL02 - Chief Engineer - Limited (Alt)

The Correct Answer is D **Why option D ("a short") is correct:** A semiconductor diode is designed to allow current flow easily in the forward bias direction (low resistance) and block current flow significantly in the reverse bias direction (high resistance). An ohmmeter test measures the resistance across the diode when the leads are swapped (forward and reverse bias). Finding a **low resistance in *both*** the forward and reverse directions indicates that the diode is no longer functioning as a unidirectional gate; current is flowing freely in both directions. This lack of resistance to reverse current flow signifies that the junction has failed and effectively created a continuous, low-resistance path (a short circuit) through the device. **Why the other options are incorrect:** * **A) good resistive quality:** A diode with good quality would show a low resistance in one direction (forward bias) and a very high (ideally infinite) resistance in the other direction (reverse bias). Finding low resistance in both directions indicates poor, not good, resistive quality. * **B) an open:** An "open" diode means the circuit is permanently broken. If the diode were open, it would show a very high resistance (often infinite) in *both* the forward and reverse directions, as no current would be able to flow at all. * **C) good capacitive quality:** While diodes do exhibit some inherent capacitance, the primary function and the most indicative failure mode tested by an ohmmeter is its ability to rectify (allow current in one direction only). The ohmmeter test focuses on the resistive state of the junction. Furthermore, an ohmmeter test that shows low resistance in both directions is a clear indication of a resistive fault (a short), not a confirmation of good capacitive quality.