Question 51 CEL02 - Chief Engineer - Limited (Alt)

The Correct Answer is A ### Explanation of Correctness (Option A: 100:1) When using an analog ohmmeter (or a DMM in resistance mode, though analog meters often use a higher voltage) to test the forward-to-reverse (front-to-back) resistance ratio of a standard silicon PN junction diode: 1. **Forward Bias:** The ohmmeter's leads are connected to allow current to flow easily (forward bias). The meter measures a low resistance (typically tens to hundreds of ohms, depending on the meter's range and internal battery). 2. **Reverse Bias:** The leads are reversed, blocking the current flow (reverse bias). The meter measures a very high resistance, often appearing as "infinity" or "OL" on a digital meter, or megohms on an analog meter. 3. **Ratio:** The ratio is calculated as $R_{reverse} / R_{forward}$. A ratio of **100:1** (or greater, up to perhaps 500:1 for general-purpose diodes) is the minimum commonly accepted standard in introductory electronics and troubleshooting to confirm a good diode. $100:1$ is the most conservative and typical answer expected in a multiple-choice setting for the "roughly" expected ratio. ### Why Other Options Are Incorrect **B) 500:1:** While a ratio of 500:1 is certainly possible and often seen in high-quality or specific power diodes, it is not the *minimum* or *typical benchmark* used for the generic "roughly what ratio" assessment. 100:1 is the lower limit used to define a healthy diode response. **C) 1000:1:** A 1000:1 ratio implies that the reverse resistance is exceptionally high relative to the forward resistance. This level of ratio is achievable, especially when using sensitive measuring equipment and testing specific rectifier diodes, but it is less common as the generalized minimum standard compared to 100:1. **D) 5000:1:** A ratio of 5000:1 is achievable only with highly sensitive meters and diodes that exhibit near-perfect reverse blocking (extremely high $R_{reverse}$). This ratio is far too high to represent the general or minimum "roughly" accepted standard found during common troubleshooting with standard ohmmeters.