Ohm's law fails in

Options

A. Diode
B. Thyristor
C. PN junction system
D. All of these

Answer

### Step-by-Step Solution:

1. **Understanding Ohm's Law**:
- Ohm's Law states that the current (I) flowing through a conductor between two points is directly proportional to the voltage (V) across the two points, provided the temperature and material properties remain constant. This is mathematically expressed as:
\[ V = I \cdot R \]
where R is the resistance.

2. **Identifying Ohmic and Non-Ohmic Conductors**:
- **Ohmic Conductors**: These are materials that obey Ohm's Law. Their resistance remains constant regardless of the voltage and current. Examples include metals like copper and aluminum.
- **Non-Ohmic Conductors**: These materials do not follow Ohm's Law. Their resistance can change with voltage, current, or temperature. Examples include diodes, transistors, and certain types of semiconductors.

3. **Conditions Where Ohm's Law Fails**:
- Ohm's Law fails in scenarios involving:
- **Non-ohmic resistors**: Such as diodes and transistors, where the relationship between voltage and current is not linear.
- **Semiconductors**: Materials like silicon and germanium that exhibit non-linear current-voltage characteristics.
- **Thyristors**: These are also considered non-ohmic devices as they have a threshold voltage and do not conduct until this voltage is reached.

4. **Conclusion**:
- Therefore, Ohm's Law fails in the presence of non-ohmic materials, which include semiconductors (like diodes and p-n junctions) and thyristors. The correct answer to the question would be option 4, which encompasses all these non-ohmic devices.

### Final Answer:
Ohm's law fails in non-ohmic conductors such as diodes, semiconductors, and thyristors.