Match List-I with List-II and select the correct answer using the given lists: List – I (SCR rating) List – II (Protective element) A. di/dt limit 1. Snubber B. dV/dt limit 2. Heat sink C. i2t limit 3. Series reactor D. Junction temperature limit 4. Avoid runway speed on no load
A - 3, B - 1, C - 4, D - 2
This question requires matching the operational limits and ratings of a Silicon Controlled Rectifier (SCR) with the appropriate protective elements or concepts used in circuit design. Understanding these parameters is crucial for reliable SCR operation.
Understanding SCR Parameters and Protection
Let's analyze each item from List-I and find its corresponding match in List-II based on the provided correct answer:
1. Matching Items from List-I and List-II
| List-I (SCR Parameter) | List-II (Protective Element/Concept) | Explanation |
| A. $\frac{di}{dt}$ limit | 3. Series reactor | The $\frac{di}{dt}$ limit specifies the maximum allowable rate of rise of current through the SCR after it has been triggered. A Series reactor (inductor) is used to limit this rate of current increase, especially during the turn-on process, preventing damage to the SCR. |
| B. $\frac{dV}{dt}$ limit | 1. Snubber | The $\frac{dV}{dt}$ limit refers to the maximum rate of rise of voltage across the SCR in the forward blocking state. Exceeding this limit can cause spurious turn-on. A Snubber circuit, typically composed of a resistor and capacitor (RC network) in series, is connected across the SCR to suppress voltage spikes and limit the $\frac{dV}{dt}$, thereby preventing unintended triggering. |
| C. $i^2t$ limit | 4. Avoid runway speed on no load | The $i^2t$ rating represents the maximum energy (proportional to the integral of current squared over time) that an SCR can withstand during a fault condition or surge current event without sustaining damage. While fuses or special semiconductor fuses are the typical protective devices, the provided match relates this limit to preventing hazardous operational states such as runway speed on no load, suggesting protection against transient overcurrents in specific scenarios. |
| D. Junction temperature limit | 2. Heat sink | The Junction temperature limit ($T_j$) is the maximum temperature that the semiconductor junction within the SCR can safely reach. During operation, power dissipation causes the junction to heat up. A Heat sink is a passive heat exchanger attached to the SCR to dissipate this heat effectively into the surrounding environment, ensuring the junction temperature stays below the specified maximum limit and preventing thermal failure. |
2. Correct Match Selection
Based on the detailed analysis, the correct matching is:
- A matches with 3 (Series reactor)
- B matches with 1 (Snubber)
- C matches with 4 (Avoid runway speed on no load)
- D matches with 2 (Heat sink)
This corresponds to option 3: A - 3, B - 1, C - 4, D - 2.
Important Questions from Thyristor Protection
Which of the following is a safety device connected in series to the circuit?
What is the term for the minimum anode current below which a thyristor will turn OFF and stay OFF, even if a gate pulse was previously applied?
The Snubber circuit is used to thyristor circuits for -
For series connected SCRs, static equalization is obtained with the help of
Which of the following is the function of an R-C snubber circuit connected in parallel to an SCR?
