3) Explain the construction and working of an SCR. (6 marks) b) Explain forward blocioing and conduction mode of operation of SCR ( 5 marks) c) Draw the V-I characteristics and explain how gate current affects switching of SCRs ( 5 marikt) d) Using equivalent circuit for UJT, derive the expression for intrinsic standoff ratio (4 marks) QUESTION FOUR (20 MARKS) a) Explain the procedure involvedi in troubleshooting of medical electronic gadgets (4 mardes) b) Discuss how electronic component fatlures can be avoided (4 marks)

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### 3) Explain the construction and working of an SCR. (6 marks)

**Construction of SCR:**
- An SCR (Silicon Controlled Rectifier) is a four-layer semiconductor device made up of alternating P-type and N-type materials, forming three junctions (J1, J2, J3). The layers are arranged as follows: P-N-P-N (or alternatively N-P-N-P).
- The terminals of the SCR are:
  - Anode (A): Connected to the P-type layer.
  - Cathode (K): Connected to the N-type layer.
  - Gate (G): Connected to the P-type layer, which controls the device.

**Working of SCR:**
1. **Forward Blocking Mode:** When a positive voltage is applied to the anode with respect to the cathode, the SCR is in a forward blocking state. The junctions J1 and J2 are forward-biased, while J3 is reverse-biased, preventing current flow.
2. **Triggering:** When a small positive voltage is applied to the gate terminal, it allows a small current to flow, which triggers the SCR into conduction. This is due to the reduction of the reverse bias at J3.
3. **Conduction Mode:** Once triggered, the SCR enters the conduction mode, allowing a large current to flow from anode to cathode. The SCR remains in this state even if the gate current is removed, as long as the anode current remains above a certain threshold (holding current).
4. **Turn-off:** To turn off the SCR, the anode current must be reduced below the holding current, or the device can be turned off by applying a reverse voltage.

### b) Explain forward blocking and conduction mode of operation of SCR (5 marks)

**Forward Blocking Mode:**
- In this mode, the SCR is forward-biased, meaning the anode is at a higher potential than the cathode. However, the SCR does not conduct current because junction J3 is reverse-biased.
- The SCR can withstand a certain amount of forward voltage (breakdown voltage) without conducting. This is the region where the SCR is off, and it behaves like an open switch.

**Conduction Mode:**
- When the SCR is triggered by a gate current, it enters the conduction mode. In this state, all three junctions are forward-biased, allowing current to flow from the anode to the cathode.
- The SCR will continue to conduct as long as the anode current remains above the holding current. If the current falls below this level, the SCR will turn off and return to the forward blocking mode.

### c) Draw the V-I characteristics and explain how gate current affects switching of SCRs (5 marks)

**V-I Characteristics of SCR:**
- The V-I characteristics of an SCR consist of three regions: forward blocking, forward conduction, and reverse blocking.
- In the forward blocking region, the SCR does not conduct until the breakdown voltage is reached.
- In the forward conduction region, the SCR conducts with a low voltage drop across it.
- The reverse blocking region shows that the SCR can withstand reverse voltage until the breakdown point.

**Effect of Gate Current:**
- The gate current (Ig) plays a crucial role in the switching of SCRs. A small gate current can trigger the SCR into conduction, reducing the required anode current for triggering.
- The higher the gate current, the lower the anode current required to turn on the SCR. This means that the SCR can be turned on with less anode voltage when a sufficient gate current is applied.
- The gate current also affects the holding current; a higher gate current can lead to a lower holding current, allowing the SCR to remain in conduction with a lower anode current.

### d) Using equivalent circuit for UJT, derive the expression for intrinsic standoff ratio (4 marks)

**Equivalent Circuit of UJT:**
- A UJT (Unijunction Transistor) consists of a single N-type material with a P-type material (the emitter) diffused into it. The equivalent circuit can be represented as a voltage divider formed by two resistors (R1 and R2) connected to the base terminals.

**Intrinsic Standoff Ratio (η):**
- The intrinsic standoff ratio is defined as the ratio of the voltage across the base terminals to the supply voltage. It is given by the formula:
  
$$
\eta = \frac{V_B}{V_{BB}}
$$

Where:
- $$ V_B $$ is the voltage across the base terminals.
- $$ V_{BB} $$ is the supply voltage.

- The expression for $$ V_B $$ can be derived from the voltage divider rule:

$$
V_B = \frac{R2}{R1 + R2} \cdot V_{BB}
$$

- Substituting this into the equation for η gives:

$$
\eta = \frac{R2}{R1 + R2}
$$

This shows that the intrinsic standoff ratio depends on the resistances in the UJT circuit.

---

### QUESTION FOUR (20 MARKS)

### a) Explain the procedure involved in troubleshooting of medical electronic gadgets (4 marks)

1. **Preparation:**
   - Gather necessary tools and documentation (manuals, schematics).
   - Ensure safety protocols are in place, especially for devices that may be connected to patients.

2. **Visual Inspection:**
   - Check for obvious signs of damage, such as burnt components, loose connections, or corrosion.

3. **Functional Testing:**
   - Power on the device and observe its operation. Use test equipment to measure voltages, currents, and signals at various points in the circuit.

4. **Isolation of Fault:**
   - Use a systematic approach to isolate the faulty component or section of the circuit. This may involve testing individual components or sections of the circuit.

5. **Repair or Replace:**
   - Once the faulty component is identified, repair or replace it as necessary. Ensure that all connections are secure.

6. **Final Testing:**
   - After repairs, conduct thorough testing to ensure the device operates correctly and safely.

### b) Discuss how electronic component failures can be avoided (4 marks)

1. **Quality Components:**
   - Use high-quality components from reputable manufacturers to reduce the likelihood of failure.

2. **Proper Design:**
   - Ensure that the circuit design is robust, with appropriate tolerances and margins for voltage, current, and temperature.

3. **Thermal Management:**
   - Implement adequate cooling and thermal management strategies to prevent overheating, which can lead to component failure.

4. **Regular Maintenance:**
   - Schedule regular maintenance and inspections to identify and address potential issues before they lead to failures.

5. **Surge Protection:**
   - Use surge protectors and fuses to protect against voltage spikes and surges that can damage components.

6. **Environmental Considerations:**
   - Ensure that devices are used in appropriate environmental conditions (temperature, humidity) to prevent stress on components.
**3) Construction and Working of an SCR:** - **Construction:** An SCR is a four-layer semiconductor device with P-N-P-N layers, having anode, cathode, and gate terminals. - **Operation:** - **Forward Blocking:** No current flows when forward voltage is applied without gate current. - **Triggering:** A small gate current triggers conduction, allowing current to flow from anode to cathode. - **Conduction:** Once triggered, the SCR remains on as long as anode current exceeds the holding current. - **Turn-off:** The SCR turns off when anode current drops below the holding current or when a reverse voltage is applied. **b) Forward Blocking and Conduction Modes:** - **Forward Blocking:** The SCR does not conduct when forward voltage is applied without gate current. - **Conduction:** Triggered by gate current, allowing current to flow from anode to cathode. The SCR remains on as long as the anode current is above the holding current. **c) V-I Characteristics and Gate Current Effect:** - **V-I Characteristics:** Shows forward blocking, forward conduction, and reverse blocking regions. - **Gate Current Impact:** A small gate current can trigger the SCR into conduction, reducing the required anode current. Higher gate current lowers the holding current, allowing the SCR to remain on with less anode current. **d) Intrinsic Standoff Ratio of UJT:** - **Equivalent Circuit:** Consists of a voltage divider with two resistors (R1 and R2). - **Intrinsic Standoff Ratio (η):** Defined as the ratio of the voltage across the base terminals to the supply voltage. - **Expression:** $$ \eta = \frac{R2}{R1 + R2} $$ This ratio depends on the resistances in the UJT circuit. --- **Question Four: Troubleshooting and Component Failure Prevention** **a) Troubleshooting Procedure:** 1. **Preparation:** Gather tools and documentation, ensure safety. 2. **Visual Inspection:** Look for damage or loose connections. 3. **Functional Testing:** Power on and observe operation, use test equipment to measure voltages and signals. 4. **Isolation of Fault:** Systematically identify the faulty component or circuit section. 5. **Repair/Replace:** Fix or replace identified components. 6. **Final Testing:** Ensure the device operates correctly after repairs. **b) Preventing Electronic Component Failures:** 1. **Use Quality Components:** Choose high-quality parts from reputable manufacturers. 2. **Robust Design:** Ensure circuits are designed with appropriate tolerances and margins. 3. **Thermal Management:** Implement cooling to prevent overheating. 4. **Regular Maintenance:** Schedule inspections and maintenance to catch issues early. 5. **Surge Protection:** Use protectors and fuses to guard against voltage spikes. 6. **Environmental Controls:** Use devices in appropriate conditions to minimize stress on components.

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