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How to Prevent BC847 B transistor Failures Due to Voltage Spikes

How to Prevent BC847B Transistor Failures Due to Voltage Spikes

Introduction

The BC847B transistor is commonly used in various electronics, such as amplifiers, switches, and small-signal applications. However, like most electronic components, it can fail if exposed to conditions that exceed its ratings, particularly voltage spikes. Voltage spikes are brief, high-voltage surges that can damage sensitive electronic components. In this article, we will explore the causes of BC847B transistor failures due to voltage spikes, explain the reasons behind such failures, and provide a step-by-step guide on how to prevent these issues.

1. Understanding the Causes of BC847B Transistor Failures

Voltage Spikes and Transistor Vulnerability:

The BC847B transistor is rated for a maximum collector-emitter voltage (Vce) of 45V. If a voltage spike exceeds this value, it can break down the internal junctions of the transistor, leading to permanent damage. Voltage spikes can occur in many scenarios, such as: Power supply fluctuations Inductive load switching (e.g., motors or relays) External disturbances like lightning or nearby electrical equipment A sudden surge in voltage can overwhelm the transistor's ability to dissipate the excess energy, causing the junctions to short, burn out, or fail.

Why Voltage Spikes Are a Problem:

The BC847B transistor’s internal structure is sensitive to voltage changes. A quick spike in voltage can cause the collector-emitter junction to break down, leading to failure. The transistor may overheat, resulting in a thermal failure. In extreme cases, it can even lead to a fire hazard if the spike is severe enough.

2. How Voltage Spikes Affect the BC847B Transistor

When voltage spikes occur, they affect the BC847B transistor in several ways:

Breakdown of the Junctions: The collector-base junction and collector-emitter junction can undergo a breakdown if the voltage exceeds the rated maximum. This could lead to leakage currents or a short circuit. Thermal Overload: If the transistor tries to dissipate the excessive energy generated by the voltage spike, it may overheat, damaging the transistor’s internal structure. Permanent Damage: Repeated exposure to voltage spikes, even if they are below the breakdown level, can degrade the transistor’s performance over time, causing reduced gain or failure to switch correctly.

3. How to Prevent BC847B Transistor Failures Due to Voltage Spikes

Now that we understand how voltage spikes can harm the BC847B transistor, here are steps you can follow to prevent such failures:

Step 1: Use Transient Voltage Suppression ( TVS ) Diode s

TVS Diodes are designed to protect sensitive components from voltage spikes. They are very effective in clamping voltage spikes to a safe level before they reach the transistor.

How to Implement TVS Diodes: Select a TVS diode with a clamping voltage just above the BC847B’s maximum Vce (45V). Connect the TVS diode in parallel with the transistor’s collector-emitter terminals. Choose a diode that can handle the power rating of your circuit. Step 2: Add a Zener Diode for Voltage Clamping

Zener diodes can also be used to clamp excessive voltages. A Zener diode with a breakdown voltage close to the transistor’s maximum Vce will protect it by limiting the voltage across the transistor.

How to Implement a Zener Diode: Select a Zener diode with a breakdown voltage of around 40V to 45V. Connect the Zener diode across the collector-emitter of the transistor (in reverse bias). This will clamp the voltage to a safe level, preventing the transistor from seeing a voltage spike higher than its rated maximum. Step 3: Use Snubber Circuits for Inductive Loads

When driving inductive loads, such as motors or relays, voltage spikes can be particularly severe when the inductive load is switched off.

How to Implement a Snubber Circuit: Add a resistor- capacitor (RC) snubber circuit in parallel with the inductive load. This snubber will absorb the energy from the inductive kickback and prevent the transistor from being exposed to dangerous voltage spikes. Step 4: Use Proper Power Supply Decoupling

Voltage spikes can also be introduced through the power supply lines. Decoupling capacitors help smooth out voltage fluctuations and prevent spikes from reaching sensitive components.

How to Implement Decoupling: Place a combination of small (e.g., 0.1 µF) and larger (e.g., 10 µF) capacitors close to the power input to filter out noise and transients. Ensure that the capacitors are rated for the voltage in your circuit. Step 5: Ensure Proper Grounding and Shielding

Poor grounding and inadequate shielding can allow external voltage spikes to affect the transistor.

How to Improve Grounding and Shielding: Use a dedicated ground plane to connect all ground connections in the circuit. Implement shielding in high-frequency or high-power sections of the circuit to prevent external voltage spikes from affecting the transistor. Step 6: Choose Higher-Rated Components

If your application is prone to high voltage spikes, consider using a transistor with a higher voltage rating.

For instance, you could use a BC337 transistor (which has a higher voltage rating of 80V) to prevent failure in applications where voltage spikes are frequent.

4. Step-by-Step Troubleshooting Guide for a BC847B Transistor Failure

If you suspect a BC847B transistor has failed due to voltage spikes, follow these steps to troubleshoot and identify the problem:

Step 1: Check the Voltage Rating Verify that the voltage in your circuit does not exceed the BC847B’s maximum collector-emitter voltage (45V). Use an oscilloscope to check for voltage spikes that may occur during switching operations. Step 2: Inspect the Circuit for Protection Check if you have implemented proper protection components like TVS diodes, Zener diodes, or snubber circuits. If no protection is present, consider adding it based on the recommendations above. Step 3: Examine the Power Supply Look for any irregularities in the power supply, such as noise or fluctuations that could lead to voltage spikes. Ensure that decoupling capacitors are in place and functioning properly. Step 4: Replace the Damaged Transistor If the BC847B transistor has failed, replace it with a new one, ensuring it is properly oriented and correctly connected. Consider using a transistor with a higher voltage rating for better protection against spikes.

Conclusion

BC847B transistor failures due to voltage spikes are a common issue in electronic circuits, especially in environments with fluctuating voltages or inductive loads. By understanding the causes of these failures and implementing the right protection strategies, you can significantly increase the reliability and longevity of the transistor in your designs. Using components like TVS diodes, Zener diodes, snubber circuits, and proper decoupling can effectively mitigate the risk of voltage spikes and prevent transistor failures.