Transistor Protection from Relay Voltage Spikes

Understanding the Problem

When a transistor switches OFF an inductive load like a relay coil, the collapsing magnetic field generates a large reverse voltage (back EMF or inductive kick). This voltage spike can exceed the transistor's breakdown voltage, causing damage.

Analyzing Protection Methods

The goal is to provide a safe path for the energy stored in the inductor's magnetic field when the current is interrupted.

• Capacitor in series: Incorrect. A capacitor in series would block the DC current needed to energize the relay and doesn't effectively suppress voltage spikes across the switch.
• Resistor in series: Incorrect. A series resistor limits current but doesn't dissipate the inductive energy spike effectively, potentially reducing relay performance and still exposing the transistor to high voltage.
• Inductor parallel: Incorrect. An inductor in parallel with the relay would essentially short-circuit the relay coil, preventing it from operating correctly. It does not protect the transistor from the inductive spike.
• Diode parallel: Correct. A diode placed in parallel with the relay coil, oriented to be reverse-biased during normal operation, provides a path for the current to circulate safely when the transistor switches off. The diode clamps the voltage spike to its forward voltage drop (typically ~0.7V for silicon diodes), protecting the transistor. This is commonly known as a flyback or freewheeling diode.

Solution Explanation

A diode connected in reverse parallel across the relay coil is the standard method for protecting the switching transistor.

• Normal Operation: The diode is reverse-biased and does not conduct. The transistor controls the current flow to the relay.
• Switching OFF: When the transistor turns OFF, the relay coil's collapsing magnetic field induces a voltage spike. The polarity of this spike forward-biases the diode.
• Protection Mechanism: The diode conducts, creating a closed loop allowing the inductor current to decay gradually through the diode and the coil, rather than forcing a high voltage across the now-open transistor switch.

Therefore, a diode parallel to the relay protects the transistor from the damaging voltage spike.