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Difficulty: HardDiodes

When a freewheeling (flyback) diode is installed across an inductive DC relay coil to suppress transient voltage spikes, its cathode must be connected to the positive supply rail so that the diode remains reverse-biased during normal energized operation and becomes forward-biased only when the collapsing magnetic field inverts the coil's terminal polarity.

Answer: Answer

Answer

True: In a flyback protection configuration, connecting the diode's cathode to the positive supply rail keeps it reverse-biased during steady-state energization, while allowing inductive collapse back-EMF to forward-bias the diode and clamp harmful voltage transients.
The statement is true because connecting the cathode of a freewheeling diode to the positive DC rail holds the diode in reverse bias during normal circuit operation, preventing unwanted conduction. When the control switch opens, the inductor's collapsing magnetic field inverts the terminal voltage polarity (back-EMF), which makes the anode positive relative to the cathode, placing the diode in forward bias to clamp voltage spikes and dissipate stored inductive energy.

Step-by-Step Solution

1
Analyze the biasing condition of the diode during normal steady-state energization of the inductive relay.
With the cathode tied to the positive DC supply rail and the anode connected to the switched low side, the diode is reverse-biased and draws only negligible leakage current, allowing full operating voltage across the coil.
A semiconductor diode conducts only when its anode is positive relative to its cathode; applying a higher potential to the cathode establishes reverse bias.
2
Determine the electrical response of the relay coil when the control switch opens to de-energize the circuit.
In accordance with Lenz's Law, the collapsing magnetic field induces a high-voltage back-EMF across the coil that reverses the terminal polarity, driving the switched terminal significantly positive relative to the supply rail.
An inductor opposes instantaneous changes in current by reversing its induced terminal voltage.
3
Evaluate the diode's conduction state during the induced back-EMF transient.
The reversed polarity places a higher positive voltage at the diode's anode than at its cathode, forward-biasing the diode to create a circulating discharge path that clamps the voltage spike to approximately 0.7 V0.7\text{ V} above the supply rail.
Forward conduction through the diode safely dissipates the stored magnetic energy and protects switching components from overvoltage breakdown.

Key Concept

Flyback diode polarity and inductive transient voltage clamping in DC circuits
Estimated Time:1m 30s
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