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Difficulty: MediumDiodes

In a direct-current (DC) control circuit, a silicon rectifier diode is connected in parallel across the terminals of an electromechanical relay coil, with its cathode connected to the positive supply rail. What is the primary function of this diode in the circuit?

  1. A
    To convert the direct-current supply into alternating current needed to keep the relay energized
  2. B
    To conduct continuous forward-bias current around the coil while the circuit is actively powered
  3. To safely dissipate high-voltage transient spikes generated by the collapsing magnetic field when the coil is de-energizedAnswer
  4. D
    To step up the supply voltage across the coil terminals to increase contact closing speed

Answer

To safely dissipate high-voltage transient spikes generated by the collapsing magnetic field when the coil is de-energized
An electromechanical relay coil is an inductor that stores energy in a magnetic field. When the control switch opens to turn off the relay, the sudden interruption of current causes the magnetic field to collapse rapidly. In accordance with Lenz's Law, this collapse generates a high-voltage back-EMF spike of reverse polarity. Because the diode is connected in anti-parallel (cathode to positive rail, anode to switched side), this reverse voltage places the diode into forward bias. The diode conducts, clamping the voltage spike to its forward drop (approximately 0.7 V0.7\text{ V}) and circulating the current through the coil until the energy safely dissipates as heat, protecting upstream transistors or switch contacts from electrical breakdown.

Step-by-Step Solution

1
Analyze the biasing condition of the diode during normal steady-state operation
The diode's cathode is connected to the positive supply rail and its anode to the switched ground side, placing it in reverse bias with negligible leakage current.
Understanding normal circuit operation confirms that the diode does not interfere with energizing the relay coil.
2
Analyze what happens when the circuit switch opens to de-energize the relay coil
The current suddenly drops to zero, causing the magnetic field around the inductive coil to collapse rapidly.
Inductors resist instantaneous changes in current by generating an induced electromotive force (back-EMF).
3
Determine the polarity of the induced voltage and the resulting state of the diode
The collapsing magnetic field reverses the polarity across the coil terminals, making the anode positive relative to the cathode and placing the diode into forward bias.
The forward-biased diode clamps the inductive spike to approximately 0.7 V0.7\text{ V} and circulates the decay current safely until the stored magnetic energy dissipates.

Key Concept

Flyback (freewheeling) diode protection across inductive loads
Estimated Time:1m 0s
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Diodes

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