In practical electronics, a diode is often placed directly in parallel (reverse-biased with respect to the supply) across a relay coil. What is the primary purpose of this diode in the relay driver circuit?

Introduction / Context:
Relays are inductive loads. When the driving switch or transistor turns off, the relay coil's magnetic field collapses and the inductor attempts to keep current flowing, producing a large voltage spike (inductive kick). A diode placed across the coil is commonly called a flyback (freewheel, snubber) diode and is essential for protecting the driving device and reducing contact arcing.

Given Data / Assumptions:

• Inductive element: relay coil modeled as an inductor.
• Driver opens current path abruptly.
• Diode is wired in reverse polarity across the coil under normal operation (non-conducting when powered).
• Goal: identify the diode’s main protective function.

Concept / Approach:
Inductor voltage follows v = L * di/dt. When current is interrupted, di/dt is large and positive in magnitude, so the inductor generates a high voltage of opposite polarity to sustain current. The flyback diode provides a low-impedance loop for the decaying coil current, clamping the voltage near the diode drop and safely dissipating energy in the coil resistance and diode.

Step-by-Step Solution:
At turn-off, the coil current cannot instantly go to zero.Without a diode, the coil drives the switch node to a high voltage, causing arcing across mechanical contacts or overstress of a transistor.With the diode, current freewheels through the diode and coil, limiting voltage to approximately the supply plus one diode drop.Lower voltage stress means reduced arcing at the opening contacts and protection of semiconductor drivers.

Verification / Alternative check:
Scope measurements show that with no diode, switch node voltages can exceed hundreds of volts for a 12 V coil. With the diode installed, the node clamps near 12–13 V and the current decays exponentially with time constant L/R.

Why Other Options Are Wrong:
Regulate input voltage: Regulation is not the diode’s function.Protect the coil from arcing: The coil does not arc; the opening contacts or driver do.Ensure most current passes through the coil: That is determined by circuit design, not the diode.Increase pull-in force: The diode only conducts on turn-off, not during energizing.

Common Pitfalls:
Placing the diode in the wrong polarity (then it shorts the supply), omitting it entirely, or assuming a Zener is required in low-speed relay applications (a simple diode is usually enough).

Final Answer:
to reduce arcing across the relay contacts