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Self-Induction

  • 3 min
  • Beginner

The self-induction is the opposition of a coil to changing its current through an induced voltage.

We already know that if we bring a magnet close to a coil, we generate voltage (Faraday). But when we pass current through a coil to create an electromagnet, the coil itself is creating a magnetic field that grows and cuts its own turns.

Therefore, according to Faraday, the coil induces voltage in itself. While the current is constant, nothing happens. But at the moment of turning on and, especially, of turning off the circuit, the coil reacts with great violence.

The phenomenon: magnetic inertia

A coil does not want the current to change abruptly. If you try to increase it suddenly, it opposes. If you try to cut it off suddenly, it also opposes.

That is why it is often said to have a kind of electrical inertia:

  • If the current goes up, the coil slows it down.
  • If the current goes down, the coil tries to maintain it.

The basic relationship is:

Where:

  • is the inductance.
  • is the rate at which the current changes.

The more abrupt the change, the greater the induced voltage.

The real problem: the inductive kick

When we turn off a coil, the magnetic field collapses rapidly. The current wants to continue flowing, and since the circuit has been opened, the coil raises its voltage as much as necessary to try to do so.

This is where the famous inductive kick appears:

  • Sparks jump in switches and relays.
  • Transistors and MOSFETs get punctured.
  • A microcontroller’s pins can be damaged.

In a small coil, the peak can already be serious. In a large load (relay, contactor, motor or solenoid) it can be directly destructive.

The freewheeling diode

In a DC coil with unidirectional switching, a common protection is to place a diode in antiparallel.

  • The cathode to the positive.
  • The anode to the negative.

How does it work?

  1. Switch closed: current flows normally and the diode is reverse-biased, so it does not conduct.
  2. Switch open: the coil reverses its polarity to maintain the current. At that instant, the diode becomes forward-biased and offers it a safe path to recirculate.

The current leaves the coil, passes through the diode, and re-enters the coil, gradually dissipating the stored energy. Instead of trying to jump across the transistor or switch, it goes around in a controlled loop.

Diode selection

To choose the right protection diode, at least two things should be considered:

  1. Current: it must withstand, at minimum, the current the coil was consuming.
  2. Speed: for normal relays, a 1N4007 is usually sufficient. For fast applications (like motor PWM), fast or Schottky diodes are preferable.

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