Master the Basics: Protect Against Flyback Voltage
When working with inductive components like motors, relays, or coils, one of the biggest risks is a phenomenon called flyback voltage—a sudden voltage spike that can damage sensitive components. Fortunately, with a simple and inexpensive addition to your circuit, you can protect it from harm.
What Is Flyback Voltage?
An inductor stores energy in a magnetic field when current flows through it. But when that current is suddenly interrupted—such as when a switch opens—the magnetic field collapses rapidly, releasing the stored energy as a high-voltage spike. This spike, known as flyback, tries to maintain the current flow and can reach voltages far higher than the original supply.
Why Is It Dangerous?
This sudden voltage spike can:
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Damage sensitive components like transistors, microcontrollers, or power supplies
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Break down insulation in wires or circuit traces
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Create unpredictable circuit behavior
Real-World Example: Motor Flyback
Motors contain coils of wire and behave electrically like an inductor and a resistor in series. When powered on, they draw steady current and store magnetic energy. But when power is suddenly cut off, the inductive coil releases that energy in the form of a reverse polarity voltage spike.
For example, when disconnecting a 5V computer fan motor, the flyback voltage measured was around –13V, significantly higher than the motor’s normal operating voltage. That kind of spike can easily damage other parts of the circuit.
How to Prevent Flyback: Use a Flyback Diode
The most common way to protect your circuit is by adding a flyback diode in parallel with the inductive load (e.g., the motor). This diode provides a safe path for the current when the circuit opens:
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Under normal operation, the diode is reverse biased, so it does nothing.
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When current is cut, the diode becomes forward biased, allowing the energy to loop through the diode and motor until it safely dissipates.
This type of diode may also be called a snubber, suppressor, or clamp diode.
How It Works
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Switch closed: Current flows through the motor. The diode is off.
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Switch opens: The motor's collapsing magnetic field generates a spike. Instead of damaging components, the current flows through the diode loop.
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Energy dissipates: Resistance in the diode and wiring slowly bleeds off the current over a few milliseconds.
Demonstration
Using an oscilloscope:
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Without a diode, disconnecting power from a fan motor produces a –13V spike.
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With a diode in place, the voltage across the fan drops safely to 0V, without a damaging spike.
Key Takeways
If your circuit includes inductive loads like motors, always add a flyback diode to prevent voltage spikes. It’s a simple, low-cost component that can save you from major headaches—and protect your circuit from permanent damage. A small precaution that makes a big difference in reliability and safety.
