How can a diode defend the coil during transistor cutoff?

Thread starter Femme_physics
Start date May 14, 2012
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Coil Diode Transistor

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SUMMARY

The discussion centers on the role of a diode in protecting a relay coil during transistor cutoff. When the transistor switches off, the coil's magnetic field collapses, inducing a high voltage that can damage components. The diode provides a path for the induced current, allowing it to dissipate safely and preventing voltage spikes that could harm the transistor. This mechanism is crucial in automotive applications and other circuits utilizing inductive loads.

PREREQUISITES

Understanding of inductive components and their behavior, specifically inductors and coils.
Knowledge of transistor operation, particularly cutoff states.
Familiarity with Faraday's law of electromagnetic induction.
Basic principles of diode operation and biasing.

NEXT STEPS

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Investigate the design considerations for protecting circuits with inductive loads.

USEFUL FOR

Electronics engineers, automotive electricians, and anyone involved in circuit design or maintenance, particularly those working with inductive loads and relay systems.

May 26, 2012

truesearch

Before the days of readily available, cheap semiconductor diodes one way to protect circuits, relay or magnet coils and people from high induced voltages at switch off was to have a resistor (a light bulb was common) permanently connected across the coil.
This provided a path for current so that the induced emf was kept low.
The disadvantage was that, unlike a diode, current was always flowing through the protective component and this was wasted energy.