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What Are Flyback Diodes and Why Are They Used?

Hardware: Signal Conditioning Other>>SSR Series>>SSR-OAC-5, Signal Conditioning Other>>SSR Series>>SSR-OAC-5A, Signal Conditioning Other>>SSR Series>>SSR-ODC-5

Problem:
I have an output SSR module and I was told that I need to use flyback diodes to protect the system. What are these diodes and why are they used?

Solution:
Flyback diodes are normal diodes that are used in conjunction with any system that is used to drive motors as a protection circuitry. The reason we need to use flyback diodes is to avoid damage due to Flyback Action of the motor. The winding in the motor is essentially an inductor. When the flow of current in an inductor is suddenly cut off, the current cannot suddenly go to 0. As a result, a voltage is induced opposing this current. This voltage can be so high that it destroys the transistors. The flyback diodes provide a safe return path for this current that bypasses and protects the transistors.

Related Links:
Developer Zone Tutroial: High-Voltage Flyback Measurements
Developer Zone Tutorial: Protecting Inductive Loads on the NI-6509

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Report Date: 10/23/2004
Last Updated: 07/24/2007
Document ID: 3EMAMO3F

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