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October 2, 2021

Here is a website that provides a short explanation of flyback diodes:

Inductive Flyback and Flyback Diodes

It is not too rigorous, but it gives a high level explanation that is helpful to grasp the concept.

Installing flyback or “freewheeling” diodes on DC relays, or RC snubber circuits on AC relays, is a common practice for protecting contacts from damage and sensitive electronic circuits from interference.

When used in nuclear, however, design scrutiny intensifies, even in non-Class 1E applications. You need to pay attention to potential failure modes and incorporate them into your design evaluations. What happens if the diode fails open or closed, for example?

Depending on the application, this may drive more complex snubber designs. Or, it may drive alternate designs that eliminate the snubbers.

In applications where timing is crucial, such as isolation valves that must meet tech spec time limits, the time delay to achieve actuation introduced by the flyback circuit comes into focus.

In a Class 1E application, especially where limits on open or closure time exist, laboratory testing is likely the best option to demonstrate acceptance. This should be planned from the outset and written into a specification if possible.

Sincerely,

John "Snubbed" Livingston
Editor
www.NuclearElectricalEngineer.com

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