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, Switches
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 or freewheeling diodes are used as protection for any system that has highly inductive circuits such as coils on relays, solenoids, and motors. The reason flyback diodes are used is to avoid damage to the circuitry connected to the inductive load due to flyback effects of inductive loads. For example, the winding in a motor is essentially an inductor. Circuit theory for inductors shows that when the flow of current to an inductor is suddenly stopped a large voltage appears across that inductor in the opposite direction of the current that was stopped. This is called flyback. The voltage appears because energy is stored in the inductor’s magnetic field before the current is stopped. After the current has stopped, the magnetic field in the inductor releases its energy as electricity back into the circuit but in the reverse direction that it was supplied. For electronic switching circuits this flyback voltage can damage the switching circuitry. In order to mitigate the flyback action of the inductor a flyback diode is placed in parallel with the inductor to provide a current path back through the inductor. The diode needs to be placed correctly in the circuit so that during normal operation is it not activated but during the flyback state of the inductor the diode becomes active and conducts current way from the rest of the circuit and back through the inductor. Related Links: Developer Zone Tutroial: High-Voltage Flyback Measurements Developer Zone Tutorial: Protecting Inductive Loads on the NI-6509 KnowledgeBase 1EPBH9LK: Can I Connect a Coil Relay Directly to my FP-DO-400 or FP-DO-401? Attachments:
Report Date: 10/23/2004 Last Updated: 05/27/2009 Document ID: 3EMAMO3F |
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