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What Is a Flyback Inductor?

By Paul Scott
Updated May 16, 2024
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A flyback inductor is a transformer designed to generate high voltage, high frequency sawtooth signal patterns. Also known as flyback transformers (FBT) or line input transformers (LOPT), flyback inducers supply the the specific signal requirements of a cathode ray tube (CRT) in televisions and computer monitors. The flyback inductor typically also features secondary, lower voltage outputs which supply other parts of the monitor circuit. Flyback inductors differ from conventional transformers in that they are able to momentarily store energy in their windings when the power supply is cut. This is achieved by placing the primary and secondary windings onto a ferrite core with a carefully calculated air gap between the two which serves to increase the reluctance, or ability to store energy.

The CRTs in televisions and monitors used in computers, ATMs, gaming machines, and vending stations require extremely high voltages at high frequencies to operate, typically 20,000 to 30,000 volts and 15 to 150 kilohertz (kHz). A specific signal profile known as a sawtooth waveform is also required to control the horizontal deflection or movement of the tubes' electron beam. Both of these requirements are met by the flyback inductor. This component is basically a specialized transformer consisting of a primary and secondary winding placed on a ferrite core with an air gap between them. The main difference between the construction and operation of the flyback inductor and the conventional transformer is the carefully calculated air gap between the two windings and the resultant increase in the reluctance of the inductor.

Reluctance is a term that describes the ability of the inductor to store the charge induced in its secondary windings momentarily after the power has been cut. This characteristic along with the relationship between wire thickness and number of turns on the primary and secondary windings allows the flyback inductor to maintain the high voltage, high frequency signal required by the CRT. The sawtooth pattern of the signal is achieved via the monitor's circuitry which switches the flyback inducer supply on and off in a continuous, controlled manner. This switching is typically achieved with transistors which allow the supply to ramp up to a peak with an abrupt cut-off, thereby giving the inductor output signal its characteristic sawtooth profile.

The secondary winding of many flyback inductor models include taps or secondary outputs used to supply lower voltages to other parts of the CRT circuit. Care should always be exercised when disconnecting a flyback inductor from the CRT even if the power to the monitor has been disconnected. The residual charge stored in the inductor can produce a nasty shock and electrical burn if the CRT lead is touched prior to full discharge.

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