A flyback transformer or line output transformer (LOPT) is a type of transformer used to generate the high voltage needed for driving a cathode ray tube or "picture tube". It converts the input voltage, usually mains voltage in the range 120 to 240 volts, to an output voltage in the range 10 to 20 kilovolts for a colour tube (or just a few kilovolts for a monochrome tube). Unlike mains transformers and audio transformers, a LOPT is designed to have windings with high inductance. This is achieved by winding the coils on a ferrite core with an air gap, which increases the reluctance of the magnetic circuit and therefore its capacity to store energy.
The primary winding of the LOPT is driven by a low-voltage direct current that is switched on and off by the horizontal flyback pulse of the CRT. This is a pulsed waveform that repeats at the line frequency of the display, and is produced by the circuitry that makes the electron beam "fly back" from the right-hand side of the picture to the left after tracing each horizontal line. In televisions, this frequency is about 15 kilohertz, but in computer displays it varies over a wide range. (It is this frequency that creates the whining noise that can be heard from most CRT displays.) The current pulse creates a magnetic field in the ferrite core. When the pulse ends, the field collapses, inducing a high-voltage sawtooth current in the secondary winding. This alternating current is then converted to direct current by a high-voltage rectifier. If the output voltage of the LOPT is not high enough by itself, a voltage multiplier is placed before the rectifier to increase the voltage. The rectified voltage is then used to charge the anode of the cathode ray tube. There are often auxiliary secondary windings that produce lower voltages for driving other parts of the display's circuitry.
In modern displays, the LOPT, voltage multiplier and rectifier are often integrated into a single package on the main circuit board. There is usually a thick grey wire from the LOPT to the anode terminal (covered by a rubber cap) on the side of the picture tube. The thickness of this wire is mostly due to the thickness of the plastic insulation, the copper conductor inside being much thinner as it carries only a small current.
The main advantages of the flyback transformer over a standard mains transformer are its small size and light weight, which are the results of operating it at the line frequency (many kilohertz) instead of the mains frequency (50 or 60 Hz). A secondary advantage is that it provides a failsafe mechanism – should the horizontal deflection circuitry fail, the flyback transformer will cease operating and shut down the rest of the display, preventing the screen burn that would otherwise result from a stationary electron beam.
WARNING: A flyback transformer and its associated circuitry operate at lethally high voltages. Only trained persons should touch or modify these devices, after first ensuring that the transformer is switched off and any stored energy has been safely discharged.