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A flyback transformer (FBT) or line output transformer (LOPT) is a type of transformer used to generate the high voltage needed for driving a cathode ray tube (CRT) 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 30 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 high amplitude pulsed waveform that repeats at the line frequency of the display, and is produced by the circuitry that makes the electron beam "fly back" rapidly 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 (about 30kHz to max. 150kHz in modern monitors). (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 and air gap. 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 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.
Failure
Flyback transformers are a frequent source of failure for television sets; the high voltage present in the many turns of wire with the somewhat thin insulation required for the transformer to be of reasonable size is likely to eventually result in leakage at one point or another; as the leakage heats the insulation it carbonizes and conducts more, which leads to even more heat and carbonization, until the leaked current is high enough for the high voltage to cease to function. As a result, replacement flyback transformers for almost every set on the market are available through dealers in electronic parts, typically for a few tens of dollars. The problem is exacerbated by the tendency of the flyback to accumulate a coating of dust due to electrostatic attraction, which serves as a path to ground for leaks which might otherwise not be of sufficient magnitude to initiate the chain of events leading to destructive failure, as described. As a result, occasional cleaning of the accumulated dust from the high voltage circuitry inside a television can be beneficial, provided the proper precautions are taken, as below.
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. The CRT has an inherent capacitance which can hold a high voltage charge for a period after the power is switched off. Often a high resistance bleeder resistor is in parallel to ensure the charge is safely grounded when not in use, but it is not wise to assume this is the case.
In many recent televisions, after replacing the flyback transformer, the control firmware must be recalibrated to account for slight differences in performance between transformers in order to maintain accurate color reproduction.
Reference
Patents
- - "Television sweep transformer" - Theodore J. Godawski
See also
Flyback converter Transformers (electrical)
"Flyback transformer"