Flyback transformer
The line transformer or horizontal output transformer (English: flyback transformer or line output transformer ) is a component of a television / monitor with a picture tube . It serves to supply the line deflection coil of the deflection system and at the same time usually also to generate the high voltage of 20 to 30 kV required for operating the picture tube and other voltages necessary for operating the device.
Line transformers work with the line frequency , in European TV sets with 15.625 kHz. Line transformers in 100 Hz televisions operate at twice the frequency, i.e. at 31.25 kHz. The line transformer in monitors is operated at different frequencies, which depend on the resolution of the image sent by the computer. For example, the line frequency of a monitor with a resolution of 1024 × 768 pixels and a vertical frequency of 85 Hz is around 68.7 kHz. The line output stage uses these frequencies to switch a switching tube or, today, a switching transistor, which is used to control the line transformer.
The whistling noise of some older monitors and most of the older, conventional television sets is caused by the fact that mainly the line transformer, but also other components such as coils and capacitors, are mechanically stimulated to vibrate by the magnetic and electrostatic forces that occur. The whistling has a frequency of 15.625 kHz due to the European television standard. 100 Hz televisions and most high-resolution computer monitors whistle outside of the listening area .
Historical versions from the 1950s to the 1970s
At that time, a line transformer consisted of two separate coils that were placed on a ferrite core provided with an air gap . The primary coil (1) in the right picture is supplied with a square-wave voltage via a switching tube (3); The line generator serves as the frequency source, which generates the line frequency synchronously with the TV broadcast signal. It carries taps or separate windings that feed the line deflection coils. The line deflection coils receive a current with a sawtooth shape from the line transformer. The switching tube conducts during the rising ramp of the saw tooth (line trace). The steeply sloping section is caused by their fast locking, it causes the line return. The magnetic field of the transformer and the deflection coils coincide, creating a high voltage pulse ( self-induction , Lenz's rule ). This is stepped up in the secondary coil (2) and used to generate the picture tube anode voltage.
Since coils always have a certain winding capacity due to their mechanical design , a coil is always an oscillating circuit with a corresponding resonance frequency. The collapsing magnetic field of the coils involved therefore leads - in contrast to the ideal shape of a narrow, high square pulse - to an overshoot of the line return. This and the influence of the ohmic resistance components of the coils can become noticeable in the image through distortions. This is counteracted by circuit measures that ensure a linear increase in current during the visible line lead.
The destruction of the magnetic energy stored during the line search in vibration-damping resistors during the short return time has been replaced by a more economical process. To put it simply, the cathode of a tube diode (booster diode , left in the picture) is connected to the anode of the line end tube , the anode of which is in turn connected to the other end of the primary winding (the base of the line transformer) via a capacitor .
The voltage peak that occurs during the return is applied to the diode in the correct polarity. This conducts, charges the mentioned capacitor from the energy of the collapsing magnetic field and thus effectively dampens the vibrations that otherwise arise. With a technical trick, the operating voltage of the device and the so-called booster voltage generated in this way (from to boost , amplify) can be connected in series, so that an operating voltage of 500..800 V results, whereby the line output stage works more effectively and linearly. This is why this diode is also called a booster diode , more rarely also a saving diode , because the line return energy is not converted into heat, but is available to the device itself and thus effectively reduces the power consumption from the mains.
The secondary coil for high voltage generation is cast in synthetic resin or solidified beeswax because of the risk of flashover or, in very early versions, wound as a very flat cross-wound coil and partially encased in plastic. From there, the high voltage is conducted via a short cable (5) to the anode of the high-voltage rectifier tube (4), which is also insulated against flashovers with plastic parts. The high voltage is conducted from its cathode to the anode of the picture tube via a cable (6) .
The line transformer also provided the heating voltage of around 1.25 V for the hot cathode of the high-voltage rectifier tube with a single turn . The separate heating of the high-voltage rectifier diode (such as DY86 or DY802) is indispensable in order to supply its high-voltage cathode isolated - the heating winding is made from high-voltage insulated stranded wire. The heating voltage was set by visually comparing the brightness of the filament with a second, battery-heated tube, possibly by inserting a series resistor. In the early days of television technology, tubes with 6.3 V heating voltage (EY51, EY86, ...) were used at this point, but these required more turns on the transformer and were soon replaced by the D-types as technology developed.
In order to shield the strong electromagnetic fields and also to protect against the high voltage, the line transformer with the associated tubes was in a so-called line cage . It consisted of a metal housing which was provided with numerous perforations to ensure that the heat given off by the tubes could escape. In addition, it provided a certain shielding against the X-rays that may arise in a stabilizing ballast triode (usually PD500 or PD510) .
Execution in the 1980s and 1990s
Most line transformers contain high-voltage diodes that rectify the transformer's AC voltage in a potted housing. The anode voltage of the picture tube and the focusing voltages for supplying the focusing electrodes (electrostatic focusing) in the beam system of the picture tube are generated in a high-voltage cascade . The variable resistors for the fine adjustment of the focus and screen grid voltage are also integrated in today's line transformers. The flyback transformers are controlled with switching transistors ( power bipolar transistors ).
Newer line transformers, as they are common in Trinitron picture tubes, for example , are designed as so-called diode split transformers (DST). In these, the high-voltage winding is divided into several sections, each of which supplies a rectifier circuit. The complete arrangement, as shown in the adjacent circuit diagram, is completely encapsulated. In addition to the diodes, it also includes circuit components for generating and adjusting the focusing voltages and control connections. The diodes for rectifying the high voltage are connected in series between several secondary windings, eliminating the need for a high-voltage cascade. This makes it possible to divide the high-frequency alternating voltages so that they only have to be isolated from direct voltage in the amount of the anode voltage.
This is associated with a lower load on the insulation material and a lower tendency towards pre-discharges, which lead to insulation material damage. Furthermore, with this circuit, a picture tube flashover (electrical discharge inside the picture tube) does not lead to an overload of the rectifier diodes, as was the case with the high-voltage cascades used earlier. The number of high-voltage capacitors required can also be reduced by half.
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- Heinz Richter : television experimentation practice . Franckh'sche Verlagshandlung W. Keller & Co., Stuttgart 1952.
- Heinz Richter: TV for everyone . Franckh'sche Verlagshandlung W. Keller & Co., Stuttgart 1952.
- Otto Limann: TV technology without ballast . Franzis-Verlag, Munich 1969.
- Otto Limann: radio technology without ballast . Franzis-Verlag, Munich 1963.
- J. Jager: Data and Circuits of Television Receiver Valves . In: Series of Books of Electronic Valves . IIIc. Philips Technical Library, Eindhoven, NL 1959 ( PDF, 16 MB ( Memento of January 8, 2009 in the Internet Archive )).