Flyback transformer

The line transformer or horizontal output transformer (English: flyback transformer or line output transformer ) is a component of a TV / monitor with picture tube. It is used to supply the line deflection coil of the deflection system and usually also for generating the same necessary for the operation of the picture tube high-voltage 20-30 kV and further necessary for the operation of the device voltages.

Line Output Transformers work with the line frequency in European TVs with 15.625 kHz. Line Output Transformers from 100 Hz televisions work with twice the frequency, ie at 31.25 kHz. In monitors the flyback transformer is operated at different frequencies, which depend on the resolution of the image sent from the computer. For example, is the line frequency of a monitor with a resolution of 1024 × 768 pixels and 85 Hz vertical frequency is about 68.7 kHz. With these frequencies, the line output stage switches a switch tube or today a switching transistor, which are used to control the flyback transformers.

The whistling sound of some older monitors and most of the older, conventional televisions arises from the fact that it is mainly the line output transformer, but also other components such as inductors and capacitors are excited by the occurring magnetic and electrostatic forces to vibrate mechanically. The whistle has by European television standard, the frequency of 15.625 kHz, TVs with 100 Hz technology and the most high-resolution computer monitors whistle outside the audible range.

Past versions of the 1950s to the 1970s

A line output transformer then consisted of two separate coils stuck on a equipped with an air gap ferrite core. The primary coil (1) in the left image is supplied via a switch tube ( 3) with a square-wave voltage; serves as a frequency source of the line generator to produce the line frequency in synchronism with the TV broadcast signal. She wears taps or separate windings which feed the line deflection coils. The line deflection obtained from the flyback transformer a current with a sawtooth profile. During the rising ramp of the sawtooth ( Zeilenhinlauf ) leads the interrupter. The steep section is caused by their rapid locks, it causes the carriage return. Here, the magnetic field of the deflection coils of the transformer and collapses whereby a high voltage pulse is formed ( self-induction, Lenz's law ). This is transformed up in the secondary coil (2) and used to generate the picture tube anode voltage.

Since coils always have their mechanical construction, a certain winding capacitance, an inductor is always a resonant circuit with a corresponding resonant frequency. The collapsing magnetic field of the coils involved leads therefore - contrary to the ideal shape of a tall, narrow rectangular pulse - an overshoot of the line flyback. This, together with the influence of the ohmic resistance of the coil portions may be noticeable by distortions in the image. The counteract circuit measures that provide a linear current rise during the visible lines runnings.

The destruction of the stored magnetic energy in the Zeilenhinlauf vibration-damping resistors during the short payback time was replaced by a more economical process. Sets out simplified to the anode of the cathode of a thermionic diode Zeilenendröhre (booster diode, left) is connected, the anode of which is in turn connected via a capacitor to the other end of the primary winding connected ( the base point of the flyback transformer ).

The flyback voltage generated at the tip is applied to the diode in a suitable polarity. This leads, charges the capacitor mentioned from the energy of the collapsing magnetic field and thus dampens the vibrations usually occur effectively. Due to a circuitry trick the operating voltage of the device and the so-called booster voltage thus generated can (from to boost, boost ) are connected in series, so that an operating voltage in the amount of 500 .800 V results, making the line output stage works more effectively and more linear. Therefore, this diode is also called booster diode, rarely also saving diode because the flyback energy is not converted into heat, but the device itself is to be available and thus effectively reduces the power consumption from the power grid.

The secondary coil of the high voltage generation is cast because of the rollover risk in resin or solidified beeswax or wrapped in very early copies as very flat cheese and partially covered with plastic. From there the high-voltage via a short cable (5) to the anode of the high voltage rectifier tube (4) is passed, which is also insulated with plastic parts against arcing. Of its cathode, the high voltage via a cable (6) is directed to the anode of the picture tube.

The line continued to provide transformer with a single turn the heating voltage of about 1.25 V for the hot cathode of the high voltage rectifier tube ready. The separate heating of the high voltage rectifier diode (such DY86 or DY802 ) is essential to supply the high voltage and cathode electrically isolated - the heater winding is manufactured from this high-voltage insulated stranded wire. The setting of the heating voltage was carried out by visual comparison of the Heizfadenhelligkeit with a second, battery- heated tube, where appropriate, by inserting a series resistor. In the early days of television technology tubes with 6.3V heater voltage ( EY51, EY86, ...) were used at this point, but this required more windings on the transformer and the advancement of technology were soon replaced by the D- types over.

In order to shield the strong electromagnetic fields and also to protect against high voltage put the line output transformer then with the corresponding tubes in a so-called line cage. It consisted of a metal box, which was provided with many perforations, in order to guarantee the escape of the heat emitted by the tubes. He also presented some shielding against in any existing stabilization Ballasttriode ( PD500 or PD510 mostly ) resulting X-rays dar.

Execution in the 1980s and 1990s

Most flyback transformers contain high voltage diodes that rectify the AC voltage of the transformer in a potted enclosure. In a high-voltage cascade, the anode voltage of the picture tube as well as the focus voltages for the supply of the focusing are generated (electrostatic focusing) in the blasting system of the picture tube. The variable resistors for fine adjustment of the focus and screen voltages are integrated in today's flyback transformers. The line transformers are driven by the switching transistors (power bipolar transistors ).

Newer flyback transformers, which are common for example in Trinitron tubes are designed as so-called diode split transformers (DST). In these, the high-voltage winding is divided into several sections, each of which provide a rectifier circuit. The complete assembly, as shown in the adjacent circuit diagram, is completely molded. It not only covers the diodes yet circuit components for the generation and adjustment of the focus voltages and control connections. The diodes for rectification of the high voltage is connected between a plurality of secondary windings connected in series so that the high voltage cascade is omitted. This makes it possible to divide the high frequency AC voltages, so that must be isolated only with DC voltage with the anode voltage.

This is associated with a lower Isolierstoffbelastung and a lower tendency to pre-discharges that lead to Isolierstoffschädigungen. Furthermore, in this circuit performs an image tube over ( electric discharge inside the CRT) to no overload of the rectifier diodes, as was the case with the previously used high-voltage cascade. , The number of necessary high-voltage capacitors may be reduced to half.

Swell

• Heinz Richter: TV - experimental practice. Franckh'sche Verlagshandlung W. Keller & Co., Stuttgart, 1952.
• Heinz Richter: TV for all. Franckh'sche Verlagshandlung W. Keller & Co., Stuttgart, 1952.
• Otto Aeschlimann: TV technology without ballast. Franzis -Verlag, Munich 1969.
• Otto Aeschlimann: 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 ( translated by J. Jager, Hartley Carter ) (PDF, 16MB ).

• Televisions Technology
• Transformer