Pulse transformer test methods

The author believes that the methods for testing pulse transformers with low-level signals without desoldering from the circuit are unreliable. It offers two simple methods for near-operational transformer testing. Of course, their dismantling is required, but the reliability of the test results is guaranteed!

   Pulse transformers of power supplies and line scanners fail most often due to overheating of the windings. With the breakdown of the power switches, the current in the winding rises sharply, which leads to its local heating with a subsequent violation of the insulation of the winding wire. Most often this happens in small-sized transformers wound with a thin wire, for example, in power supplies of modern video recorders, video players and line transformers (TDKS) of TVs. As a result of overheating of the winding wire, turn-to-turn closures occur, which sharply reduce the Q-factor of the transformer, which disrupts the operating mode of the auto-generator of a pulsed power supply (SMPS) or a horizontal scanning stage.
   Checking pulse transformers of power supplies and TDKS is a rather relevant topic, a lot of methods for detecting turn-to-turn short circuits have been described. The results of testing pulse transformers by measuring the resonant frequency, inductance or quality factor of the winding are unreliable. The resonant frequency of a transformer is particularly dependent on the number of turns, the capacitance between the layers of the windings, the properties of the core material and the height of the gap. Turn-to-turn closures do not eliminate resonance, but only increase the resonant frequency and reduce the Q-factor of the coil. The shape of the test sinusoidal voltage is not distorted by short-circuited windings, and it is generally unreasonable to use rectangular pulses due to the occurrence of shock excitation pulses. There are also devices on this principle, but they are ineffective. The saturation of the core can affect the pulse shape, but in this case, a high-power generator is needed. Apparently, for these reasons, the efficiency of the known methods is very low, and the test results are not very reliable.
   The following are simple, reliable methods for testing pulse transformers in a near-operational mode. The output stage of the horizontal scan of the TV or its switching power supply (SMPS) is used as a signal generator. The proposed methods make it possible to safely detect the places of breakdown of the TDKS housing insulation, the so-called "fistulas".
   To check the first method, a working TV is required, the horizontal scan of which is used as a generator. The tested TDKS must be dismantled, and its filament winding connected to the filament voltage terminals on the kinescope board, as shown in Fig. 1.
   For the second method, a working SMPS is used as a generator, it is even possible from a repaired TV. To check the TDKS, the winding designed to connect the line transistor is connected to the secondary winding of the SMPS transformer, designed to generate a voltage of 110 ... 140 V (Fig. 2].
Трансформаторы ТДКС

                  Connecting the tested TDKS through the filament winding                                                                   строчный трансзистор

   In both cases, the TDKS turns out to be in a mode close to the working one, and the criterion for its serviceability can be considered the appearance of a high voltage at the anode output, capable of "piercing" 2 ... 3 cm of air space. For the manufacture of the arrester, you can use a wire with two crocodile clips. One "crocodile" is connected to the negative terminal of the anode winding, and the second is hung on the "suction cup", where the spark gap is formed. The presence of short-circuited turns is easily determined by the overload of the generator [line scan or SMPS] and the absence of discharges in the high-voltage circuit.
   Suspicious SMPS transformers can be checked using the second method, connecting a winding designed for a power switch to the generator output. An indication of the presence of short-circuited turns in the transformer under test is the overload of the SMPS, the breakdown of generation and the operation of the protection.
   A final reminder: when working with high voltages, remember the safety rules!

                  Alexander Omelyanenko
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