All the variety of today's household radio equipment has one common, unifying element - a power source. For the so-called "stationary" equipment, such a source is an AC power network of industrial frequency (50 or 60 Hz), the voltage of which is transformed to the required value using "power" transformers. And even for most portable battery-powered devices, power adapters are often provided to allow such equipment to be connected to the mains.
The heart of any mains power supply is a power transformer. Under normal operating conditions (rated voltage of the source, moderate ambient humidity, etc.), power transformers are extremely reliable parts that do not fail “just like that”, for no apparent reason.
This fact allowed the designers to abandon the traditional "margin of safety" in the design and manufacture of power transformers and thereby significantly reduce their dimensions and weight.
However, every medal has a flip side. Bringing the utilization factor of the transformer in terms of power and permissible current to 80 ... 90%, the designers had to switch to the use of extremely thin winding wires (with a diameter of about 0.05 ... 0.07 mm) for the primary windings of low-power transformers, to an increase in the current density in the secondary windings, as well as the use of moisture-proof compound fillings of ready-made transformers, which significantly impair the heat transfer of the windings, especially internal (primary) ones. As a result, such transformers have become very critical to various abnormal, emergency situations - increased (even short-term) voltage in the network, accidental short circuits in the apparatus circuit (especially in the rectifier circuit). And very often, as a result of such random situations, power transformers are irreversibly damaged, usually due to the burnout of the internal terminal of the primary winding.
And here the service technicians face certain difficulties. The fact is that in modern ultra-compact equipment, a minimum volume of internal space is allocated to a power transformer, which does not allow replacing a burnt-out transformer with a transformer of another type with the same electrical (winding) data. And if exactly the same new transformer does not appear in the workshop, the only way to bring the device back to life is to rewind the burned-out transformer.
However, for this, it is necessary to know exactly the number of turns and the diameter of the wire of each winding, which may be impossible in the absence or burning of the label.
In this case, the two tables below, in which there are data that allow without any
calculations to make any power transformer with a rated power from 1 W to 1 kW. The tables are compiled on the basis of an analysis of the winding data of numerous real transformers (mainly of Western European and partly Japanese production), as well as interpolated mathematical calculations, and are confirmed by repeated tests in practice.
For each standard size of the transformer (see Table 1) the rated power (Pnom), the cross-sectional area of the core (Scepa), the number of turns per volt of voltage (in no-load mode), the total number of turns and the diameter of the winding wire (dnp) for primary windings for mains voltage in NO and 220 V, as well as similar data for the most "typical" windings with voltages of 6.3; 12.6; 25 and 250 V (the latter for lamp equipment). The tables are extremely easy to use. Having disassembled the burned-out transformer, measure the cross-sectional area of the core with maximum accuracy, find this (or a neighboring larger) value in the table and wind the primary or two primary windings according to the data corresponding to this section. Secondary windings for "standard" voltages are wound directly according to the tabular data, and if the required voltage differs from the standard, then the required number of turns is determined through the "number of turns per volt" indicator. If there is a problem with determining the wire diameter for "non-standard" secondary windings, use the second table (Table 2).
REPAIR & SERVICE-08-2001