Theory: network power supplies with a transformer. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Beginner radio amateur
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When powering electronic devices from the network, it is first of all necessary to convert the voltage of 220 V to the required one, usually much lower, and to ensure galvanic isolation (no direct contact) with the network. Both of these functions are performed by a network transformer. As a rule, constant supply voltages are needed, so the next task is to turn the 50 Hz alternating voltage into a constant one. This is a rectifier function with a smoothing filter.
Consider first the transformer (Fig. 61).
Since the mains frequency is quite low, magnetic circuits (cores) are used from stamped W-shaped plates of transformer steel. Twisted split or toroidal magnetic circuits made of furnace steel tape are less commonly used. The size of the magnetic circuit determines the power of the transformer, and therefore the entire power supply. It is tentatively considered that for magnetic circuits made of stamped plates, the power in watts is equal to the square of the coil window area (in cm2).
The number of turns of the windings must be proportional to the voltages at their terminals. The proportionality factor is the number of turns per volt. It is determined again by the cross section of the magnetic circuit S and is found by the empirical formula 40 ... 45 / S. If the number of turns per volt is not enough, the no-load current of the primary winding increases sharply and the transformer stray field, which creates pickups on the sensitive cascades of radio equipment, increases the heating of the wire.
As an example, let's say that you need a 12 V transformer with a load current of 1 A. Its power will be 12 W, and the cross-sectional area of the magnetic circuit must be at least 3,5 cm2. Suitable plates W12 with a set thickness of 30 mm or W16 with a thickness of 23 mm The number of turns per volt will be 14. Therefore, the primary winding should contain 220x14 = 3080 turns, the secondary - 12x14 = 168 turns. To compensate for the voltage drop across the resistance of the wires, the number of turns of the secondary winding is increased by 5 ... 10%, that is, it must be increased to 180 turns.
Now about the diameter of the wire. The permissible current density in the wire of low-power transformers is 2,5 A/mm2. Hence d(MM) = 0,7√l (A). In our example, the diameter of the secondary wire will be 0,7 mm. The current in the primary winding will be as many times less as its voltage is greater, i.e. 1 Ah 12/220 = 55 mA. But it is also necessary to take into account the no-load current of the order of 15 ... 20 mA and from here calculate the diameter of the primary winding wire. It turns out about 0.18 mm.
It is not difficult to wind the transformer on your own, but often radio amateurs try to find a suitable ready-made transformer. This is easy to do. Even if the transformer data is unknown, you can determine the winding leads with an ohmmeter and then apply an alternating voltage to the highest resistance, but not 220, but 20 ... 30 V. By measuring the voltage on other windings with an alternating voltmeter, the transformation ratios are determined.
Is it permissible to apply 220 V to the primary winding? This can be determined by measuring the no-load current of the primary winding, gradually increasing the voltage on it. The no-load current should not exceed 50% of the rated current (under load) for the smallest transformers and 10 ... 20% for more powerful ones.
If the transformer is suitable for the voltage of the windings, it must be loaded with rated current, using, for example, incandescent lamps, and allowed to work for several hours. The absence of excessive heat will indicate the suitability of the transformer.
Author: V.Polyakov
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