Repair of network transformers. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Power Supplies

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No matter how popular switching power supplies are and no matter how many advantages they have over conventional power supplies (continuity of operation), the latter are extremely durable. And this is understandable. Ease of manufacture always attracts a radio amateur. And the material side of the issue forces us to repair what is available.

In publications devoted to the calculation and manufacture of network transformers (ST), many radio amateurs are frightened off by the abundance of formulas, graphs and tables. Let's try to approach this issue purely practically, that is, consider simple techniques.

First and most important. To engage in the restoration (rewinding) of ST, it is not at all necessary to be a specialist in the field of radio engineering. In the studio where I worked, there was a man who rewound any transformers, having no knowledge of radio engineering at all. This means that if your power supply (adapter) is out of order, then do not rush to give it for repair, but rather try to repair it yourself. In addition, the cost of repairs can be quite comparable to the price of a brand new ST or even the entire power supply unit (PSU). If we decide to independently manufacture a stabilized power supply unit, a charger or a voltage converter (50 Hz) (12 ... 220 V), then we will have to "make friends" with transformers.

Let's start with low-power transformers. Most often, the radio amateur "stumbles", rewinding the ST "one to one", if the ST is "burned out". The fact is that usually STs are always underwound (especially new ones, of recent years of production, and, of course, of Asian origin). Engineering calculation implies the optimization of the ST parameters. Practice shows that such optimization (mainly in household RES) contributes to overheating of the ST due to saving copper. An experienced radio amateur will take iron of a larger cross section (a margin for the overall power of the transformer) and wind the primary (I) and secondary (II) windings of the transformer with a certain margin, providing a lower idle current (Ixx). The heating of the windings will be less, and the reliability of the winding product will be higher.

And if the ST is installed in a stabilized power supply, then an increase in the voltage drawdown of the secondary windings does not play any role at all.

Let's consider a practical case. Smoke came out of the transformer in the two-cassette "Internationale" (this happens often here, especially if there is a 110 V switch, users usually put it in this position). In principle, half of household RESs, as well as "Chinese" who have similar STs, have such properties.

Small-sized STs are installed in chargers (unfortunate devices), in power supplies of receivers, etc. After the "fireworks", the primary winding of the ST burns out and it becomes impossible to know how many turns it contained.

I wound on a similar "hardware" (Sh13Ch18) the primary winding of 4500 turns D0,08 mm (even D0,09 mm may not fit). It is very good if the ST frame is preserved (not burned out, not melted), otherwise there will be more fuss.

For the manufacture of the frame, fiberglass 1 mm thick and a jigsaw are well suited. Winding II contained 260 turns of D0,23 mm wire. It is clear that winding 4500 turns with a "hair" is not a pleasant occupation. Therefore, I used an electric drill with a voltage regulator (all new electric drills have such a regulator). It is important to center the frame relative to the axis of rotation of the electric drill chuck. The enamel wire D0,07 ... 0,08 mm (I’m not talking about thinner ones) breaks off very easily, especially at increased drill speeds. And soldering is not only a nasty business (tinning requires patience and accuracy), but also contributes to an increase in the diameter of the coil, if only because of the introduction of additional insulation.

Anyone who likes to fuss will not stand such work for a long time. Often the plates of the CT magnetic circuit are connected by welding. It is easy to make a cut with a hacksaw and remove the burnt CT winding. The simplest formula, proven by practice in the repair of transformers:

N=50/S,

where N is the number of turns per volt in both I and II windings of the ST; S is the cross-sectional area of ​​the magnetic circuit (cm2).

For W-shaped iron "Chinese" 13Ch18 we have S = 2,34 cm2, and N = 21,37 turns per volt. The number of turns I of the winding n=21,37×220=4700. Since the steel here is of high quality (with such a number of turns Iхх<3 mA), in order to place the second winding, the number of turns of the primary was reduced to 4000, obtaining Iхх=5 mA.

The result is excellent when compared with the factory winding option (about 3000 turns) and the value of Iхх=20...40 mA.

Now they talk and write a lot about transformers, but people need practical information (less calculations, more specifics). So, if in a powerful ST the increased current Ixx can even be beneficial, for example, lead to an increase in efficiency (of course, if without severe overheating), then in a low-power ST, failure is guaranteed. Why? If an increase in current Iхх from 100 to 120 mA for such a "monster" as TS-180 causes some additional heating, then for Ш13Ч18 an increase in Iхх from 5 mA to 10 mA is like death! The power dissipated by the ST in the Ixx mode increases sharply. In this "Asian" mode, the power may exceed the power drawn by the load. A person in the market buys a "Chinese" power supply unit, and after half an hour smoke comes out of the power supply unit (at Ixx = 40 mA, the power in the ST reaches 8 W, and we are talking about 5 ... 10-watt STs).

If you want your small-sized CT to work long and trouble-free, it should not heat up at all in the Ixx mode. If it heats up in this mode so that it cannot be held by hand, then it was made incorrectly. In order not to risk, we connect the primary winding to the network through a table lamp. In this case, the turn-to-turn short circuit is not terrible - almost all the voltage will “sit down” on the light bulb.

Based on my experience with large transformers, I can say that, for example, in the TS-270 there is a large spread in Ixx. In some cases, in ST it is necessary to sort out the magnetic circuit. Before disassembly, it is necessary to measure and remember the value of Ixx. It is unlikely to become smaller after assembly and disassembly. No matter how you tighten the ST with the appropriate fasteners, but reducing Ixx to the "native" value is not an easy task. I have already talked about this topic [1].

It is necessary to thoroughly clean both halves of the magnetic circuit. I have seen cases when the halves of the magnetic circuit were of different sections, so these specimens got very hot.

It is not difficult to reduce the value of Ixx by winding the primary winding. We wind all the secondary windings, carefully remove the screen. Next, in the TS-180, we wind one layer of the winding with a D1 mm wire (about 60 turns). Further winding will do more harm than good.

Measurements in circuits with ST should be made using a laboratory autotransformer LATR, introducing voltage from zero [2]. And it is necessary to have an ammeter, with which you can measure Ixx and determine short-circuited turns.

References:

1. Zyzyuk A.G. About the power supply, published in RA 7/2001//Electric. 2001. - No. 12. - C.5.
2. Zyzyuk A.G. Network ammeter to LATR//Elektrik. - 2000. - No. 12. - P.11.
3. Zyzyuk A.G. Repair of power supplies type IPT-3//Electrician. - 2002. - No. 4. P.14.
4. Zyzyuk A.G. About transformers // Radioamator. - 1998. - No. 2. - P.37.

Author: A.G. Zyzyuk

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