Network adapters. Reference data

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In shops, kiosks of underground passages, on the radio markets of many cities in the country, you can buy remote "socket" power supplies (often called adapters), designed in the form of a kind of enlarged mains plug. Regardless of the name of the manufacturer, they are usually made in China. What are these blocks, what are their real possibilities, how to use them? You will find answers to these questions in the article.

Remote power supplies are divided into two types - "universal" and specialized. A typical diagram of the universal block is shown in fig. 1. The device contains a step-down network transformer T1 with a large number of taps in the secondary winding, an output voltage switch SA1, a diode rectifier bridge VD1 - VD4 (usually from 1N4001 diodes for a voltage of 50 V and a current of 1 A), a smoothing capacitor C1, a power-on indicator - LED HL1 with current-limiting resistor R1, switch SA2 for the polarity of the output voltage and a set of output connectors at the end of the output cable (only one of them, X2, is conventionally shown in the diagram). For other units, the number of switch positions SA1 is less, there may be no indicator of inclusion in the network. "Universal" blocks are designed to work with various loads.

Specialized blocks are focused on one or another specific load, therefore they do not have secondary winding taps, a polarity switch, there is only one output connector, and often there is no on indicator.

Usually, the inscriptions on the nameplates of the blocks promise very good characteristics, which, however, are not confirmed in practice. The load characteristics of nine types of remote units were measured by the author in a laboratory (see table). The test results are shown in fig. 2 - 8. The graphs were taken at a reduced mains voltage - 205 V. This is close to the minimum value at which the power supplies should still work normally. The output voltage and current values ​​indicated in the table correspond to the inscriptions on the case.

(click to enlarge)

What conclusions can be drawn from these characteristics? The declared values ​​of the output voltage are provided at an output current that is much less than the promised one - two times or more. The minimum voltage (1,5 and 3 V) is given by FIRST units at a current equal to only 5% of the specified rating. At the maximum load current, the output voltage is one and a half to two times less than the nominal one (and even more for small values ​​of the output voltage).

The characteristic of the SLD MW108 universal block was only able to be taken for the "12 V" position of the output voltage switch (Fig. 7). During the measurements, the transformer warmed up so that the outer plastic insulating tape wound over the coil began to melt (and this is with the housing cover removed!). When a voltage of 150 V was applied to the primary winding instead of the nominal 220 V, the transformer practically did not heat up without load. This indicates that the transformer is calculated incorrectly. In addition, the output voltage drops so quickly with increasing load current that this indicates an excessively high resistance of the transformer windings.

The best parameters, first of all, the lowest output impedance, is possessed by the PPI-1280-TUV unit, which was equipped with active loudspeakers for the IBM PC computer. The output impedance of the RW-900 and *28 blocks, calculated, according to the seller, only for powering "Dendy" game consoles, is significantly higher. When comparing these three devices, which are close in their declared characteristics, it becomes obvious that the larger the mass of the block, the lower its output impedance, i.e., the better the load capacity.

On fig. 7, along with others, shows the characteristics of a mock-up sample of a self-made remote unit assembled on the basis of a standard TPP21 1 transformer with secondary windings connected in series and a diode bridge with a 1000 microfarad capacitor. Its output impedance is significantly less than that of the RW-900 or *28, but the mass is much larger. Note that the bottom line of the table shows the dimensions and weight of only the TPP211 transformer.

When using remote blocks, it should be borne in mind that the considered graphs in Fig. 2 - 7 illustrate the dependencies for the average value of the output voltage. In reality, a ripple voltage is superimposed on it, its shape at low current is close to sawtooth. On fig. 8 shows peak-to-peak (peak-to-peak) ripple versus output current for some of the tested devices. For FIRST units, dependences are shown for two positions of the switch SA1 - the upper curve corresponds to the "12 V" position, the lower one - "6 V". As can be seen from these graphs, the dependence of the ripple voltage on the current is determined mainly by the capacitance of the filter capacitor.

It should be noted that the nominal voltage of oxide capacitors installed in remote units often does not exceed 16 V (and even 10 V for SLD MW108). This is not enough for reliable operation of the unit at idle or at low current consumption. This is confirmed by Fig. 2 - 7. Cases have been noted when capacitors failed. In units with a 12 V output voltage, it is recommended to install capacitors with a nominal voltage of at least 25 V.

Even with a current equal to only 10% of the maximum, the ripple voltage reaches 0,5 V, which is too much to power most types of consumer radio-electronic equipment that do not have a built-in stabilizer in the power supply unit. Therefore, it is practically impossible to use remote units without a multiple increase in the capacitance of the filter capacitor or without an intermediate voltage regulator.

The design of some remote units allows you to build in them a simple stabilizer of very high quality, assembled on microcircuits of the KR142 group and others.

The most convenient for this purpose are microcircuit stabilizers of the KR142EN5 and KR142EN8 series [1]. If the required output voltage does not match one of the standardized values, you can use the KR142EN12A or KR142EN12B microcircuits with a resistive divider [2]. At the output of the stabilizer, you need to turn on an oxide capacitor with a capacity of at least 10 microfarads. The microcircuit should be installed on the heat sink, and to facilitate the thermal regime in the case, drill a dozen ventilation holes with a diameter of 6 mm.

You can determine the fundamental suitability of a particular remote unit for building a stabilized power supply as follows. With the required load current (it should not exceed half the limit, see table), the output voltage of the remote unit at the minimum mains voltage should be more than the required value by half the ripple voltage plus the minimum allowable voltage on the microcircuit used (about 2 ... 2,5 IN).

A practical example of the modernization of a remote unit is the design described in [3], the diagram in Fig. 10. Here it is convenient to use the diode bridge and capacitor C1, already available in the remote unit. Another example of using a generic block is presented in [4].

Literature

1. Shcherbina A., Blagiy S. Chip stabilizers series 142, K142, KR142. - Radio, 1990, No. 8, p. 89, 90; No. 9, p. 73.
2. Nefedov A., Golovina V. Chips KR142EN12. - Radio, 1993, No. 8, p. 41.
3. Alekseev S. Chargers for Ni-Ca batteries and batteries. - Radio, 1997, No. 1, p. 44 - 46; No. 2, p. 44 - 46.
4. Dolgov O. Charger with stabilized charging current. - Radio, 1996, No. 4, p. 72.

Author: S. Biryukov, Moscow

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