Tehnium Magazine no. 2 - 2006

Tehnium Magazine no.2 - 2006 - Filtration of rectified voltages

Reading time: 4 minute

The main topics discussed

1. Filtration of rectified voltages

The vast majority of electronic applications require DC power. As the electrochemical sources of direct current (batteries, accumulators) have limited capacity and are relatively expensive, we often use the alternating current network, which through rectification can provide us, practically unlimited, the necessary energy.

When analyzing recovery schemes, the resulting voltage is pulsating, and even if a capacitor is connected to the output of the rectifier that "smoothes" to some extent the resulting voltage, most of the time the situation is unsatisfactory, requiring a more efficient filtration.

In order to better understand the phenomena and ways of filtering, the article presents a brief analysis of the nature of the electrical signals resulting from the recovery process.

2. Charger for lead-acid batteries - sulfuric acid paste

Where are lead-sulfuric acid batteries used?

Batteries with lead-sulfuric acid paste, crimped models (hermetically encapsulated), with usual capacities of 4-7Ah, are usually used as emergency power supplies, at relatively short intervals, in case of accidental drop of mains voltage (for example, for computers, lighting installations, operating rooms in hospitals, etc.).

Of course, they have also attracted the attention of amateur builders, who use them for various other purposes, such as powering experimental assemblies in their own laboratory or with some devices they take with them on the trip (for example, of the lamps for lighting the tent, of the chargers for the mini-accumulators from the mobile phones, electric shavers, massagers) etc.

The advantages of lead-paste sulfuric acid batteries

These batteries are compact, of reasonable size and not too heavy, but especially are "clean" (acid does not flow or splash from them), which makes them easy to handle, transport and store.

In addition, at short intervals, they are able to provide currents with intensities of up to 10A, which an ordinary laboratory source does not allow.

Let us now turn to the actual subject of the article, namely the description of the charger proposed for 12V / 7Ah battery model.

Normally, the charging of such a battery - previously discharged - is done with a constant direct current with the intensity of the 0.7 A, for a period of 14 hours. Practically, however, few builders struggle to achieve a constant current source.

Most use a rectifier with or without filtering, which will discharge a "covering" DC voltage at the output, following that the determination of the desired charging current (average intensity, ie that indicated by DC ammeters) is done by introducing in series some adequate limiting resistors (coiled resistors or incandescent bulbs) of appropriate wattage.

The configuration of the first scheme presented in the article

The configuration of the scheme (rectifier without filtering, 12 V / 21 W car bulb as series limiting element) was imposed by the transformer used: an old, robust model, with good quality sheets, tightened in screws (practically, without vibrations), with a secondary of 2 x 9.7 V, for a maximum current of about 1.5 - 2 A (so without significant heating at 0.7 A).

3. Audio amplifier - Hi-Fi

The audio power amplifier presented in this article complies with international HI-FI standards, being in the class of the most advanced products in the field. It has the following features:

Nominal power PN = 50W;
OVER-POWER - OP = 10%;
Input impedance - Day = 47k kOhm;
Output impedance - Ze = 8 Ohm;
Input signal - Ui = 200mV;
Frequency band - BW = 16Hz - 25kHz;
Attenuation at the ends Bw - A / = 25V / us;
Signal-to-noise ratio - S / N> / = 80 dB;
Total harmonic distortions - THD
Intermodulation distortions - TID
Supply voltage UA = +/- 32V

4. Simplified calculation of toroidal transformers

Toroidal transformers are increasingly used by amateur manufacturers due to some their advantages as well:

  • small size;
  • low weight;
  • high efficiency;
  • low escape field;
  • better cooling.

The complete calculation of a toroidal transformer is complicated and involves the knowledge of data that are not accessible to amateur manufacturers.

The article will present the simplified calculation of toroidal transformers with P, with sufficient accuracy for amateur builders. Due to the fact that in our country there are transformer cores from the USSR-CIS, when making this material were used data from graphs 1 and 2 of the article; for cores of other origin, the same calculation method can be applied, provided that the net section and the induction in the core are determined.

5. Electric fences

For many years, electric fences for guarding animals or removing criminals have been used around the world. In order not to have a "Nazi" fence, of sad memory, we must know some strictly necessary elements, for an efficient fence and of course non-lethal.

In the European Union, the necessary conditions are regulated by the norm CENELEC (European Committee for Electrotechnical Standardization) EN 61011: 1992, which provides for the following:

  • duration between pulses: minimum 1 s;
  • max. voltage peak Vp: 10 kV;
  • pulse duration: 0.1 s max .;
  • amount of electricity: 2.5 mC max. (Q = C: V);
  • pulse energy 5J max. (E = CxV ^ 2/2);
  • period in which I> 300 mA: 1.5 ms;
  • Vp is measured on a capacitive load varying between 0.2 and 10 uF, in steps of XNUMX nF;
  • duration (U> 12 V), the amount of electricity and energy is measured on a resistive, non-inductive load of 500 Ohm.

Approved industrial constructions must withstand direct lightning.

6. Universal device for electric drilling machine

Often the amateur electronics technician, handyman or modeler is put in the situation to process small rotating parts or to cut to precise dimensions different non-metallic materials.

A medium power electric drilling machine (250 - 400 W) can do these processes if there are the necessary devices, using as tools a circular saw blade, a perforation, a few files, two or three disc cutters, a grinder stone of maximum 125 mm diameter and possibly some wood lathe knives.

In order to meet such requests, the article presents the construction of such a device.

7. FM tuner

It is proposed to make a FM tuner in the 88-108 MHz band. The material is addressed to builders with some experience, but also to beginners who have mastered the basics in the field of radio reception and want to approach a more complex assembly.

That is why a not very complicated scheme was chosen, with integrated circuits, which would allow the realization of a simple wiring in amateur conditions.

The most difficult problem for many is the realization and especially the adjustment of the coils in the oscillating circuits. Especially in ultrashorts, where a tiny displacement of a coil means an appreciable jump in frequency.

The proposed scheme includes only two such coils that are made easily, by rolling on the cylindrical side of a drill with a diameter of 6 mm.




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