Conex Club Magazine - no.4 - 2000

Conex Club Magazine - no. 4 - 2000 - Stabilized source 12V / 5A

Reading time: 3 minute

The main topics presented

1. 2x30W stereo amplifier

The series of presentation of power amplifiers continues with a high-performance model that offers a musical power output 2 x 30W with low distortion of 0.08%.

The amplifier is configured with standby and muting functions and accepts both a monophonic and a stereo signal source.

The stereo amplifier is made with the integrated circuit TDA7394 which has some special technical features that it recommends to Hi-Fi audition enthusiasts.

2. Check DTA / DTC transistors

The structure of such transistors is presented DTC (npn) or DTA (pnp) made in Asia. Also here is the arrangement of the terminals to the D-PAC capsule - a plastic capsule similar to T092.

Table 1 (in the article) presents the catalog data regarding the resistors included on the structure for some of the usual types of DTC / DTA transistors.

On most transistors of this type the maximum collector current is approx. A 0.1, the maximum emitter collector voltage is 50V and the maximum power dissipated by 0.2.

Figure 2 (from the article) shows a simple circuit for checking these transistors. A circuit is used MMC4047 (CD 4047) connected as stable, oscillating on a convenient frequency bore from group R1-C1 (1 = 1 / 4.4 x R1 x C1);

3. Microphone preamplifier

In the case of frequency modulated radio broadcasts, the frequency curve of a microphone amplifier must have a certain alignment.

It must be ascending to the high frequencies in order to obtain an intelligible and penetrating modulation.

The amplifier is made with 2 transistors of type BC171-173. Preferably the first transistor is BC173.

The resistor is connected to the emitter circuit of the first amplifier transistor 1 KOhm which is taken by a capacitor with the value of 1pF. This makes the amplification increase in proportion to the frequency of the input signal, in our case, from the microphone.

The second amplifier transistor has a 100 Ohm semi-adjustable resistor connected to the emitter circuit. By acting on it you can adjust the amplification of the entire assembly, about 4-5 times, in the sense of decreasing the degree of amplification.

This amplifier is recommended when using dynamic microphones.

4. Prescaler 1Ghz

The assembly is designed to extend the frequency measurement range up to 1GHz, having a good rejection of disturbing signals and a sensitivity of 10 mV / 1 MOhm. It is based on integrated circuits SAB6456 and 74LS90.

5. FI amplifier

A subassembly of great importance, from the radio receivers destined for radiocommunications is the intermediate frequency amplifier.

This amplifier must provide a substantial amplification of the signal from the mixer, an amplification that must reach values ​​of 80 - 120dB, because the signal at the output of the mixer has very small values.

Using discrete components, these values ​​for amplification were obtained with several stages, but large manufacturers, such as Motorola, have created specialized integrated circuits such as MC1350P.

In addition to a pronounced amplification, approx. 50dB, this circuit has its own low noise and can be controlled by the automatic amplification adjustment system.

The amplifier presented in this article, intended especially for shows J3E (SSB), experimentally works excellently being tested in traffic and compared to industrial products.

These qualities could be obtained using modern circuits, the ingenuity and skill of the author.

Analyzing the principle electrical diagram of the intermediate frequency amplifier, it is observed that it contains 5 integrated circuits that ensure all the functions of the intended purpose.

Two noted circuits IC1 and IC2 type MC1350P provides amplification, a noted circuit IC4 LM358 performs automatic control of amplification, circuit IC3 MC1496 is the SSB demodulator and the circuit IC5 LM386 which is the audio frequency amplifier.

6. 12V / 5A stabilized source

The voltage source shown is a typical application of the integrated voltage regulator with adjustable output, LM317.

The increase of the current debited at the output, as well as its limitation is achieved with the help of a external transistor (type BD912) which takes over the excess current carried by the load.

The principle electrical diagram of the voltage source is presented in figure 1 (from the article), and its main technical parameters are:

  • 9-15V output voltage (typically 12V);
  • maximum output current 5A;
  • short circuit protection, I-limit = 6A;
  • load stabilization 0.8% (D: I = 5A).

7. SMD technique

SMD… Under this name hides the biggest revolution in the current electronics industry. If we paraphrased Discovery Chanel we could say "tourning point of electronic technology".

The rapid transition from the classic components provided with tinned copper terminals or legs to the miniaturized ones, directly weldable on the wiring, produced an enormous qualitative leap both in terms of increasing reliability and decreasing the dimensions of the subassemblies.

The innumerable advantages of this new technology explain the explosive penetration in all fields of electronics: industry, microcomputers, telecommunications, consumer products, etc.

The article presents the advantages (but also the disadvantages) of these components, their characteristics, the laying technique.


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