Conex Club Magazine - no.6 - 2003

Conex Club Magazine - no.6 - 2003 - Thermal protection of power amplifiers

Reading time: 4 minute

The main topics presented

1. Wall-mounted electrical cable detector

A useful tool for technicians in the field of electrical installations of strong and weak currents. mounting detects live electrical cables mounted in the wall.

How many times have electricians been put in trouble when repairing an interrupted power line route and mounted in the wall?

Or how often are the cases when the installers of data networks or low currents, which mount applied on the wall, by means of screws, PVC cable channels, ask themselves if electric cables pass through there?

The presented assembly comes as a simple solution in solving this problem.

This kit signals optically and audio whether or not an electrical cable is live. It is recommended to use it to detect the wiring routes of power lines mounted in plaster (in the wall) or to detect defects along a line.

An LED (see figure 1 in the article) indicates the presence of cable current. The closer the installer (with the antenna of the kit, made on the printed wiring) to the electrical cable, the more the LED emits light "flashes" with a higher frequency.

Optionally, if you want to emit an audible signal, there is a place reserved on the wiring for mounting a buzzer.

2. Thermal protection of power amplifiers

Are shown some suggestions for thermal protection of audio power amplifiers.

Due to the subunit efficiency, any power amplifier dissipates some of the energy consumed in the form of heat. This is roughly the case with power supplies, including those that power amplifiers.

If this dissipated power is not properly discharged, it can become dangerous, shortening the life of semiconductors and other components or even causing their destruction. As a result, measures must be taken to prevent these undesirable effects.

The simplest method is to use thermal fuses (with normally open or closed contact and having different temperature thresholds), such as those used in household appliances (washing machines, microwave ovens, etc.). They are located in the areas of interest, ensuring the best possible thermal contact with the parts that emit heat (radiators, transformers, etc.).

3. Dynamic noise reducer

Interspersed in the audio chain, between the signal source and the tone controller, the dynamic noise reducers (in short DNR) aim at attenuating the background noise, "white", which is heard annoyingly in the acoustic enclosures or the speakers of the electronic devices.

As direct applications, it is used for portable cassette players, Hi-Fi audio systems with magnetic tape, videocassettes or TV / FM receivers.

Assembly equipped with integrated circuit LM1894 can be used in stereo audio systems to reduce noise. The utility of this installation is found in FM receivers, TV receivers, cassette players, video cassette players, etc.

For FM group L1 applications, C14 forms a pilot frequency filter 19kHz. Remember that a signal smaller than 100mVrms is not enough to activate the noise detector.

Depending on the signal source, the noise level will be adjusted from the semi-adjustable RV1. The assembly is interconnected after the signal source preamplifier and before the circuit that includes the volume and tone adjustment.

Technical data

  • Compatible with all pre-recorded audio tapes;
  • Reduces by 10dB the noise generated by the audio tapes;
  • Supply voltage: 4.5… .18V;
  • Input voltage: 300mVrms (max. 1Vrms);
  • Dimensions: 51 x 47mm.

4. 13.8V / 3A source with backup battery

Voltage sources are a very important chapter in the field of warning and surveillance systems, they require special attention from the design phase to the execution and testing.

They must be provided with a good thermal regime, for a long period of uninterrupted operation, sometimes even years. In addition, electronic mount protection is recommended in case of system defects (such as the battery).

13.8V / 3A source it is often used in radio transmitters for rapid intervention dispatchers, it providing the current necessary for the transmitter to transmit.

The montage presented comes as a continuation of the article from the previous issue. This is a 13.8V stabilized source at a limited maximum current of 3A and with a backup battery (12V / 3… 7Ah).

The output voltage can be adjusted in a narrow range around 13.8V. The assembly has protection against short circuit and overload.

5. Intercom system for 12 stations

The intercom system presented is suitable for use in buildings with a relatively small number of families, and can be adapted for a smaller number of stations.

The system consists of two main blocks: Central Unit (UC), which is mounted at the entrance to the building and contains a keyboard (which can have a number of buttons equal to the number of stations) and Individual Position (PI) which is mounted in each apartment.

6. Inflation barrier

The installation was created in order to signal an event, in this case to limit the access of some people in a certain space.

But the assembly can also be used for many other purposes, such as: counting objects, counting people in a performance hall in order to obtain statistics, etc., the fields of applicability being quite large.

The infrared barrier presented works on the principle of modern barriers in the field of "security".

Detection of objects metered by the barrier is not affected by errors (for example due to falling leaves from a tree or the passage of small animals) and this is due to the otherwise simple principle of using more than one transmitter-receiver IR pair.

The pairs of transmitters are coupled, in modern barriers, in an SI or OR logic or any other combination, precisely to minimize errors, respectively false alarms, depending on the purpose or function for which the barrier is mounted.

By using special lenses, current barriers can work up to 100m distance between transmitter and receiver.

In order to avoid the purchase of some quite expensive and pretentious components as a way of functioning, but also sometimes a bit difficult to procure, the variant with classic components was chosen (reference to the infrared transmitter and receiver).

7. Alternating voltage regulator with PIC16F84

The device allows adjusting the light intensity of filament bulbs. It is a demonstration of how microcontrollers can be used in an application such as light intensity control.


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