Conex Club Magazine - no.1 - 2000

Conex Club Magazine - no.1 - 2000 - VHF Transceiver 144 - 148MHz

Reading time: 4 minute

The main topics presented

1. Amplifier 2 x 15W

Integrated circuit TDA2005 is an audio power amplifier, stereo operating in class B, specially designed for applications to car radio cassette players: power booster.

The circuit has a current capacity of up to 3.5A and can work well on a load whose impedance drops to 1.6 Ohm (in stereo applications) obtaining a musical power greater than 20W (bridge configuration). It is presented in a MULTIWATT capsule, the configuration of the pins being offered in figure 2 (in the article).

TDA2005 has a short circuit protection of the outputs at ground, short-term overvoltages, mostly inductive loads and chip overtemperature caused either by an output overload or by an incorrect cooling.

The maximum voltage up to which the integrated circuit can work is 18V.

2. Advantages of using MOSFET transistors

It is known that bipolar power transistors require for the control an important base current of the order of hundreds of mA, because the amplification factor in direct current B (Beta) is small and there is the problem of creating an expensive control scheme that provides the necessary basic current.

MOSFET transistors have the great advantage of being voltage-controlled semiconductor devices, the current absorbed by the grid being very small (pA order). The value of the grid-source control voltage (UGs) is usually between 3-15V.

An important quality of MOSFET transistors is that they have drain current (ID) consisting only of majority carriers. Thus, the switching times (both at conduction and at blocking) are shorter than at bipolar transistors.

With these transistors, switching voltage sources can be made with a very high working frequency, and their size decreases significantly. On the feature ID = f (UDS) at constant UGS, MOSFET transistors have a linear portion being recommended in the realization of linear voltage sources.

The drain-source voltage drop in the conduction is slightly higher (1-2V) than for saturated bipolar power transistors (UCEsat = 0.2 - 0.1V), which correlated with the drain current considerably increases its dissipated power.

The electrical resistance of the channel when the transistor conducts RDSon is usually between 0.1 - 5 Ohm depending on the drain-source working current and voltage.

Because MOSFET transistors made in the horizontal structure have the electrical resistance of the large channel, for high powers they are made in the vertical structure in double layer, DMOS or with the gate electrode made in depth in a V-shaped channel, VMOS.

3. Thermostat

The electronic thermostat is used when the aim is to obtain and maintain a constant, predetermined temperature in an enclosure, such as: the incubator, the aquarium, a corrosion tank or a living room.

The assembly shown uses the integrated circuit as a temperature sensor LM135 (or LM335) known to readers from the presentation made in issue 2 of the magazine in the material entitled "Electronic thermometer". The transmission of the control to the heating element of the monitored enclosure is done by an electromagnetic relay.

The main technical data of the thermostat are:

  • Supervised temperature range: 0 - 100C;
  • Adjustable hysteresis: 0.5 - 10C;
  • Output on a relay with NI / ND contacts, 6A / 250V;
  • 5V / 200mA power output for an electronic voltmeter;
  • Supply voltage: 220V.

4. 4-tone siren

Of particular amusement, this electronic montage can generate signals that mimic groups of sounds already established and known such as that of the medical ambulance, of the intervention machines of the police or firemen or that of the machine gun in action.

The electronic assembly presented is made with the specialized integrated circuit for this purpose UM3561A noted in the electrical diagram with U1.

Depending on the logic level of pins 6 and 1 at the output of the circuit UM2561 (pin 3) we will have the electrical signal of one of the 4 possible tones.

There are four such possibilities presented in the table in the article. The terminals corresponding to pins 1 and 2, respectively, were marked with S6 and S1; they can connect to either Vcc or GND, but never connect to the same potential. The combination of both unconnected (NC) has the effect of generating a tone similar to that emitted by the police car. For easy switching on these working modes it is recommended to mount two dip switches.

It is possible to supply the assembly with electrical voltage within the range 3 - 12V. An electroacoustic transducer is used low power speaker (1VV) with impedance of 4-8 Ohm. It is mounted at terminals OUT1 and OUT2, between the plus terminal (Vcc) and the collector of the darlington group Q1 - Q2 (BC547B - BD135). This group acts as an audio frequency amplifier.

5. VHF Transceiver 144 - 148MHz

This narrowband frequency modulation (NBF) transceiver is suitable to work in the field reserved for radio amateurs, ie 144 - 148MHz, but also outside this field within limited limits.

It is made on distinct functional blocks and incorporates high performance components, being located through the electrical performances in the category of professional equipment.

Obviously, such a construction can be approached by people with experience in the field and with a passion for amateur radio.

6. DC / DC converter

The new control circuits for switching sources and the accessibility of MOSFET power transistors have led to the simplification of power supply schemes.

This article presents a simple DC / DC converter, usable for powering from a direct current source 12V of circuits that require power supply voltage 25V DC.

It is often necessary to supply circuits or portions of circuits with different voltages. An example can be a radio transmitter that uses type transistors in the output floor 2N3632 fed to 24-25V, while the rest of the transmitter components are powered at 12V.

In stationary mode, with mains supply, it is easy to build a supply circuit that generates the two voltages.

When working in a laptop, or for various other reasons, if the entire device is to be powered from a single voltage source, we need a DC / DC converter.

As the "standardized" voltage is that of the usual batteries, most of the time the only voltage available is that of 13.8V.

The diagram presented in the article ensures an output voltage of 24-25V, a maximum output current 3A and absorbs from the 13.8V source a current lower than 8A.

7. Alternating voltage inverter

Alternating Voltage Inverters (ATVs) are an integral part of the class of power energy converters generically called static power converters which also include rectifiers, choppers and inverters.

VTA performs the conversion of alternating current - alternating current, more precisely, it modifies, within certain limits, the effective value of the fundamental on the load, the final result being the adjustment of the electric power dissipated on it.

VTA can be used to adjust the light intensity of a light bulb or the speed of an AC motor and more.


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