Conex Club Magazine - no.5 - 2002

Conex Club Magazine - no.5 - 2002 - Charger for NiCd batteries with U2400B circuit

Reading time: 4 minute

The main topics presented

1. Are mobile phones harmful?

The recent development of cellular telephony and, in general, a mobile radio communications systems (SCRM), led to an increase in public interest in assessing the possible harmful effects of the electromagnetic field (cem) of radio frequency (RF) emitted by the devices used in these systems on the human body.

The issue of the biological effects of cem at very high powers (of the order of megawatts and gigawatts), which appeared as a result of the development of radar systems, were approached by a wide circle of specialists in the related fields (especially Americans and Russians). One of the results of this research was the establishment of norms (standards) for staff protection.

SCRMs, made relatively recently (2001), use low power and obviously need to be analyzed in terms of biological effects. Consequently, without taking into account the possible harmful effects on health due to the so-called non-thermal effect of cem, it is necessary to evaluate the quantitative cem induced in the various tissues of the human body.

This assessment is imposed by performing the most significant biological experiments. On the other hand, an efficient, accurate and reliable assessment is required by the assessment of standardized compliance tests for mobile telecommunications equipment (ETM) with basic protection limits.

The analysis of the possible range of variations of the intensity of the induced fields in various tissues requires a great effort, because the intensity of the local field strongly depends on a large number of parameters such as:

  • working frequency and antenna input power;
  • the position of the device in relation to the user's head;
  • device construction;
  • the external shape of the user's head;
  • distribution of various tissues in the head;
  • the electrical properties of these tissues.

2. Charger for NiCd batteries with U2400B circuit

Batteries have a lifespan usually expressed in charge-discharge cycles. Usually commercially available NiCd cells have a lifespan of approx. 300-500 cycles and this in the conditions of a neat, controlled charging of the respective battery.

By doing otherwise, a new battery may lose capacity or malfunction after a few dozen cycles. Careful charging means ensuring a regime that prevents overcharging or raising the internal temperature of the battery beyond the allowable limits.

Usually, during fast charging, detecting a slightly negative slope of the voltage on the battery can determine its termination and avoid overcharging.

This type of charging termination is known as the -dV criterion and gives good results in combination with a maximum charging time criterion.

Battery memory effect

Because some batteries may have a so-called memory effect, or their state of charge is unknown, a controlled discharge of the battery before charging is sometimes useful.

This unloading regime is especially recommended when using fast loading methods, realizing precise "initialization" of the battery status.

Specially designed to ensure long battery life, the U2400B circuit is a example of a modern circuit for charging NiCd batteries.

The block diagram of the circuit is the one in figure 1 (from the article), its capsule being of the type DIL with 16 terminals.

How does the circuit work?

The circuit can be supplied with voltages between 5V and 25V, allowing direct charging of batteries up to 12V.

Using level transfer circuits (output pin 12 can support a maximum of 26V) can be charged batteries with higher voltages.

The circuit has an internal voltage reference, with a nominal value of 3V, the references of the internal comparators (maximum temperature, discharge, charging) being Analog output signals for the voltage range of 0.498V - 0.552V., the hysteresis being 15mV.

3. RS232 to RS485 converter

Electronists who have tried at least once to transmit TTL logic signals of the "DATA" type on long lines have been able to find with regret that they are distorted and are disturbed by the inductions of the environment.

Methods of amplifying, filtering or using shielded cables do not always give the best results. Therefore, in the race against the clock of the development of cable data teletransmission, famous companies in the construction of electronic components also participate.

One of these, easy to find on the Internet, is also MAXIM, the famous manufacturer of MAX xxx chips.

The most frequently used port for such communications is the serial one which, in addition to the signals strictly necessary for RX and TX communication, also has a set of auxiliary signals. DCD, DTR, DSR, RTS, CTS and Rl.

They have standardized voltage levels at VL = -12V and VH = + 12V, the reference being the common table. The converter used in this case is a MAX232 which is able to make CMOS and TTL signals compatible in both directions.

4. Numeric interface

The article addresses the methodology of creating a numerical interface, not only as an extension of the existing ports typically in a computer, but also as a component of a standard acquisition system.

This interface is presented as a starting point for the development of applications that communicate with computer buses.

5. BA6209 (Reversible motor driver)

This name represents a specialized circuit designed to control the speed and direction of rotation of brush motors.

With two pulses applied to the circuit input, three utility modes of the motor are controlled: forward, reverse and stop.

The circuit is recommended for applications in cassette players, video recorders or other apparatus incorporating motors.

If the controlled motor requires higher currents than those provided by the BA6209 circuit, then suitable transistors are interposed. The circuit can be interfaced with elements in CMOS technology.

6. Automatic for lighting

It should be noted that the installation is connected directly to the alternating current network using the existing installation in the building.

On each floor there is an actuation button that controls the supply of all bulbs on all floors for a predetermined time depending on needs. The timing element is the LM555 integrated circuit.

The closing of the circuit for powering the bulbs is done through the contacts of a relay. I'm using a type relay MILLIONSPOT H100F23-1-C which is fed with 12V and which through its contacts can feed a load of 1kW at 220V.

The relay is mounted in the collector circuit of the transistor Q1 type BC171. The LM555 circuit is in the configuration of monostor. The duration of the timing is established by the elements R4 and C3 and is calculated with the formula T = 1.1C3R4.


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