Conex Club Magazine - no.10 - 2005

Conex Club Magazine - no. 10 - 2005 - VIPer22A, low power switching source

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The main topics presented

1. Measuring temperature performance by non-contact methods

Thermography (thermography) is a term that refers to a modern, high-performance technique that allows the visualization and generation in real time of thermal maps ("thermal images", thermograms) of the technical (and even biological) systems under investigation.

For performing the thermal scanning activity specialized equipment called thermovision / thermography cameras is used, similar in size and appearance to the well-known video cameras in everyday life.

What is thermal vision?

Thermovision is a the method of viewing objects from the point of view of infrared radiation (IR) emitted by them and not by that of visible radiation which can be detected without any difficulty by the human eye.

In the ordinary situation man can see the surrounding objects due to the light reflected by them. The human eye has the ability to see a narrow portion of the electromagnetic spectrum, called the "visible" (Figure 2 in the article).

In the absence of the Sun, the main source of illumination, there is no light to be reflected and most mammals are virtually unable to see anything (some species of snakes have the ability to see infrared).

It has long been known that any body with a temperature higher than OK (-273.15 ° C) emits infrared energy. Unfortunately, the human eye does not have the ability to see the rest of the electromagnetic spectrum, so neither does the area of ​​the spectrum that includes the IR radiation emitted by bodies.

It seems surprising but even an eternal glacier in the Antarctic lands emits infrared radiation. The primary source of IR radiation is body heat.

How is IR energy generated?

IR energy is generated by the vibration and rotation of atoms and molecules in any biological or technical system. The laws on which thermovision is based are Planck's law, which introduced the energy quantum hypothesis and established for the spectral density of the emitter of a body, a formula that verified the experimental data in the whole frequency range, Law Stefan - Boltzmann, which established the connection between the total energy flow of the body and its absolute temperature and Wien travel law, which established the connection between the body temperature and the wavelength of the maximum spectral density of the emitter.

His contribution must not be omitted Einstein - the association of quanta with particles, called photons, which move at the speed of light.

2. Switching voltage source 1.25… 35V / 3A

The switching voltage regulators from the Simple Switchers range are grouped according to the current capability of the internal switching element, respectively LM2574 for 0.5A, LM2575 for 1A and LM2576 for 3A.

Each circuit in the respective range is made for a fixed value of stabilized voltage, respectively 3.3V, 5V, 12V and 15V, these can be differentiated by the suffix from the component name (example LM2575T-12, for 12V output voltage).

Within these product groups there are also variants for variable output voltage, recognized by the marking LM257 xT (or S) -ADJ. The application offered is based on the latter variant of the circuit, respectively the version for maximum 3A at the output (limited internally!), LM2576T-ADJ.

The assembly is supplied directly from a transformer that can supply the necessary power in the load (Attention !, the efficiency is about 70% -80%). The rectifier bridge used must support at least 3A.

3. Digital thermostat with 3 1/2 digit display

The electronic thermostat is recommended for use in an air conditioning system or boiler, but the applications do not stop with the examples here. The temperature is clearly displayed on a 3 and 1/2 digit LCD display, in degrees Celcius or degrees F.

A functional block that allows the adjustment of the hysteresis at trigger, ensures the installation a high degree of precision and safety in operation.

The thermostat made in the Velleman kit (unassembled) measures temperatures in the range of -50… + 150C, the range in which control is also ensured. A single strap allows the display to change from grade C to grade F.

The resolution of the device is +/- 0.1C (or +/- 1F) and allows the adjustment of the hysteresis range, around the controlled temperature, in a wide range, from 0.2 to 10C.

4. VIPer22A, low power switching source

VIPer22A combines in a single capsule (DIP8 or S08) the functions of a PWM controller and a MOS power switch, for low power switching sources, with power supply directly from the mains, usable in most network adapters for charging batteries, at TV receivers or for engine control.

The power dissipated in the DIP capsule is 20W, and in the SMD, S08, 12W. The switching frequency is 60kHz, and the voltage on the switching element (Vdd) can vary between 9 and 38V.

The start circuit is activated at high voltage. It offers various protection modes: overvoltage and undervoltage (with hysteresis), overtemperature and overcurrent in load (with autorestart).

5. Analyzer for batteries

The aim is to make a "smart" tester for batteries (accumulators) of various types (capacities) and nominal voltages generated at the terminals.

The test must be performed at different load currents, and the indication must be optical.

A battery has at its terminals, without charge, a quasi-constant voltage, a situation noted until close to "discharge" to complete. Measuring the voltage at the empty battery terminals with a voltmeter hardly indicates its condition at all.

There are still electronics today (many beginners) who "fall into the trap" of an erroneous conclusion, following such a measurement. The only viable solution to know the real state of an electric battery is to connect to its terminals a resistive load (a resistor) through which to circulate a certain electric current, correlated with the normal possibilities (in terms of initial capacity - nominal) of the battery and measuring the voltage at its terminals (immediately or after a few minutes of operation "in charge").

6. Impedance meter for antennas

Determining the input impedance of an antenna requires specialized, complex equipment, difficult to procure for the amateur builder.

How adapting the antenna impedance to the output impedance of the transmitter represents the success of achieving an optimal energy efficiency, this desideratum preoccupies to a great extent all radio radiators.

As the measuring instruments have an essential contribution to the desire to know with enough precision the input impedance of an antenna, in order to come to the aid of the readers faced with such problems, an impedance meter is published with good practical results.

The assembly recommended to be built by those interested allows the measurement of antennas in the HF and VHF frequency range, which is a great advantage.

In addition, with this tool you can accurately determine the transformation ratio of a balloon and the length of a coaxial cable section to 1/4 of the wavelength, to be mounted as an impedance transformer.

In essence, a bridge mounting capable of measuring a wide range of impedances is presented.


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