The main topics presented
1. Practical measurement of impedances in electronic equipment (II)
Principles of operation with an impedance meter
The self-balancing bridge method is commonly used in modern low frequency impedance meters.
They will be presented operating principles with an impedance meter working on this method, as a good example of operation to evaluate impedances.
Table 2 (in the article) provides some of the main specifications of such equipment. The impedance measurement circuit is functionally divided into three blocks (figure 12 in the article).
"Signal source" generates the test signal applied to the unknown component, the impedance of which is to be determined. The signal frequency is adjusted in the measuring range, checking in the technical book of the device if the resolution is acceptable.
The test signal is generated by a synthesizer controlled by a microprocessor / microcontroller. Output signal level, usually adjustable in the 10mV - 1V range, is obtained by means of an attenuator. In addition to generating the test signal, this block also generates the internal reference signals.
The "self-balancing bridge" balances the current through the range resistor with the current in the DUT (DUT - Device Under Test, component / measuring device, in Romanian) in order to maintain a zero potential on the lower terminal.
A detector circuit detects the potential on the lower terminal and controls the amplitude and phase of the output so that the detected potential becomes zero. When the bridge is unbalanced an IU converter detects an error current and a phase detector separates the 0 ° and 90 ° vector components.
The output signals of the phase detectors are applied to the modulator to control the 0 ° and 90 ° component signals. The resulting signal is amplified and sent back through the Rr range resistor to cancel the current through the DUT so that no error current flows through the IU converter.
At modern impedance meters the balancing operation is performed automatically in the entire frequency range.
Vector report detector
The "vector ratio detector" measures two vector voltages, on DUT (Educated) and on the series resistor series circuit (Err). Since the value of the gamma resistor is known, the measurement of the two voltages will give the impedance vector Zx of DUT as follows: Zx = Rr x (Edut / Err).
2. AC voltage regulator - voltage controlled
Once the new concepts of construction or redevelopment of buildings have taken place in our country, the modern wiring of strong and weak power networks, respectively the command and control equipment of utilities, must offer innovation and reliability.
Remote controls (wired or radio) of the devices are recommended. A voltage control of the lighting, for example, has the advantage of safety (no electric arc in the switch), the sources of fire are reduced.
The assembly we present, made in a kit not assembled by Velleman, under the code K8003, "DC Controlled dimmer" (DC voltage controlled voltage regulator), is ideal for replacing classic switches or dimmers in a building's lighting system.
The control of the light intensity of an incandescent bulb is performed in voltage, in the range 0… 10Vdc. The mounting dimensions are small, which makes it easy to use in practice. It can be mounted in a box of electrical appliance or in an electrical panel.
Voltage control is performed on two wires and can be obtained from the low power voltage source, the consumption being of 1… 2mA.
3. Electromagnetic relays - measurement of own actuation and return times.
The measurement of short time intervals of the order of milliseconds - as is the case of the own actuation and return times of electromagnetic relays - can be done with the help of stopwatches or universal counters.
Using the principle of measurement of these devices, an own scheme was elaborated, based on an industrial microcontroller, the obtained measurement precision being the same with the precision of the quartz crystal used: 100ppm or 0.01%.
Connecting to the computer makes it possible to remove the display and perform floating point calculations, which directly affect the result obtained.
The presentation of the source codes of the programs used - the program run by the microcontroller and the one associated with the graphical interface - facilitates the understanding of the operation modes of the serial port of the microcontroller and the "MSCOMM" control of the program Visual Basic.
Constructive and functional description
The electrical diagram of the measuring interface is presented in figure 1 (from the article).
The microcontroller used is AT90S2313, the first in the AVR series with UART serial port. The Reset circuit consists of R8, D2, KO and C5, which also performs the auto-reset of the microcontroller during the coupling of the supply voltage.
The time base uses a quartz crystal with a resonant frequency of 7.3728MHz, facilitating a serial communication with the baud rate of 9600 bps with error 0.
The RS232 converter is built around the integrated circuit MAX232 (or any other equivalent). The connection to the computer is made through a standard DB-9 (mother) connector. Please note that incorrect installation of one of the capacitors C14 - C17 will lead to the destruction of the integrated circuit!
The voltage of + 5Vdc necessary for the operation of the assembly is obtained with the help of a series integrated circuit 7805 (0.1A or 1A variants), mounted in a classic configuration.
4. CNX201 - Audio amplifier
There are situations when amateur electronics manufacturers need a medium frequency, linear audio amplifier to test various constructions, such as: tone correctors, checking various receivers or for other purposes.
Most electronic components can be found in specialty stores and in the laboratory of each electronics technician, and its realization is available to everyone.
The convenience of using this audio frequency amplifier is that it does not require a stabilized rectifier. The only requirement for the power supply: it must be well filtered.
The rated supply voltage is 24V, but it works under normal conditions and when the voltage is within limits 20-28V.
Of course, the power will differ accordingly from the 10W that can be obtained when the supply voltage is 24V. The sensitivity of the amplifier is of the order of 40-50mV, on an input impedance of at least 10 KOhm.
The quiescent current of the amplifier is stabilized as a result of the use of the 3 diodes inserted D1… D3. Because they are located in the immediate vicinity of the radiators of the final transistors (see photo in the article), a temperature stabilization of the current of the final transistors is also achieved.
The final transistors used
A wide range of types can be used as final transistors: 2N3055, 2N3055T, BDX95, 2N3442, 2N3767, 2N3054, practically, any transistor of radiated power of at least 20W, of NPN structure, with collector voltage of minimum 40V.
The printed wiring allows the use of all the mentioned types. If we limit ourselves to a power of 5W, transistors can also be used BD441.
5. ICL8038 - Function generator (II)
In the first episode of this article, published in the previous issue of the magazine, we presented the description of the ICL8038 function generator (currently manufactured by HARRIS SEMICONDUCTOR and EXAR, under the code XR-8038A), the advantages offered by it, the main electrical characteristics and the operation.
This second part of the material offers especially to practicing electronics a series of very interesting and useful applications with this IC
6. MAX732 / MAX733 - Switching voltage stabilizers
In some practical applications powered by two or at most three 1.5V batteries (or accumulators) we need DC voltages of + 12V or even + 15V.
How can they be obtained? The MAX732 / MAX733 series of voltage stabilizing integrated circuits, produced by the well-known American corporation MAXIM, comes to the aid of the manufacturers.
These ICs are step-up regulators ("boost"), ie voltage elevators of + 12V / + 15V, made in CMOS technology.
7. TLE 4905/35/45 L - Proximity transducers with HALL effect
The TLE4905 integrated circuit (and the other members of the TLE49X5 family) is a proximity transducer that works on the basis of the Hall effect and was designed for industrial applications regarding precise positioning, brushless DC motor control, rotation angle determination, etc.
It is protected in case of accidental reversal of the polarity of the supply voltage and in the event of electrical disturbances. In table 1 (from the article) you can see the significance of each terminal.
The capsule includes the Hall sensor, an amplifier stage, and a Schmitt trigger circuit (figure 1 of the article). The output is made through a floor with the collector empty, and to minimize the influence of the supply voltage on the switching levels, the blocks mentioned above are supplied by means of an internal stabilizer.
8. 2-zone alarm interface
The interface for the presented alarm is distinguished by simplicity and reliability, being able to be part of a complex system together with the Electronic Cipher K6400 from Velleman (presented in the magazine a few issues ago), sensors (magnetic contacts, infrared motion sensors - PIR, barrier infrared, vibration sensor), an electronic siren and a back-up battery power supply.
It has 2 surveillance areas: one with timing at the entrance without self-restraint (so as not to complicate the scheme), and the other with immediate actuation.