Conex Club Magazine - no.2 - 2006

Conex Club Magazine - no.2 - 2006 - Freeze indicator for the car

Reading time: 6 minute

The main topics presented

1. Alternative technologies for connecting electronic components (II)

The mechanisms of electrical conduction in isotropic conductive adhesives for connecting electronic components are presented.

Manufacturers of conductive adhesives recommend specific applications for each type of adhesive. When choosing to use a certain adhesive in a given application, these indications must be observed in order to optimize contact.

In table 5 (from the article) the characteristics of conductive adhesives are presented, the most important being the heat treatment temperature, the volumetric resistivity, the thermal conductivity, the maximum use temperature and the viscosity.

Conduction mechanism in isotropic conductive adhesives

An isotropic conductive adhesive is composed of a mass of epoxy resin into which conductive particles, usually silver, are inserted. Unlike similar products used in electromagnetic shielding, where the content of metal particles is relatively low, in conductive adhesives this percentage is usually higher, approx. 80%.

On the other hand, in order to ensure the adhesion and rigidity of the solder for the components connected by conductive adhesives, it is necessary that the percentage of epoxy resin be high.

In figure 6 (of the article) it is presented graph of the strength of a conductive adhesive as a function of the metal fraction (p) contained in the volume of the adhesive.

It is observed that the transition that takes place at p = pc is abrupt. Adhesives are usually made with p values ​​much higher than pc to cover manufacturing tolerances.

The shape of the curve can be well predicted by Percolation Theory. The theory has applicability in all fields where transport processes occur in random environments and explains the realization of conductive paths according to a model similar to the path that fluids find when they enter through porous rocks.

2. Simple and economical bargraph

A specialized integrated circuit, for example LM3914, costs around 2 Euro. The use of a bargraph for different indications in automation (advance of a power tool, speed of a digitally controlled electric motor, etc.) significantly increases the cost of the project due to the use of LM3914.

Introduces himself an interesting idea that saves over 50-60% of a project budget; to indicate the position of a digital switch controlled by the counter CD4017 connected RD networks can be used as in the diagram presented in the article.

Resistors R29… R33 are probed, possibly with semi-adjustable. CD4050 amplify the current of the LEDs, so as not to excessively force its outputs CD4017.

3. ICL7665 - Electric voltage monitor with dual output for under and over-voltage

Although designed to monitor the supply voltage of microcontrollers (for which it generates an error signal), the ICL7665 can be used in various applications that require monitoring the supply voltage and switching to a backup source.

Designed for supervision of CPU systems, ICL7665 operates with low current consumption (3uA). Regardless of the mode of presentation (DIP8 / S08 or TO-99), the significance of the pins on the capsule keeps the standardized arrangement order.

An overview, in the internal functional diagram (figure 2 of the article), builds the designer with the working mode of the ICL7665.

Two comparators with the same 1.3V internal voltage reference, two hollow drain outputs (OUT1 and OUT2) and two outputs for programming (optional) hysteresis domains, are the basic elements of the integrated circuit.

pins SET1 and SET2 are the inputs (inverters) of the two comparators. These are made in CMOS technology.

ICL7665 can be supplied with voltage in the range 1.6… 16V and monitors the two inputs, SET1 and SET2, DC voltages from 1.3V to hundreds of volts. The accuracy is 2%. The response time (propagation) is 75us.

4. Stepper motors - Structure and control

Electronists less familiar with Stepper Motors (MPPs) are discouraged from using them when they discover that they have 4 to 6 wires for power.

The article presented aims to do an introduction to the simplified structure of MPPs and their power supply and control mode.

5. Stepper motor driver with MOSFET transistors

Like any motor, stepper motors consist of a stator and a rotor, but without brushes. Unlike conventional brushless DC motors, the variable electromagnetic field that controls movement is formed not by the switching of the coils by the brushes on the rotor, but by the way the pulses are applied on the stator coils.

The stepper motor rotor can be a permanent magnet, a variable reluctance or a mixture of the two. By controlling the field between the stator and the rotor, it is possible to obtain a movement in one direction or another, extremely precise.

Because, stepper motors are supplied with rectangular voltage pulses of appreciable current.

They are stepper motors whose step is 0.9 °; 1.8 °; 3.6 °, etc. Compared to the 360 ​​° of a circle, a step of 0.9 ° implies, at a complete rotation, the execution of 400 individual steps, controlled by the control logic of the impulses applied on the four coils.

Loss of power

Regarding the power losses, especially due to the thermal effect, which are important for a stepper motor (MPP), the following observations are made: at low travel speeds (low frequency control) MPP develops at a shaft or a very high torque.

If they are supplied with fixed (stabilized) voltage sources, no low problems occur at low frequency; but if the control frequency increases (increases the speed of movement) the torque decreases even more as a result of the high impedances of the coils, effect manifested at high frequencies.

Three power modes are possible, as follows:

  • powering the coils from a "chopper" (switching source), with very high voltage, so that at high speeds the filling factor increases in the source control;
  • power supply from the high value voltage source in series with a ballast resistor - the resistor limits the current through the stator;
  • power supply from a constant current source that will keep the current through the coils (the same, regardless of speed) - if the speed increases, the voltage increases accordingly.

The assembly presented in the article has the following technical characteristics:

  • MOSFET drivers for single-pole stepper motors (IRFZ44);
  • direction inputs - DIR and step - STEP;
  • integrated, low frequency oscillator for manual control of the PAS;
  • supply voltage in the high range, 7… 35V.

IRFZ44 Features:

  • MOSFET channel n;
  • drain voltage - max. 55V, max. 41A, 83W:
  • channel resistance 0,02452;
  • capsule TO220AB.

6. Simple protection for acoustic enclosures

This protection system for acoustic enclosures, simple but effective, acts immediately and disconnects the acoustic enclosure when a higher voltage of 10Vdc is detected on the speakers.

If, for example, a bridge power amplifier fails in operation (short circuit), all the supply voltage (from the positive or negative pole) is present at the output terminals, directly on the speakers, and as this voltage is often over 25Vdc, is enough to destroy the speaker coil (speakers can be destroyed even mechanically).

The intelligent connection of the six diodes, as noted in the diagram in Figure 1 of the article (two groups of three Zener diodes in series, the groups being mounted in antiparallel, respectively D1-D2-D3 and D4-D5-D6), makes that the relay to operate at a voltage higher than 10Vdcc.

This value results approximately from the sum of the threshold voltages of the Zener diodes in a group (3 x 3.3V = 9.9V) plus the sum of the opening voltages of the diodes in

second group, directly polarized (approx. 3 x 0.6V = 1.8V), total 11.7V. However, this value depends on several parameters of the Zener diodes (directly and inversely polarized) and cannot be calculated very exactly if they are not known (they are catalog data and also depend on the polarization currents).

On closer inspection, this is a symmetrical double parametric stabilizer, from which the coil of a relay is supplied. So, when a direct voltage of about 10Vdc and over is detected, the relay disconnects the acoustic enclosures from the power amplifier.

7. Compact Flash / IDE interface

Compact Flash is a flash memory that can be electrically written and read keeps the information stored even in the absence of supply voltage.

Compact Flash offers a capacity of tens, hundreds or thousands of MB, and the data write speed is about 10MBps. These memory "cards" are the result of digital photos, but also of audio, video, etc. files, the reading and writing speeds offering optimal performances.

The technology of these cards is of the IC Tower Stacking type, which allows the grouping of several Nand Flash components, thus obtaining an increased capacity.

Principle of operation

In general, the basic structural and functional unit of memories is represented by an assembly consisting of a transistor and a capacitor, an assembly that can store a quantity of one-bit information.

This ensemble is called memory cell. The working mode of a memory cell is very simple: the capacitor can keep, as long as it is powered, a negative or positive electric charge, corresponding to the bit with the logical value 0 or 1.

The transistor allows access to the stored value or writing a new value. However, current memories have quantities of millions of bytes, which means that a byte consists of eight bits, so the mechanism of operation is much more complex, but it works on the principle described above.

The role of external memory (Compact Flash, Hard disk, etc.) is to store and manage as much data as possible. The components responsible for how they are managed and stored are the operating system and the file system.

8. Automatic night lighting powered by the mains

Compared to other assemblies that offer the same functions, the novelty element in the electrical diagram is switching function performed by static switch consisting of triac and optotriac (T1 - IC2).

The optotriac used is of the type MOC3041. Its use ensures reliability and safety in operation, switching the load by zeroing the voltage of the power supply network (220Vac / 50Hz).

Technical data:

  • power supply directly from the mains, without transformer;
  • static relay switch consisting of optotriac and triac;
  • detection when the network voltage passes through zero (ZCD);
  • with radiator can switch loads of approx. 600W;
  • securely fused on the input.

9. Car freeze indicator

The main purpose for which Velleman designed this module is its use in the car, to signal the danger of frost, but the assembly can be found many uses in other areas of activity.

The installation instructions are optical, via LED LD1, as follows:

  • + 3 ° C or above, LD1 is quenched;
  • at + 3 ° C LD1 emits short flashes of light;
  • at 0 ° C LD1 emits regular flashes, with a filling factor of approximately 50%;
  • at -3 ° C and below this temperature the LED is permanently lit.

The NTC thermistor used is a metal encapsulated model B57045Ky.

Technical data:

  • supply voltage 10… 15Vdc (12V from battery);
  • current consumed 25mA max .;
  • sensor - NTC thermistor;
  • signaled temperature range (optical, flashing) -3… 30 ° C;
  • dimensions: 56 x 42 x 20mm.


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