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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Timer thermostat. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Power regulators, thermometers, heat stabilizers Quite often among home radio electronics there are things that serve more as decorations. How to turn a souvenir thermometer timer into a temperature controller is described in this article. The author of the article was asked to make a thermostat, and as the main unit of the design, they offered to take a timer-thermometer purchased in the trade network under the trade name VICING AB type BOX 554S-631-07 (a mistake may have crept into the name of the device due to the poor quality of the marking sticker ). Before proceeding to the description of the developed circuit, a few words should be said about the device and the capabilities of the timer-thermometer itself. And this is done not for the purpose of advertising, but to make it easier to understand the principle of operation of the entire device as a whole. The timer-thermometer performs two functions: countdown time, in timer mode, with an audible signal at the end of a previously set time period, and also measures temperature up to 300 ° C, using an external sensor made in the form of a pointed metal rod. In the temperature measurement mode, an audible signal also sounds when the previously set threshold is reached. All manipulations are carried out by a switch and buttons located on the front panel, and are indicated by a digital LCD indicator. The timer-thermometer is powered by a 1,5 V element of the R6 type. Unfortunately, the thermometer measures only positive temperatures, which somewhat narrows the scope of its application. Strictly speaking, in everyday life it is difficult to find a use for this product that corresponds to its price. Rather, it can be seen as a useful souvenir. After finding out the capabilities of the temperature meter, the customer was offered and then successfully implemented a circuit (Fig. 1).
First of all, about the issues of electrical safety and grounding. The diagram does not show the elements and circuits of protective grounding, although they were provided for in the real design. To use the thermostat in domestic conditions, it is necessary to use a T1 transformer with a separate section on the frame for the secondary winding, this will increase the safety of the structure. The body is made of plastic. You should also pay attention to the fact that the temperature sensor housing and the shielding braid of its cable cannot be grounded or connected to any metal structures: the thermometer begins to lie insidiously, or even refuses to work at all. So, about the diagram above. As already mentioned, the main function of temperature measurement is performed by VICING AB. Using the thermometer signal, upon reaching a predetermined temperature threshold, you can control the heating element by connecting or disconnecting it from the 220 V network. This task is handled by the triac VS1, controlled by a generator on a unijunction transistor VT1, which is turned on, in turn, by transistors VT2, VT3. Transistor VT2 receives pulse packets generated by the transistor of the temperature meter itself. Since the thermometer beeper is connected to the board with a flexible wire, it is not difficult to remove the necessary information from it, as can be seen from the diagram. The periodically opening transistor VT2 charges the capacitor C4 to a voltage sufficient to create an opening current in the base of the transistor VT3. The opened transistor VT3 shunts the emitter of the transistor VT1, thereby interrupting the operation of the generator and turning off the triac VS1. As a result, the heater is turned off and cooling of the heated object begins. When the temperature drops, the switch-on process occurs in the reverse order. As in any closed automatic control system, self-oscillations may occur, which leads to temperature overshoot above a given threshold. To prevent the negative consequences of this phenomenon, the temperature sensor should be placed as close as possible to the heater (not forgetting the sensor insulation!). A large mass of the temperature control object leads to the inertia of the system as a whole and also leads to an increase in temperature. This effect can be minimized by a reasonable choice of heater power. When checking the parameters of the working sample (the power of the heater was chosen as 60 W), the temperature exceeded the specified threshold by 2°C, and the temperature instability did not exceed ±1°C. The thermostat provides a power source for the meter, made on a VD6 zener diode and a VD7 diode. For safety net, you can connect a VD8 stabistor in parallel with them. When checking, it is necessary to adjust the voltage of this source as close as possible to 1,55 V. In this buffer mode, the battery will last a long time, and in the event of a power failure, information about the set heating threshold of the object will not be lost. During operation, special attention should be paid to the manipulation of the START / STOP button: For the normal operation of the thermostat, the indicator must have a sign of turning on the sound alarm. You can check this by turning on switch S1 (when the set temperature is reached, of course). It will not be superfluous to remind you that without a thorough check in operation, the thermostat should not be left for a long period unattended in the on state. A few words about radioelements. Transformer T1 is taken from the TOSNA power supply, standard size ShL 8X16. It makes no sense to make such a transformer yourself: labor costs are high, and the results can be disastrous, the primary winding wire is too thin and there is too much of it. A transformer with a power of 1,5 ... 2 VA and with a secondary winding voltage of 8 ... 10 V is suitable for replacement. The transformer core is made up of two ferrite rings of the M2000NM1 brand with a size of K12x8x3, you can also take one ring with a size of K16x10x4,5 from the same material. The sharp edges of the cores should be blunted with sandpaper and then carefully wrapped with two layers of varnished cloth. When winding the wire, it is necessary to ensure the distance between the extreme turns of the windings is at least 3 mm. The windings are wound in bulk with a PETV939 wire with a diameter of 0,22 mm. The primary winding contains 50 turns, the secondary 70. After winding, the transformer is dipped in PVA glue and dried thoroughly. If the triac TS106-10 is used in the design, then the output stage should be assembled according to the circuit shown in Fig. 2.
The matching transformer has the same data, but the phasing of the secondary winding should be changed. Triac KU208G at a load of up to 200 VA can be used without a radiator. For ease of adjustment, VICING can be connected to the actuating part with a small-sized connector. During the operation of the thermostat, interference that occurs at the moments of switching penetrates into the network. They can be completely eliminated only by synchronizing the moments of turning on and off the triac when the mains voltage passes through zero. In our case, interference can be reduced by installing a passive surge protector. Author: L.F.Lyaskovsky
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