In the automatic regulation of the temperature is used the most a variety of sensors from simple mercury thermocontact TK and TPK to specialized chips. But if you want to accurately maintain the target temperature within a time interval lasting from the day before decades, need only provide stability of the device on the basis temperature-sensitive quartz resonators subsequent transformation hermosawave frequency in the signal control heaters or coolers thermal chambers.
Scheme quartz self temperature transmitter in the frequency (CAPTCHAS) shown in Fig. 1.
The serial sensing quartz the resonator cut Bang at the frequency of 5 MHz with a linear temperature-frequency characteristics and temperature coefficient of 185 Hz/°C [1]. You can replace produced by the plant in Volzhsky, Volgograd region thermosensitive resonator RK-112 containing piezoplates slice ухbI/10° 54'/9° 45'51".
CAPTCHAS are in the form of an independent site that is hosted inside the chamber, connecting with the main unit thermostat coaxial cable. By cable on the generator serves the supply voltage and receiving a signal dependent on the temperature quartz resonator ZQ1 frequency f(T)=5000000+185T, where f - frequency, Hz; T - temperature, °C.
The main unit of a thermostat constructed according to the scheme shown in Fig. 2.
Resistor R2 serves as a load for CAPTCHAS. Further, the signal frequencies through hermosawave the amplifier-shaper on the VT2 transistor and the element DD1.3 is input With second trigger circuits DD2. On a similar login her first trigger signal standard frequency from the generator on the elements DD1.1, DD1.2 thermostable quartz resonator ZQ1 cut AT.
Frequency phase detector [2] consists of two triggers circuit DD2, element DD1.4, resistors R9, R11, diodes VD2, VD3 and capacitor C5. His output voltage is a logic low if frequency higher thermostability exemplary, and high if it is lower. The intermediate voltage level possible only in case of exact equality of frequencies, which in practice does not happen as oscillations of synchronous generators.
At a high level at the output of the frequency detector transistors VT4 and VT5 open, feeding the tension mounted in the combustion chamber heater. Simultaneously HL1 led is lit.
Through the emitter follower transistor VT3 signal frequency hermosawave may be served on electronic frequency counter. The Zener diode VD1 with the transistor VT1 form a voltage regulator power supply circuits.
Adjusting the thermostat, first of all choose a resistor R3, achieving the symmetrical shape of the pulses at the exit of the element DD1.3. The capacitance of the capacitor C3 choose such that at a given temperature an exemplary frequency was equal hermosawave generated CAPTCHAS.
The heat stabilizer is operated at the stand of the certification of medical electronic digital thermometers. The thermal chamber of the stand has a volume of 60 cm3, its walls of foam thickness of 30 mm. the Heater consists of two resistors MLT-2 56 Ohm, connected in parallel.
45 minutes later inclusion in the chamber is set and maintained temperature of 40 °C. Tests have shown that the withdrawal of the temperature stabilization is not exceed 0,005 °C in one degree of temperature change of the reference resonator (ZQ1 in Fig. 2) and 0.02 °C per year. Thus, for five years while working the device in the indoor environment the error of maintaining the temperature in the chamber is not exceeds 0.1 °C.
Literature
Author: V. Solodovnik, M. Cheban