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One thermostat - several objects. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Power regulators, thermometers, heat stabilizers

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To maintain the desired temperature of several objects or control zones, sometimes you can get by with a single thermostat. The essence of the method is that the temperature sensors of each object are connected to the control device in turn. In a few seconds, the device measures the temperature of the first object and generates a command to turn on or off the corresponding heater. The command is "remembered" by the memory cell, and the device switches to servicing the next object. A complete cycle takes less than a minute, which is quite acceptable if the thermal inertia of the objects is large and during this time their temperature does not have time to noticeably change.

The figure shows a diagram of a device that serves three objects of regulation. Their temperature sensors are RK1-RK3 thermistors, connected in turn to the thermostat by relay contacts K2.1-K4.1. The thermostat is suitable for almost any industrial or amateur production with a relay output and capable of operating in the desired temperature range. Sensors can be not only thermistors, but also thermocouples, resistance thermometers, and even specialized microcircuits.

(click to enlarge)

Simultaneously with the sensors, contacts K2.2-K4.2 switch variable resistors R1-R3, which set the stabilization temperature for each object. If the characteristics of the sensors are identical, and the temperature of all objects must be the same, there is no need to switch variable resistors. You can leave one already installed in the regulator.

One more group of contacts of the same relays (K2.3-K4.3) is intended for connection to the output of the temperature controller of storage capacitors C3-C5. Depending on the position of the armature of the executive relay K1 in the controller (it depends on the ratio of the actual and set temperature of the currently served object), the corresponding capacitor will be charged to 12 V or discharged to zero. It will keep this state until the next cycle. Resistor R6 limits the charging or discharging current.

The voltage from the capacitors is supplied to electronic keys (transistors VT7-VT12), which control the electric heaters of the corresponding objects using the K5-K7 relay. LEDs HL4-HL6 signal the inclusion of heaters.

The distributor of pulses that control relays K2-K4 is made on microcircuits DD1 and DD2. With the values of the resistor R4 and capacitor C1 indicated on the diagram, the repetition period of the generator pulses on the elements of the DD1 microcircuit is 6 ... 8 s. This will be the time interval allotted to each object. It is undesirable to increase it, as temperature "overshoots" will increase. And if the period is reduced, the control device may not have time to react to the changed temperature. Switch SA1 serves to stop the switching of objects if necessary, for example, to analyze the operation of the device or find a fault. Pulses from the outputs of the counter DD2 through the keys on the transistors VT1 -VT6 alternately turn on the relay K2-K4 and simultaneously the LEDs HL1-HL3.

Relays K2-K4 - reed switches RES44, version RS4.569.251. Each of them can be replaced by three RES55A, version RS4.569.600-01 or RS4.569.600-06, by connecting their windings in parallel. Other suitable relays can be used, but it must be remembered that the contacts switching the sensors must have a minimum resistance so that the control accuracy is not reduced.

Relay K5-K7 - RES22, version RF4.523.023-01. With a high power of the heaters, the relays must control them through additional, for example, thyristor switches of the appropriate power, assembled according to one of the known schemes.

The +12 V voltage is taken from any source capable of delivering the required current (it is consumed mainly by the K2-K7 relay). You should not use the power supply of the temperature controller itself for this, voltage fluctuations caused by sharp drops in the current consumed at the time the relay is triggered will inevitably worsen the accuracy of regulation.

With proper installation and serviceable parts, the adjustment comes down to setting the desired maintenance period for objects by selecting the resistor R4. The number of adjustable objects can be increased to ten by using the free outputs of the DD2 chip and changing the number of other nodes accordingly. The author has a similar device that regulates the temperature of molten polypropylene in eight heating zones. For two years of operation, there were no complaints about his work.

Author: V.Naryzhny, Bataysk, Rostov region.

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