Thermostat for welding plastics

Description of the simple and reliable design of a temperature regulator for welding plastics, for example, plastic frames.

Thermostats. Appointment and scope

It would seem a simple thing temperature controller, and its main purpose is to maintain a given temperature. But there are many areas of technology or simply households where a stable temperature should be maintained, and in a fairly wide range.

For example, it could be warm floor, an aquarium with goldfish, an incubator for removing chicks, an electric fireplace or boiler in the bathroom. In all these cases, the temperature must be maintained different. For example, for aquarium fish, depending on their type, the temperature of the water in the aquarium can be in the range 22 ... 31 ° C, in the incubator within 37 ... 38 ° C, and in an electric fireplace or boiler about 70 ... 80 ° C.

There are also temperature controllers that maintain the temperature in the range from one hundred to a thousand or more degrees. Creating a temperature regulator with a range from several degrees to several thousand is impractical; the design will turn out to be too complicated and expensive, and even, most likely, inoperative. Therefore, thermostats are produced, as a rule, over a fairly narrow temperature range.

Many processes also use temperature controllers. This soldering equipment, injection molding machines for molding plastic products, equipment for welding plastic pipes, so fashionable recently, and no less popular plastic windows.

Modern temperature controllers of industrial production are quite complex and accurate, made, as a rule, on the basis of microcontrollers, have a digital indication of operating modes and can be programmed by the user. But, quite often there is a need for less complex designs.

This article will describe construction of a fairly simple and reliable temperature controller, available for manufacture in a single production, for example, in factory electric laboratories. Several dozen of these devices have been successfully used in machines for welding plastic frames. By the way, the machines themselves were also manufactured in a single production environment.

Description of the circuit diagram

The design of the thermostat is quite simple, due to the use of the K157UD2 chip, which is a dual operational amplifier (OA). One DIP14 package contains two independent op-amps, which combine only common power pins.

The scope of this chip is mainly sound amplification equipment, such as mixers, crossovers, tape recorders and various amplifiers. Therefore, the op-amps are characterized by a low noise level, which also makes it possible to use it as an amplifier for thermocouple signals, the level of which is only a few tens of millivolts. With the same success, the K157UD3 chip can be used. In this case, no alterations and settings are required.

Despite the simplicity of the circuit, the device maintains a temperature within 180 ... 300 C ° with a tolerance of no more than 5%, which is quite enough for high-quality plastic welding. Heater power 400 watts. Schematic diagram of the temperature controller is shown in Figure 1.

Figure 1. Schematic diagram of a temperature regulator (clicking on a picture will open a larger scale circuit).

Functionally, the thermostat consists of several nodes: a thermocouple signal amplifier on the DA1.1 op-amp, comparator on the DA1.2 op-amp, launchers triac on the transistor VT1 and the output key device made on the triac T1. This triac includes a load, indicated in the diagram as EK1.

Thermocouple

Temperature measurement using a thermocouple BK1.The design uses a TYPE K thermocouple with a thermo-emf of 4 μV / ° C. At a temperature of 100 ° C, the thermocouple develops a voltage of 4.095 mV, at 200 ° C 8.137 mV, and at 260 ° C 10.560 mV. These data are taken from a thermocouple calibration table compiled empirically. The measurements were made with compensation of the temperature of the cold junction. Similar thermocouples are used in digital multimeters with temperature meters, for example DT838. The use of TMDT 2-38 wire thermocouple is also possible. Such thermocouples are currently on sale.

Thermo-EMF amplifier

The thermocouple signal amplifier on the DA1.1 op amp is designed according to a differential amplifier circuit. This inclusion of the op-amp allows you to get rid of common-mode interference, which is necessary to amplify a weak thermocouple signal.

The gain of the differential amplifier is determined by the ratio of the resistance of the resistors R3 / R1 and at the values ​​indicated on the diagram is 560. Thus, at the output of the amplifier at a temperature of 260 ° C, the voltage should be 10.560 * 560 = 5913.6 mV, or 5.91 V. At this implies that R1 = R2 and R3 = R4.

In order to change the gain, for example when using a different type of thermocouple, you will have to change two resistors at once. Most often this is done by replacing the resistors R3 and R4. At the input of the amplifier and in the feedback circuit, capacitors C1 ... C4 are installed, the purpose of which is protection against interference and the formation of the necessary frequency response of the amplifier.

This scheme does not provide a cold junction temperature compensation scheme. This made it possible to significantly simplify the circuit, although it is not taken into account when measuring the temperature of the heating element in comparison with the simplification of the circuit.

Comparing device - comparator

Monitoring the heating temperature is carried out using a comparator (comparing device), performed on the OS DA1.2. The threshold of the comparator is set using the tuning resistor R8, the voltage from which through the resistor R7 is supplied to the non-inverting input of the comparator (pin 2).

Using resistors R9 and R6, the upper and lower thresholds of the temperature setpoint are set respectively. The amplified thermocouple voltage is fed through the resistor R5 through the inverting input of the comparator, (pin 3). The amplification was mentioned a little higher.

The logic of the comparator

While the voltage at the inverting input is less than at the non-inverting one, the output voltage of the comparator is high (almost + 12V). In the case when the voltage of the inverting input is higher than the non-inverting output of the comparator -12V, which corresponds to a low level.

Triac trigger device

The triac trigger device on the transistor VT1 is made according to the scheme of the classical blocking generator, which can be seen in any textbook or reference book. Its only difference from the classical circuit is that the bias to the base of the transistor is supplied from the output of the comparator, which allows you to control its operation.

When the output of the comparator is high, almost + 12V, an offset is applied to the transistor base and the blocking generator generates short pulses. If the output of the comparator is low, -12V, a negative bias locks the transistor VT1, so the pulse generation stops.

The transformer of the Tr1 blocking generator is wound on a K10 * 6 * 4 brand ferrite ring made of NM2000 ferrite. All three windings contain 50 turns of PELSHO 0.13 wire.

The winding is done by shuttle in three wires at once so that the beginning and ends of the windings are diametrically opposite. This is necessary to facilitate the installation of the transformer on the board. The appearance of the transformer is shown in Figure 4 at the end of the article.

Thermostat operation

When the thermostat is turned on until the thermocouple is heated, the output voltage DA1.1 is zero, or just a few millivolts in plus or minus.This is due to the fact that K157UD2 does not have conclusions for connecting a trim balancing resistor, with which it would be possible to accurately set the zero output voltage.

But, for our purposes, these millivolts at the output are not scary, since the comparator is tuned to a higher voltage, of the order of 6 ... 8 V. Therefore, at any setting of the comparator in this state, its output has a high level, about + 12V, which starts the blocking generator at transistor VT1. The pulses from the winding III of the transformer Tr1 open the triac T1, which includes a heating element EK1.

Together with it, the thermocouple also begins to heat up, so the voltage at the output of the DA1.1 amplifier increases as the temperature rises. When this voltage reaches the value set by the resistor R8, the comparator will go into a low state, which will stop the blocking generator. Therefore, the triac T1 will close and turn off the heater.

Together with it, the thermocouple will cool down, the voltage at the output of DA1.1 will decrease. When this voltage becomes slightly lower than the voltage at the engine of the resistor R8, the comparator will again go into a high level at the output and turn on the blocking generator again. The heating cycle will be repeated again.

For visual control of the thermostat, LEDs HL1 green and HL2 red are provided. When the working element is heated, the red LED lights up, and when the set temperature is reached, the green one lights up. To protect the LEDs from reverse voltage, protective diodes VD1 and VD2 of type KD521 are connected in parallel with them in the opposite direction.

Design. Printed circuit board

Almost the entire circuit along with the power source is made on one printed circuit board. The circuit board design is shown in Figure 2.

Figure 2. Thermostat circuit board (when you click on the picture, the circuit will open on a larger scale).

PCB dimensions 40 * 116 mm. The board was made using laser-ironing technology using the sprint layout 4 circuit board drawing program. In order to make a printed circuit board out of the aforementioned drawing, several steps should be taken.

Firstly, convert the picture to * .BMP format, paste it into the sprint layout 4 working window. Secondly, simply draw the lines of the printed tracks. Thirdly, print on a laser printer, and proceed with the manufacture of the printed circuit board. The board manufacturing process has already been described. in one of the articles. Green lines on the board indicate the wiring of the windings on ferrite rings. This will be discussed below.

In addition to the actual temperature controller, the board also contains a power source, which at first glance may seem unreasonably complex. But such a solution allowed us to get rid of the problem of finding and acquiring a low-power network transformer and additional "carpentry" to fix it in the case. The power supply circuit is shown in Figure 3.

Figure 3. The power supply for the temperature controller (when you click on the picture, a larger scheme will open).

A few words should be said about this block separately. The circuit was developed by V. Kuznetsov, and was originally intended to power microcontroller devices, where it proved to be quite reliable in operation. Subsequently, it was used to power the thermostat.

The scheme is quite simple. Mains voltage through the quenching capacitor C1 and resistor R4 is supplied to the rectifier bridge VDS1, made of diodes 1N4007. The ripple of the rectified voltage is smoothed by the capacitor C2, the voltage is stabilized by the analog of a zener diode made on a transistor VT3, a zener diode VD2 and a resistor R3. Resistor R4 limits the charging current of capacitor C2 when the device is connected to the network, and resistor R5 discharges the ballast capacitor C1 when disconnected from the network. Transistor VT3 type KT815G, Zener diode VD2 type 1N4749A with a stabilization voltage of 24V, power 1W.

The voltage on the capacitor C2 is used to power a push-pull oscillator made on transistors VT1, VT2. The base circuits of the transistors are controlled by a transformer Tr1. The diode VD1 protects the base transitions of the transistors from negative self-induction pulses of the windings of the transformer Tr1. Transistors VT1, VT2 type KT815G, diode VD1 KD521.

A “power” transformer Tr2 is included in the collector circuits of the transistors, from the output windings IV and V of which voltages are obtained to power the entire circuit. The pulse voltage at the transformer output is rectified by high-frequency diodes of the FR207 type, smoothed by the simplest RC filters, and then stabilized at the 12V level by the Zener diodes VD5, VD6 of the type 1N4742A. Their stabilization voltage is 12V, power is 1W.

The phasing of the windings is shown in the diagram as usual: the dot indicates the beginning of the winding. If during assembly the phasing is not mixed up, then the power supply does not require any adjustment, it starts working immediately.

The design of transformers Tr1 and Tr2 is shown in Figure 4.

Figure 4. View of the board assembly.

Both transformers (Figure 3) are made on ferrite rings made of ferrite of the most common brand НМ2000. Transformer Tr1 contains three identical windings of 10 turns on a ring of size K10 * 6 * 4 mm. The windings are wound by a shuttle in three wires at once. The sharp edges of the ring should be dulled with sandpaper, and the ring itself should be wrapped with a layer of ordinary adhesive tape. For mechanical strength, the transformer is wound with a sufficiently thick PEV - 2 0.33 wire, although a thinner wire can also be used.

Transformer Tr2 is also made on the ring. Its size is K10 * 16 * 6 mm: at an operating frequency of 40 kilohertz, 7 watts of power can be removed from such a ring. The windings I and II are wound with a PELSHO - 0.13 wire in two wires and contain 44 turns. On top of these windings is a feedback winding III, which contains 3 turns of wire PEV - 2 0.33. The use of such a thick wire also secures the transformer to the board.

The secondary windings IV and V are also wound in two wires and contain 36 turns of wire sew-2 0.2. According to the diagram in Figure 3, these windings are sealed on the board even without continuity: the beginnings of both windings are sealed together on a common wire, and the ends of the windings are simply connected to the VD3 and VD4 diodes. The relative position of the windings can be seen in Figure 4.

In the circuit board figure (Figure 2 at the beginning of the article), the windings of all transformers are shown by green lines. The beginnings and ends of the windings on small-diameter rings are diametrically opposed, so you should first solder the three wires of the beginning into the board, and then, naturally ringing the windings with a tester, the ends of the windings.

Near the printing paths where the transformer Tr2 is sealed, you can see points showing the beginning of the windings I, II, and III. The output winding, as mentioned above, is sealed even without continuity: it starts together on a common wire, and the ends to the rectifier diodes.

If this option of the power supply seems complicated or just does not want to mess with it, then it can be done according to the scheme shown in Figure 5.

Figure 5. The power supply is a simplified version.

In this power supply, you can use a step-down mains transformer with a capacity of not more than 5 watts with an output voltage of 14 ... 15 V. The power consumption is small, so the rectifier is made according to a half-wave circuit, which made it possible to obtain a bipolar output voltage from one winding. Transformers from "Polish" antenna amplifiers are quite suitable.

Verification before final assembly

As already mentioned, a properly assembled device does not need adjustment, but it is better to check it before final assembly. First of all, the operation of the power source is checked: the voltage at the zener diodes should be 12 V. It is better to do this before the microcircuit is installed on the board.

After that, you should connect a thermocouple, and set the voltage of about 5 ... 5.5 V on the engine of the resistor R8Instead of a triac, connect an LED to the output winding of the blocking generator through a resistor with a resistance of 50 ... 100 Ohms. After the device is plugged in, this LED should light up, which indicates the operation of the blocking generator.

After that, you should warm the thermocouple with at least a soldering iron - the LED should go out. So, it remains only to finally assemble the device and set the required temperature with a thermometer. This should be done when the triac and heater are already connected.

Speaking of triac. Of course, you can use the domestic KU208G, but not all of these triacs are launched, you have to choose at least one from several pieces. Imported much better are imported BTA06 600A. The maximum permissible current of such a triac 6A, a reverse voltage of 600V, which is quite enough for use in the described temperature regulator.

The triac is mounted on a small radiator, which is screwed to the board with screws with plastic racks 8 mm high. LEDs HL1 and HL2 are installed on the front panel, resistors R6, R8, R9 are also installed there. To connect the device to the network, heater and thermocouple, terminal connectors are used, or simply terminal blocks.

Boris Aladyshkin