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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Soldering station control unit on the PIC16F887 microcontroller. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Ham Radio Technologies The article discusses a self-made microcontroller control unit for a soldering station, which includes a low-voltage soldering iron and an industrial-made soldering gun. The unit can also be used as a two-channel general-purpose temperature meter with thermocouples as its sensors and as a single-channel temperature controller. In amateur radio practice, very often there is a need for a convenient miniature soldering iron for working with small radio components, which has a low supply voltage, adjustable tip temperature and the possibility of grounding it. The latter greatly reduces the risk of damage to electronic components by static electricity. Many descriptions of the designs of soldering irons and soldering dryers (hereinafter referred to as dryers) have been published in the literature, but the independent manufacture of most of them requires special equipment, suitable materials and a significant investment of time. However, today it is possible to purchase ready-made, easy-to-use soldering irons and hair dryers with interchangeable nozzles for a small price. There are two common options for the design of soldering irons, which differ in the way the tip is heated and its temperature is measured. In the first version, the heater covers the soldering rod (as in classic electric soldering irons). The temperature is measured using a thermocouple pressed against its shank, opposite the tip. In this design, the heating coil is reliably protected from mechanical stress and damage. But the readings of the temperature sensor, remote at a considerable distance from the actual place of soldering, have a noticeable inertia. It takes some time for the heat removal from the tip (sting) to reduce the temperature of the shank. In practice, this disadvantage is compensated by a certain temperature margin of the rod and its high heat capacity, which ensures fast heating of the soldering point. The control system fixes the decrease in temperature only during long continuous soldering and returns it to the set value, increasing the power given to the heater. The second option differs in that the heater is located inside the rod, and the temperature sensor is pressed against it at the heater point closest to the soldering point. This ensures a faster response to changes in tip temperature during the soldering process. Such soldering irons usually use a fragile ceramic heater, which is easily damaged when the soldering iron is dropped on a hard surface or in the event of other strong mechanical loads, or internal mechanical stresses arising from uneven heat removal (for example, when working with a non-standard tip). Another working tool of a modern soldering station is a hair dryer. With its help, the necessary sections of the printed circuit board are non-contactly heated to the melting temperature of the solder by an air flow of a given force and temperature. The hair dryer is also convenient for group soldering of passive electronic components. They are preliminarily laid out on a printed circuit board, covering the soldering points with a layer of solder paste. During the soldering process, these components self-center on the pads of the board due to the surface tension forces of the molten solder. The hair dryer gained great popularity among repairmen, since it can be used to quickly solder and solder multi-pin microcircuits with a fine lead pitch. The hair dryer is also very convenient for heating heat-shrinkable tubes and for blowing hard-to-reach areas of structures with warm or cold air. Previously, soldering dryers were powered by a compressor, which was located in a separate housing and supplied air through a hose to the dryer handle, in which a heater and a temperature sensor were installed. The need for a remote compressor and its high price hindered the spread of such hair dryers in the workplace of radio amateurs. With the advent of hair dryers with built-in fans, it has become possible to do away with bulky compressors.
On fig. 1 shows a photograph of a disassembled soldering iron from a Solomon SL-10/30 soldering station with a temperature sensor installed according to the first of the options described above, and a hair dryer from a Lukey 852D + FAN soldering station with a built-in fan. It was to work with them that the proposed control unit was developed. A nichrome heater and a temperature sensor are installed in the metal casing of the front of the hair dryer. By design, the heater is similar to those used in hair dryers. The heater supply voltage is 220 V, power is about 250 W. In the extended part of the handle of the hair dryer there is a centrifugal fan with a supply voltage of 24 V (current consumption 120 mA). I would like to draw your attention to the fact that the outer diameter of the metal part of the nozzle of this hair dryer is 25 mm, unlike the popular "compressor" ones with an outer nozzle diameter of 22 mm. As a result, it requires special nozzles, while others require an adapter to install. A self-made nozzle with a round outlet of small diameter, shown in fig. 2, the author made from an old oxide capacitor K50-3 20 uF at 350 V and a car clamp.
Given that a soldering iron and a hairdryer are usually not used at the same time, it was decided to simplify the block being developed by combining the controls for these tools and using the same indicators to display their temperature and operating mode. Main Specifications
The scheme of the soldering station control unit with a soldering iron and a hair dryer connected to it is shown in fig. 3. The button in the hair dryer, indicated in the diagram SB2, is not used. The control unit is built on the basis of the PIC16F887 (DD1) microcontroller, which has a ten-bit ADC and is configured to work from the built-in clock generator with a frequency of 8 MHz. The X4 connector is provided for programming the microcontroller. Ceramic capacitors C14 and C15 are installed as close as possible to the power pins of the microcontroller. To supply sound signals, a sound emitter with a built-in generator HA1 is designed, which is controlled by signals from pin 40 (RB7) of the microcontroller through an electronic switch on a transistor VT3.
The temperature is measured using thermocouples BK1 and BK2, installed respectively inside the hot air gun and soldering iron. Shelter DA1.1 and DA1.2 amplify their thermoEMF. Cold junctions of thermocouples are physically located in the handles of the soldering iron and hair dryer; compensation for changes in their temperature is not provided. In practice, the absence of such compensation does not cause noticeable inconvenience, since soldering is usually carried out in rooms with a little changing temperature. As the exemplary voltage of the ADC of the microcontroller, its supply voltage (5 V) was used. This did not lead to a noticeable error. The ADC external reference voltage input pin is left free and, if desired, can be used to connect an external reference voltage source of increased stability, for example, MCP1541 (4,096 V) or MCP1525 (2,5 V) microcircuits. When changing the reference voltage, an appropriate adjustment of the gains of the op-amp DA1.1 and DA1.2 will be required. These coefficients are set using resistors R4, R8 for DA1.1 and R6, R9 for DA1.2. They should be selected so that at the maximum temperature the voltage at the output of the op amp does not exceed the value of the ADC reference voltage. In the event of breaks in the thermocouple circuits (including when disconnected from the X2 and X3 connectors in the solder or hair dryer), +2 V is supplied to the non-inverting inputs of the op-amp through resistors R3 and R12. The R5C1 and R7C2 circuits are filters that suppress high-frequency interference. Resistors R10 and R11, together with protective diodes inside the microcontroller, protect the ADC inputs from overload. The soldering iron heater power control is organized using the PWM microcontroller hardware module. It generates variable duty cycle pulses at pin 17 (RC2). Using a powerful key on a field-effect transistor VT1, they turn the heater on and off, changing the average power it consumes. The average value of the voltage supplied to the fan of the hair dryer is changed using PWM implemented in software. The pulses from pin 16 (RC1) of the microcontroller are fed to the fan motor M1 through a key on a field-effect transistor VT2. The hair dryer heater power is adjusted by periodically skipping a certain number of mains voltage periods. The control signal is generated by the microcontroller at pin 10 (RE2) and enters the heater power supply circuit through the dinistor optocoupler U1, equipped with a turn-on synchronization unit with the moment of zero crossing of the voltage applied to its output circuit, and triac VS1. The HL1 LED is designed to visually control the operation of the hair dryer heater. The block uses a four-digit seven-element LED indicator HG1 - RL-F5610GDAW / D15 with common cathodes of the elements of each category. The anodes of the elements are connected to port D of the microcontroller DD1 through current-limiting resistors R24-R31, which are selected so that the total current through all pins of port D does not exceed 90 mA when any sign is displayed. The common cathodes of the indicator discharges switch the keys on transistors VT5-VT8 according to the signals generated at the outputs RC4-RC7 of the microcontroller. The HL4-HL11 LEDs are included in the general dynamic indication system as elements of an additional fifth digit, switched on by the VT9 transistor according to a signal at the RC3 output of the microcontroller. The HL4 LED is used to indicate the inclusion of the hair dryer, and HL5 is a backup, it is supposed to be used when improving the unit. LEDs HL6-HL11 form a discrete scale, turning on one at a time and showing the currently set power level of the soldering iron heater (or hair dryer, if it is turned on) in steps of 1/6 full power. Higher power corresponds to an LED with a lower position number. As U2 - a converter of mains AC voltage 220 V to DC 24 V - a ready-made switching power supply PS-65-24 [1] with a power of 65 W was used. The oxide capacitor C5 is placed next to it, and from this capacitor there are separate wires to each 24 V voltage consumer. To obtain a 12 V voltage from it, a pulsed DC-to-DC buck converter on the MC33063 (DA2) chip is used, similar to those described in [2] and [3]. The voltage divider R17R19 is selected so that a voltage of 12 V is maintained at the output of the converter. Its presence is indicated by the glow of the HL2 LED. Further, the integrated linear regulator DA3 brings the voltage to 5 V, which is necessary to power the microcontroller DD1. Mains voltage 220 V is supplied to the power supply unit U2 by pressing the button SB1. The microcontroller program, after initialization, sets a high logic level at its output RE0 (pin 8), which opens the transistor VT4. Capacitor C9 ensures that at the moment the transistor opens, the full voltage of 12 V is supplied to the relay winding and its reliable operation. Upon completion of the charging of the capacitor, the current through the winding decreases to a value limited by resistor R23, which only ensures that the relay armature is kept in the triggered state. The HL3 LED indicates that voltage is applied to the relay coil. The triggered relay K1 with its contacts K1.1 bypasses the button SB1. Now it can be released, the control unit power will remain on until the VT4 transistor is closed by the microcontroller. After the power is turned on, the HG1 indicator briefly displays an inscription with the program version number and an audible signal sounds. The mode of operation with a soldering iron is switched on, which smoothly warms up to the temperature set in previous sessions and recorded in the EEPROM of the microcontroller. The current temperature value is displayed on the HG1 indicator, and the level of power supplied to the soldering iron is displayed using the HL6-HL11 LEDs. To avoid thermal shock, before reaching a temperature of 100 °C, the power level is limited to 40% of the maximum, and in the range of 100 ... 300 °C - up to 80%. This increases the time to reach operating temperature, but prolongs the life of the soldering iron. When the set temperature is reached, it stabilizes at this level. By turning the encoder knob S1, the temperature can be changed. When you press the SB3 button, the HL4 LED turns on, the soldering iron is switched to a gentle mode (its temperature drops to 150 ° C), the hair dryer fan turns on, and then its heater. The temperature of the air flow from the hair dryer rises according to an algorithm similar to heating the soldering iron. The desired temperature is set by turning the encoder knob S1. After pressing this knob once, you can adjust the air flow intensity by turning it. By pressing the SB3 button again, the heater of the hair dryer is turned off, and the soldering iron is switched to the operating mode. The fan of the hair dryer will continue to operate until the temperature of the air flow drops to 60 ° C. After that, it will be turned off automatically. With successive presses of the encoder button, the names of the following parameters are displayed in turn on the HG1 indicator:
The coefficients A0 and A1 are used by the microcontroller program to determine the temperature of the soldering iron tip or the air flow supplied by the hair dryer according to the number N obtained as a result of the ADC operation, which linearly depends on the thermoelectric power of the corresponding thermocouple. The temperature T (in degrees Celsius) is calculated by the formula T = A0 + A1N. When the encoder knob is rotated, the value of the selected parameter changes and is displayed on the indicator in a flashing form instead of its name. If within a few seconds the knob is not rotated or pressed, the indicator will return the current value of the temperature of the soldering iron or the air flow from the hair dryer. When you press the SB5 button, the microcontroller saves the current parameter values in non-volatile memory, turns off the heaters of the soldering iron and hair dryer. If at that moment the hair dryer was active, the blowing of the heater with cold air continues until the flow temperature at its outlet drops to 60 °C, after which the microcontroller sets a low voltage level at the RE0 output. Transistor VT4 closes, and relay K1 opens its contacts, disconnecting the control unit from the mains. Button SB4 - reserve. It can be used to improve and expand the functionality of the block. Instead of the PS-65-24 (U2) power supply for the soldering station control unit, any other switching or transformer power supply unit can be used, which provides a stabilized 24 V DC voltage at a load current of at least 2 A. If you use a unit as U2, having, in addition to the +24 V voltage output, another +12 V voltage with a permissible load of at least 300 mA, the buck converter on the MC33063AP1 chip can be excluded from the device. If this converter is used, the MC33063AP1 chip in it can be replaced by the MC34063AP1. Relay K1, optocoupler U1 and triac VS1 are located on a separate printed circuit board. This is necessary to maximize the removal of low-voltage circuits from those that are energized with 220 V. A WJ112-1A relay with a 12 V winding was used. Instead, another one with contacts designed for switching an alternating voltage of at least 250 V at a current not less than that consumed by the control unit and the hair dryer heater is suitable. If a relay with a nominal coil voltage of 24 V is selected, it must be powered from a source of this voltage. Instead of the MOC3063 optocoupler, you can use any dinistor one that can directly control a triac with a permissible voltage of at least 600 V. In order not to increase the level of interference created in the network, it is advisable to choose an optocoupler with a node for controlling the transition of the voltage applied to its output through zero. The BT138X-600 triac in an insulated plastic case can be replaced with a similar BT138-600 in a conventional TO-220 case with a metal flange or another one that can withstand a voltage of at least 600 V in the off state, and a current of at least 6 A in the on state. The triac works in the control unit without a heat sink. The buttons SB1, SB3-SB5 are of the DS-502 type, but they can be replaced by others that are convenient for installation. The SB1 button must be designed for an alternating voltage between open contacts of at least 250 V and withstand the inrush current of the switching power supply U2. Be sure to make sure that the selected unit has a thermistor that limits the inrush current. In its absence, be sure to install in series with the SB1 button or in the power supply itself a thermistor with a cold resistance of 5 ... 10 Ohm (for example, SCK-052 or SCK-101). Used encoder ED1212S-24C24-30F - with mechanical contacts giving 12 pulses per revolution and built-in button. Another one can be used, including an optical encoder with the corresponding power supply units and output pulse generation. The RL-F5610GDAW/D15 indicator can be replaced by any other LED with common cathodes of the elements of each category, for example KEM-5641. A commercially available Z-1 housing is used for the control unit. Its front panel has been replaced with a transparent, cut-out polycarbonate sheet. On the reverse side, a transparent film for inkjet printing is pressed against it, on which the design of the front panel is printed. This panel has SB1, SB3-SB5 buttons and sockets for connecting a soldering iron (X2 - five-pin DIN 41524 or ONTS-VG-4-5 / 16-R, also known as SG-5) and a hair dryer (X3 - eight-pin DIN 45326 or ONTS-VG-5-8/16-R). A description of these connectors can be found in [4]. Behind the transparent panel is a board with an HG1 indicator and LEDs. The appearance of the block together with a soldering iron and a hair dryer is shown in fig. 4.
If the soldering station control unit is assembled correctly and the microcontroller is programmed, it starts working immediately, you only need to set the coefficients A0 and A1 for the soldering iron and hair dryer. To do this, immediately after power is applied using the encoder, the temperature on the HG1 indicator is set below room temperature. Then, by pressing the encoder button, the setting of the coefficient A0 for the soldering iron is selected and, by changing it, the indicator shows the current temperature in the room. Then, proceeding to the setting of the coefficient A1, by turning the encoder knob, its value of 1,0 is obtained on the indicator. After that, a thermocouple or other sensor of an exemplary temperature meter is fixed on the tip of the soldering iron. It is desirable to isolate the sting with an external sensor attached to it from the environment with some material that does not conduct heat well, while observing fire safety requirements. Using the encoder, set some not very high temperature (for example, 1 ° C) on the HG100 indicator and wait for the reference thermometer readings to stabilize. If it shows a temperature above the set value, the value of the coefficient A1 should be reduced, otherwise it should be increased. By selecting this coefficient, they ensure that the difference between the measured exemplary thermometer and the set temperature does not exceed 5 ° C. The tip temperature should not be allowed to rise above 300 ... 400 ° C (according to a standard thermometer). If this happens, you should check the voltage at the output of the op-amp DA1.2 and, if necessary, select its gain so that at the maximum possible temperature of the soldering iron, the output voltage of the op-amp does not exceed the reference voltage of the ADC of the microcontroller. Finally, it is recommended to set the tip temperature at which most soldering is supposed to be done and reselect the A1 factor. Similarly, the coefficients A0 and A1 for the hair dryer are selected. In this case, the intensity of the air flow is set to medium and the temperature sensor of the exemplary thermometer is placed at a distance of 1 cm from the nozzle of the hair dryer. After the selection of all coefficients, the soldering station is ready for operation. With the described control unit, you can use any soldering iron with a built-in thermocouple and a low-voltage heating element. The hair dryer must be with a heating element for a voltage of 220 V and also with a built-in thermocouple. You should also make sure that the fan of the hair dryer is designed to operate on 24 V voltage. 3 are not standardized and may be different. Sometimes there are soldering irons and hair dryers with thermistors as temperature sensors. It is impossible to use them with the described control unit without making significant changes to its measuring path (nodes on the DA1 chip) and adjusting the microcontroller program. An alternative application of the considered design can be a two-channel temperature meter for any objects with sensors in the form of thermocouples and a single-channel temperature controller. If temperature control is not required, then after setting the coefficients A0 and A1, the encoder can be removed. The microcontroller program of the control unit can be downloaded from ftp://ftp.radio.ru/pub/2013/10/ps01.zip. Literature
Author: S. Krushnevich
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