Soldering iron thermostat on a microcontroller. Encyclopedia of radio electronics and electrical engineering

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

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In the soldering iron that I use (Fig. 1), the heating element has four leads: two - from the heater itself, which at a temperature of 21 ° C has a resistance of about 4 ohms, two more - from a thermistor with a resistance of about 50 ohms at the same temperature. There are also soldering irons (for example, RX-70G) with three leads of the heating element, one of them is common for the heater and the thermistor. They can also be used with the proposed stabilizer with a slight change in its scheme.

Technical specifications

Stabilization temperature, °С......................150...350
Temperature setting step
stabilization, ° С ....... 10
Temperature maintenance accuracy, °С .................. ± 3
Soldering iron power, W...40
Soldering iron warm-up time
from 21 °С to 260 °С, s............. 80

The main disadvantage is due to the fact that the thermistor located in the immediate vicinity of the heater, but far from the soldering iron tip, reacts with some delay to changes in the temperature of the end of the tip. For this reason, a soldering iron with a stabilizer is more suitable for soldering small-sized parts, rather than large, heat-absorbing parts.

The scheme of the device is shown in fig. 2. It is necessary to load the codes from the Stanciya hex file attached to the article into the program memory of the DD1 microcontroller. The microcontroller configuration must match the table.

A voltage of 15 V is supplied to the voltage regulator on the DA1 chip, which supplies 5 V to the digital part of the device: the DD1 microcontroller, configured to work from an internal 8 MHz RC generator, and the HG1 indicator.

The voltage divider, formed by resistor R2 and the thermistor of the soldering iron, generates a voltage that increases with the temperature of the soldering iron. It goes to the PC0 pin of the microcontroller, which serves as the input of its built-in ADC. Based on the value received from the ADC, the microcontroller program calculates the current temperature of the heater. Depending on the difference between the current temperature and the desired one, the timer-counter 2 of the microcontroller, operating in PWM mode, generates variable duty cycle pulses at the PB1 output. They open the transistor VT1, which connects the heating element EK1 to the power source. The higher the duty cycle of the pulses, the smaller the percentage of the time the heater works and the lower the average heating power.

Information is displayed on the HL1 indicator in dynamic mode. The diagram shows the type of indicator with common cathodes of the elements of each familiarity, but it is possible to replace it with an indicator with common anodes. In the first case, the PC5 output of the DD1 microcontroller remains unconnected, and in the second it should be connected to a common wire, as shown in the diagram by a dashed line.

Fig. 3

The thermal stabilizer can be mounted on a double-sided printed circuit board shown in Fig. 3. It is designed for surface mount parts (except microcontroller, indicator and buttons) mounted on the printed circuit conductor side. On the same side there are contact pads for connecting a power source (ХТ1, ХТ2), a soldering iron (ХТЗ, ХТ4, ХТ9, ХТ10), and, if necessary, a programmer (ХТ5-ХТ8).

All resistors and ceramic capacitors C2, C0805 are of size 1. Capacitor C3 is tantalum, size A. The values ​​of resistors R9-R20 are selected for the indicator of the type indicated on the diagram. To achieve optimal brightness when replacing the indicator, they may need to be selected. However, the current flowing through each of the resistors should not exceed XNUMX mA.

There is a wire jumper on the side of the microcontroller, indicator and buttons on the board. Please note that the holes for the microcontroller pins not used according to the circuit are not provided on the board. These pins must be bent or completely removed.

The voltage source 15 ... 17 V to power the soldering iron and the heat stabilizer can be built according to the circuit shown in fig. 4. The voltage on the winding II of the transformer T1 should be in the range of 13 ... 15 V at a load current of 2,5 A. For example, a 40 V TTP-12 transformer is suitable if its secondary winding is wound up to the desired voltage. The VD1 diode bridge is designed for a voltage of 100 V and a current of 4 A. Instead, any other with the same parameters will do.

If the stabilizer is supposed to be used with a soldering iron that has a common output of the heater and the thermistor, the heater control unit should be assembled according to the diagram shown in fig. 5, excluding the former (field effect transistor VT1 and resistor R11 in Fig. 2). The new node is also suitable for working with a four-pin soldering iron, if you connect the NE2 and TR2 pins of the latter together.

After connecting to the network, the device operates in standby mode: the transistor VT1 is closed, the soldering iron does not heat up, the indicator shows the word Ghf (eng. off). To turn on the soldering iron, you need to press any of the SB1 buttons. SB2. After that, if the voltage at the PCO pin of the microcontroller does not exceed 2,5 V, the soldering iron will start heating. The indicator will display a rapidly flashing value of the stabilization temperature (when first turned on - 260 °C). A voltage greater than 2,5 V indicates an open circuit of the thermistor RK1 or a too low resistance of the resistor R2. heating will not start, and the signs will flash alternately on the indicator .

If the thermistor circuit is normal, the soldering iron heats up at maximum speed (the duty cycle of the pulses supplying its voltage is 100 ° o), and its current temperature is displayed on the indicator. Starting from a temperature 4 °C lower than the set stabilization temperature, the duty cycle of the pulses decreases, becoming equal to zero at a temperature 4 °C higher than the stabilization temperature. In this interval, the fill factor is automatically adjusted so as to keep the temperature of the soldering iron as close to the set one as possible.

If you want to increase the stabilization temperature, you must press the SB1 button, and if you decrease it, then press SB2. Its new value will appear on the indicator. Unlike the current temperature, it will flash for several seconds. Each press of the button increases or decreases the temperature by 10°C. Approximately 2 minutes after the last change, the stabilization temperature setpoint will be written to the EEPROM of the microcontroller. It will be used the next time the device is turned on.

To turn off the soldering iron and put the thermostat into standby mode, press both buttons simultaneously.
The assembled thermostat must be calibrated. The thermistor built into the soldering iron in the temperature range of 150 ... 350 ° C has an almost linear dependence of resistance on temperature. The purpose of calibration is to determine the slope of this dependence according to the method described in the book by V. Trumpert "Measurement, control and regulation using AVR microcontrollers" (publishing house "MKPRESS", 2006). You will need an exemplary thermometer with a thermocouple. It is better to place the soldering iron on an open stand.

In order for the temperature stabilizer program to enter the calibration mode, you need to turn on the device by holding down any of the buttons SB1, SB2. After releasing the button, the soldering iron will start to heat up, the duty cycle of the pulses of the voltage supplying it is 10%. The number 150 will be displayed on the indicator - the soldering iron should heat up to approximately this temperature. After 7 ... 10 minutes, its temperature will be established. It must be measured by firmly pressing the thermocouple of a reference thermometer to the working part of the tip, and set the measured value on the indicator using the SB1 and SB2 buttons.

A few seconds after the last button press, the set value will be written to the EEPROM of the microcontroller. In the future, it will be used by the program in calculations. Further, the duty cycle of the pulses will increase to 40%, and the number 300 will be displayed on the indicator. After 5 ... 7 minutes, when the temperature of the soldering iron stops increasing, it is necessary to irradiate its tip and immerse the thermocouple of the reference thermometer into the molten solder. Its readings are also entered into the heat stabilizer in the manner described above, they are stored in EEPROM and used by the program in the calculation. Upon completion of the calibration, the microcontroller program will enter the normal standby mode.

Author: D. Maltsev, Moscow; Publication: radioradar.net

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