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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Heat stabilizer for soldering iron. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Ham Radio Technologies The author of this article proposes a very interesting idea - to use its heating winding as a temperature sensor for the temperature stabilization system of the tip of an electric soldering iron. The implementation of the idea allows you to thermally stabilize the soldering iron without altering it. The device can be used to stabilize the temperature and other heating devices. Those radio amateurs who use factory-made electric soldering irons usually use a manual power regulator, rather than a temperature stabilizer, to control the temperature of their tip. It is understandable, because for the heat stabilizer it is required to install a temperature sensor on the soldering iron, which entails a change in its design. Thermal stabilizers are usually used in conjunction with low-voltage electric soldering irons, often made independently. If there is no possibility or desire to make a soldering iron with a temperature sensor, you can use a simple method that does not require any modification of the finished soldering iron. The idea is that the temperature sensor is the heating element of the soldering iron. It is known that the electrical resistance of pure metals is directly proportional to the absolute temperature, therefore, by measuring the resistance, one can judge the temperature. Although the resistance of the conductors used for heating elements is less dependent on temperature, this approach is applicable here. In the case under consideration, it is convenient to measure the temperature of the heating element by the current it consumes. The advantages of the proposed method of temperature stabilization include ease of implementation, faster heating of the soldering iron (compared to the power controller) and sufficient tip temperature stability for amateur radio practice. The disadvantage is the need for individual adjustment to the power of a particular soldering iron. A schematic diagram of a thermal stabilizer that implements the above idea is shown in fig. 1.
The temperature control of the heater occurs as a result of changing the number of mains voltage half-cycles applied to it. The output node of the device, which ensures the inclusion of the trinistor at the moments when the mains voltage passes through zero, is built according to the recommendations of the article by A. Leontiev and S. Lukash "Output node of the power controller" in "Radio", 1993, No. 4, p. 40,41. The stabilization temperature is set by resistor R4. It can be set in the range of approximately 20...100% of the maximum. The device is designed to work together with a soldering iron with a power of 30 W for a supply voltage of 220 V. The use of a thermal stabilizer with a load of a different power is described below. Consideration of the operation of the thermal stabilizer will begin from the moment when the trinistor VS1 is open. Timing diagrams reflecting the operation of the device are shown in fig. 2.
The mains voltage rectified by diodes VD1 -VD4 creates a pulsating current through the heating element Rn of the soldering iron and resistors R1 and R2. The value of this current determines mainly the resistance Rn, since it is much larger than R1 + R2. In this case, the voltage at the non-inverting input of the op-amp DA1 has an amplitude of about 3 V. The comparator, made on the operational amplifier DA1, compares this voltage with the voltage taken from the variable resistor R4 engine. At the output, the comparator generates rectangular pulses, the duration of which depends on how much the voltage across the resistors R1 and R2 exceeds the voltage taken from the resistor R4 engine. As the soldering iron warms up, the current through its heater decreases, therefore, the voltage drop across the resistors R1 and R2 decreases and the pulses at the output of the comparator become shorter. Comparator DA1 controls the operation of the transistor VT1. The Zener diode VD6 is necessary to close the transistor for a low level at the output of the comparator. When transistor VT1 is closed, capacitor C3 is charged through resistors R11 and R12. The high level voltage at the output of the comparator opens the transistor VT1, and the capacitor C3 is discharged through the resistor R12. Thus, the voltage across this capacitor depends on the duty cycle of the pulses at the output of the comparator. As long as the voltage across the capacitor is less than the switching threshold of the element DD1.3, the operation of the output node is allowed. At times when the mains voltage is close to zero, the DD1.3 element generates rectangular pulses. The differentiating circuit C4R14 and the element DD1.4 shorten these pulses, and the emitter follower on the transistor VT2 amplifies them in current. At the beginning of the half-cycle of the mains voltage, they open the trinistor VS1. As the temperature of the soldering iron increases, the amplitude of the voltage across the resistors R1 and R2 decreases, and at some point the duration of the pulses at the output of the comparator will not be enough to discharge the capacitor to the switching threshold of the logic element DD1.3. As a result, the output node will turn off the soldering iron. The device could remain in this state indefinitely. But to control the temperature, current must flow through the heating element, so a generator based on elements DD1.1 and DD1.2 is introduced into the thermostat. It generates pulses with a duration of about 0,1 ... 0,2 s and a frequency of about 1 Hz. The pulses from the output of the generator through the resistor R10 enter the base of the transistor VT1 and open it, the capacitor C3 is discharged and the output node supplies voltage to the soldering iron. If during the pause the soldering iron had time to cool down at least a little, then after the generator pulse decays, the soldering iron will not turn off until the temperature of the tip rises to the set temperature. The device uses fixed resistors MLT, tuning R2 - SP5-14, variable R4 - SP2-2-0,5. Capacitors C1, C3, C4 - from the KM series, oxide C2 - K50-35. Chip K561LE5 is replaceable by K1561LE5. K564LE5 can also be used, but correction of the printed circuit board is required. The comparator can be assembled on the OU K544UD1, K544UD2 with any letter index. Instead of KS133A, any zener diode for a stabilization voltage of 3,3 ... 5,6 V is suitable. Transistors - any of the KT315, KT342, KT3102 series. The heat stabilizer is assembled on a printed circuit board made of foil fiberglass with a thickness of 1 mm. The board drawing is shown in fig. 3.
The board is installed in a strong box made of insulating material. The knob of the variable resistor R4 and socket X1 are brought to the front panel. The plastic handle of resistor R4 must be mechanically and electrically strong. It should be remembered that all parts of the device are under mains voltage. For adjustment, it is convenient to use the LED indicator, the diagram of which is shown in Fig. 4. The indicator is switched on in series with the soldering iron.
The slider of the resistor R2 is set to the leftmost position according to the diagram, and the slider of the resistor R4 is set to the bottom, which corresponds to setting the maximum temperature of the heater. Turn on the thermostat, while the indicator LED should shine confidently. In the absence of glow, it is necessary to select a resistor R5 of lower resistance. After some time, when the soldering iron is heated to the maximum temperature, move the slider of the resistor R2 to the right according to the scheme until the LED starts blinking. If this was not achieved, you should increase the resistance of the resistor R5 and repeat the described procedure. After setting the maximum temperature, let the soldering iron cool down and check the lower limit of regulation with resistor R4. For ease of use, the regulator scale can be graduated. The resistance of resistors R1 and R2 must be such that the voltage at the non-inverting input OY DA1 is within 2,5 ... 3,5 V. The resistance of resistors R4 and R5 is chosen so that the voltage across the resistor R4 engine can be changed from the value, corresponding to the voltage drop across the resistors R1 and R2 with a cold soldering iron to the voltage drop across these resistors when heated. The device can be used not only to stabilize the temperature of the soldering tip, but also in other cases when electric heaters are used. It is only important to ensure good thermal contact between the heater and the heated medium. Author: M.Kozlov, Naberzhny Chelny, Tatarstan
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