Temperature stabilizer tip of a household soldering iron. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Temperature stabilizer tip of a household soldering iron. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Ham Radio Technologies

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This device, which is easy to repeat, ensures the stability of the temperature of the soldering iron rod set by the regulator (variable resistor) when the mains voltage changes. A miniature incandescent lamp is used as a temperature sensor.

The device brought to your attention is the result of the author's desire to obtain high-quality solder joints with a household electric soldering iron, designed to operate from a mains voltage of 220 V with its fluctuations. A temperature sensor is fixed on the soldering iron rod, according to the signals from which the device maintains the heating temperature of the rod at a given level.

Rice. 1. Stabilizer circuit (click to enlarge)

The stabilizer circuit is shown in fig. 1. The stabilizer consists of two nodes: measuring and regulating, which are galvanically separated by a network transformer T1 and optocoupler U1. The measuring unit is assembled on the op-amp DA2, included as a comparator, and is powered by the secondary step-down winding of the mains transformer. The alternating voltage from it is rectified by the VD1 diode bridge, smoothed by the capacitor C3 and then stabilized at +12 V by the DA1 microcircuit - a parallel voltage regulator.

The voltage at the inverting input of the op-amp DA2 is determined by a divider of resistors R7, R8 and an incandescent lamp EL1, the current through which is about 3 mA is set by resistors R7, R8. As you know, with a change in temperature, the resistance of the filament changes. This property made it possible to use the lamp as a temperature sensor (hereinafter referred to as the sensor), fixing it on the soldering iron rod. The heating temperature of the soldering iron rod is controlled by a variable resistor R6 included in the circuit of another resistive divider R3R4R5. Both dividers form a measuring bridge. Temperature control limits are set by resistor r4.

When the temperature of the sensor changes, the bridge is unbalanced and the voltage changes at the output of the op-amp DA2. The output of the op-amp (pin 6) controls the HL1 LED and the optocoupler U1 of the control unit, assembled on a powerful field-effect transistor VT1. The optocoupler controls the gate-source voltage of the field-effect transistor VT1. When the temperature of the sensor rises, its resistance increases and a low-level voltage appears at the output of the op-amp, the HL1 LED goes out, signaling an increase in temperature above the threshold set by the variable resistor R6, and the emitting diode of the optocoupler U1 turns on, opening its phototransistor. An open phototransistor closes the terminals of the gate and source of the field-effect transistor VT1, its channel closes, and only half the period of the mains voltage is supplied to the soldering iron heater through the diode built into the transistor.

The soldering iron rod and sensor start to cool down. After a while, a decrease in the temperature of the sensor leads to the appearance of a high-level voltage at the output of the op-amp, the HL1 LED lights up, now signaling the temperature is below the specified threshold, and the emitting diode of the optocoupler turns off. Transistor VT1 opens with a voltage of 12 V at the gate, and the full mains voltage is applied to the heater. The soldering iron tip starts to heat up. Then the process is repeated. The voltage to the zener diode VD2 to open the field-effect transistor VT1 is supplied from the network through the rectifier diode VD3 and the quenching resistor R12. Capacitor C5 - smoothing.

Temperature stabilizer tip of a household soldering iron
Rice. 2. PCB drawing

The printed circuit board drawing is shown in fig. 2. It is made of one-sided foil fiberglass and placed in a housing from a low-power power supply in place of the rectifier board removed from it with a smoothing capacitor and a sliding switch. The mains transformer of the power supply is used as a transformer T1. All resistors are installed perpendicular to the board. Under the axis of the variable resistor R6, protruding outward, a hole is drilled in the case. The electrical connection of the board with the heater and the sensor is made through the ONTS-VG-11-6/16 connector (the numbers of its contacts are shown in Fig. 2). A corresponding hole is made for the connector in the case. The connector itself is not shown in the diagram. Transistor VT1 is fixed outside the board on a heat sink - a copper plate measuring 90x12x1 mm, curved with the letter "P" around the transformer. With a soldering iron power of not more than 25 W, a heat sink is not required. Varistor RU1 is mounted directly on the terminals of the transistor VT1.

A small-sized incandescent lamp of the DL1250 series (voltage - 12 V, current - 50 mA) with dimensions of 3,2x6 mm with a lead length of 25 mm was used as a sensor. In a cold state, the resistance of the thread is about 30 ohms. At a temperature of 200 ... 230 ° C - about 50 Ohm. Heat-resistant current-carrying wires with a diameter of 0,2 ... 0,25 mm and a length of 250 mm, exposed to high temperatures, are made of constantan wire and laid along the body of the soldering iron. The connection of the wires to the lamp is made by welding, otherwise the temperature of the rod will "float" over time. Wire for wires can be wound from powerful low-resistance resistors PEV, C5-35. Nichrome wire will also work, but it has twice the resistivity and is harder to connect securely. The welded leads are isolated with pieces of a fluoroplastic tube from the MGTF wire and wrapped with a fluoroplastic tape FUM-O (PTFE) for plumbing work. Then they fasten, wrapping with the same tape, a sensor lamp pressed to the heater rod, and in two or three places along the body - current-carrying wires. It is advisable to make a small notch on the copper rod of the soldering iron for the lamp. Particular attention should be paid to the reliability of the electrical insulation of the current-carrying wires and welding points from the body.

Op-amp LM301A - general application, we will replace, for example, with KR140UD7, K153UD2, LM741. The TL431 parallel stabilizer can be replaced with a KS212Zh, KS212V zener diode or its imported counterpart. Transistor VT1 for an operating voltage of at least 500 V will be replaced by MTP6N60, BUZ90 or domestic series KP707, KP726.

Varistor RU1 is allowed not to be installed. The W08M diode bridge can be replaced with one assembled from separate low-power diodes, for example, 1N4148, KD521A. Oxide capacitors - imported, C2, C4 - ceramic KM. Resistor R6 - SP4-1. Fixed resistors - any output. The DL1250 can be replaced by a DL1265 with a current rating of 65mA at 12V (see below).

Temperature stabilizer tip of a household soldering iron
Rice. 3. Appearance of the stabilizer

The appearance of the assembled stabilizer is shown in Fig. 3.

The adjustment of the stabilizer is carried out in the following sequence. The engine of the variable resistor R6 is set to the lower position according to the diagram, and instead of the resistor R8, a variable (or tuning) resistor with a resistance of 3 kOhm is temporarily connected with a rheostat. When the stabilizer is connected to the network, the HL1 LED should not light up. Further, the resistance of the temporarily connected variable resistor is reduced until the LED turns on. The resistance of the part of the resistor introduced into the circuit is measured and a constant resistor of close resistance is soldered instead. After that, if necessary, select the desired heating temperature range with resistor R4. In addition to the filament of the lamp, especially when replacing it, the resistance of the temperature sensor is also affected by the connecting wires, so the resistances of the resistors R4, R8 may differ slightly from those indicated in the diagram.

The stabilizer has been tested with 25W, 40W and 90W soldering irons. Temperature instability was 15...20 ^оC. It mainly depends on the quality of the thermal contact between the sensor lamp bulb and the soldering iron rod. The author has been using a stabilizer with a soldering iron with a power of 25 W for more than one year. There is practically no need to adjust the temperature.

The presence of a sensor in a glass container mounted on a soldering iron rod requires, of course, some care during operation in order to avoid its mechanical damage. A special stand is required.

Author: A. Zvirbulis

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