Thermostat for 220 volt soldering iron. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Ham Radio Technologies

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Experienced radio amateurs know that the quality of soldering is determined, first of all, by the temperature of the soldering tip. Both its underheating and overheating are bad. Having set the optimum temperature of the tip, it must be maintained constant during operation. For this, temperature controllers of various complexity are used. Cool circuits use thermocouples or thermistors to control temperature. Meanwhile. The soldering iron itself can serve as a temperature sensor!

According to [1], when the temperature changes by 100°C, the thermoEMF of the "iron-constantan" thermocouple is only 5,5 mV. The change in the resistance of the high-resistance wire of the soldering iron heater is 1% per 100°C. Soldering iron 40 W (220 V) has a heater resistance of 1300 ohms, those. AR=13 Ohm at 100°C. If a measuring current of 10 mA is passed through the heater, then AU-130 mV is quite a sufficient value for the operation of a simple thermostat circuit.

The scheme (Fig. 1) works as follows. We assume that the regulator R6 is in the middle position. When the power is turned on due to the low resistance of a cold soldering iron (RH), the voltage at the non-inverting input (pin 5) of the DA1 comparator is greater than at the inverting one (pin 4). Therefore, at the output of DA1 and, accordingly, at the emitter VT1 - a high level, close to the supply voltage. Capacitor C4 is charged through VD5 and R11, C10 is charged from it through R3. When the voltage at C3 reaches the threshold of the switch on the composite transistor VT2-VT3, it opens and switches the winding of relay K1. The contacts of the relay K1.1 (1-2) are closed and the soldering iron is connected to the network through the ballast resistor R1. The voltage drop across R1 is enough to light up the VD1, VD2 LEDs, signaling the heating mode.

At the same time, the voltage at the inverting input DA1 increases and becomes greater than at the non-inverting one. As a result, a low level appears at the output of DA1, the transistor VT1 closes, and the capacitors C3 and C4 are gradually discharged through the input resistance of the key. In the end, the key closes, the relay releases, and its contacts disconnect the soldering iron from the network and connect it to the input of the comparator.

The resistance of the heated soldering iron is greater, so the voltage at the inverting input of DA1 is still higher than the level at the non-inverting input, and the output of DA1 is "0". As the soldering iron cools, its resistance decreases, and there comes a moment when the level at input 4 of DA1 becomes lower than at input 5. The comparator switches again, a high level at the output charges the capacitors, the relay turns on, and the cycle repeats.

The thermostat is powered by a simple power supply (T1, VD3. VD4, C1, C2). The voltage of the secondary winding T1 is approximately 10 V.

The device is assembled on a printed circuit board made of one-sided foil fiberglass. The board (Fig. 2) is made by cutting insulating grooves. The parts are mounted on the same side of the board (the leads are bent, well deserved and soldered to the foil). The platforms for R1, R2, VD1, VD2 are separated from the rest of the circuit by a 7 mm wide gap to protect against breakdown. The thermostat is placed in

suitable plastic housing. On the axis of the adjusting resistor R6, it is necessary to put on a handle made of insulating material.

Setting up a properly assembled device is reduced to the selection of resistance R3. R5, R7 depending on the specific soldering iron and the required temperature adjustment range. The resistance of resistors R3 and R3 "is selected from the ratio

R3+R3"=RH.

If we select the temperature adjustment range of 400°C (ARN=4%), TO for a variable resistor R6 with a resistance of 3,3 kOhm, we obtain the values ​​of R7 and R5:

R7=R6/ARH=3.3/0.04=82 (кОм); R5=R7-R6=82-3.3=79 (кОм).

Attention! The device has a galvanic connection to the network. When setting up, you must follow the safety rules.

Literature

1. Koshkin N.I., Shirkevich M.G. Handbook of Physics. - M.: Nauka, 1982, S. 114.116.
2. Handbook of the radio designer. 3rd ed. - M.: Radio and communication, 1984.S.522.
3. Radio. 1995. No. 2. P.38.
4. Radio. 1985. No. 3. P.26.
5. Radio, 2002, No. 8. P.38.
6. Radio, 2006, No. 4. P.22. 27, 39.

Author: Yu.Semenov, Voronezh

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