Thermostat for low voltage soldering iron. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Ham Radio Technologies
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Many modern radioelements are not only miniature, but also very "gentle". They are afraid of "a lot of things", but, first of all, static electricity and overheating. Therefore, the installation requirements indicate that soldering should be carried out with a low-voltage soldering iron with a rigidly fixed tip temperature. Having set the required tip temperature, it must be maintained constant during operation. For this, temperature controllers of various complexity are used. Industrial soldering stations use thermocouples or thermistors installed inside the body of the soldering iron to control the temperature.
However, an ordinary transistor can serve as a temperature sensor, since, as is known from a school physics course, its parameters depend so much on temperature that in conventional circuits it is necessary to introduce special thermal stabilization circuits. And here this thermodependence is most welcome.
The thermostat (Fig. 1) is designed to work with the soldering iron "EPSN-25/24" (25 W, 24 V).
The VT2 temperature sensor is pressed with a clamp to the tube of the soldering iron casing, and thin connecting wires to it are stretched along the handle, the soldering iron wire and secured with threads with glue and adhesive tape. VT2 together with resistors R2, R3 forms the base bias circuit of the transistor VT1. The voltage at the VT1 emitter and at R2 is stabilized by the R1-VD1 ... VD4 chain, therefore the voltage at the VT1 collector is determined only by a change (decrease) in the resistance of the VT2 temperature sensor when heated, which causes a decrease in voltage at the base of VT1. Transistor VT1 closes, and the voltage drop across resistor R4 decreases. This voltage is fed through the ripple filter C1-R5 to the transistor switch VT3-VT4, assembled according to the Darlington circuit and controlling the turntable K1. Diode VD6 increases the noise immunity of the key, and VD5 blocks the self-induction EMF of relay K1.
At the initial moment (with a cold temperature sensor), the resistance VT2 is high, the transistor VT1 is "well" open, the voltage drop across R4 is enough to trigger the key, so it is open, and relay K1 is turned on. Relay contacts K1.1 connect the soldering iron (RH) to the power connector X3. At the same time, the VD8, VD9 LEDs light up. signaling the heating mode.
As it heats up, the resistance of VT2 decreases, at some point the VT3-VT4 key closes, the relay releases, and the soldering iron turns off. Now it cools down, the resistance VT2 grows until the key works again. Resistor R6 within certain limits, you can adjust the sensitivity of the sensor (you can not set it, but connect the terminals of the collector and base VT2 short).
The thermostat is powered by a simple rectifier (VD7, C2). 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 tinned and soldered to the foil).
Setting up a properly assembled device is reduced to the selection of resistances R1 and R3. so that when adjusted with R2, the required temperature range for heating the tip is obtained, depending on the specific soldering iron and temperature sensor.
Literature
- Radio. 1998, No. 10.
- Radiomir, 2002, No. 10, p.12.
- Radio. 1973. No. 12. P.57.
- Radio. 1998, No. 6, C45.
- Radio. 1996, No. 12.С50.
Author: Yu.Semenov, Voronezh
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