Five ways to get the right soldering iron temperature. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Five ways to get the right soldering iron temperature. Encyclopedia of radio electronics and electrical engineering

Free technical library

Encyclopedia of radio electronics and electrical engineering / Ham Radio Technologies

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Many people know that in order to obtain high-quality soldering during the installation of radio components, it is necessary that the temperature of the soldering iron tip matches the operating temperature of the solder. It differs for different brands of solder. If the tip of the soldering iron is overheated, the solder will oxidize and the solder will not be strong enough. In addition, in this case, the soldering iron tip quickly burns and the solder generally ceases to stick to it. High-quality soldering has a mirror shine after cooling, and it can only be obtained at a certain temperature. So, for the most common brand of solder POS-61, the soldering temperature is 190...260 °C. The recommended temperature for soldering microcircuits is 235 ± 5 ° C for a duration of not more than 2 s.

When buying the simplest cheap soldering iron for a mains voltage of 220 V, as a rule, it turns out that it overheats and does not solder well. There are four ways to fix this problem.

Method 1. If the soldering iron has a tip in the form of a rod, which is fixed on the body with a screw (Fig. 5.7), then by adjusting the length of the rod immersed in the heater, you can easily smoothly change the temperature. But not all soldering irons have such a tip attachment design, and this method may not be acceptable.

Five Ways to Get the Right Soldering Iron Temperature
The design of the soldering iron with a movable tip

Method 2. You can use LATR or a transformer with a large number of taps. In this case, the temperature is controlled by changing the voltage applied to the heater winding.

Method 3. An additional resistor (rheostat) is connected in series with the soldering iron heater. In this case, the power of the resistor should be the same as that of the soldering iron, and we select the resistance value to obtain the desired temperature. Such an additional resistor has large dimensions and heats up, which is inconvenient.

Method 4. The electronic regulator allows you to smoothly change (with a variable resistor R2) the temperature of the heater over a wide range. The device has a transformerless power supply and small dimensions, which allows it to be placed in a stand for a soldering iron. The circuit is not critical to the types of parts, and its setting consists in selecting the value of the resistor R4 (at a zero value of R2) to obtain the maximum voltage on the heater. The connected soldering iron can have a power of 15 to 300 W, and when replacing the VD1 ... VD4 diodes with a higher current, up to 1000 W.

Five Ways to Get the Right Soldering Iron Temperature
Temperature controller circuit

If the soldering iron is designed for a lower nominal supply voltage (48 or 36 V), a voltage-reducing transformer will be required, and a reduced voltage may be applied to the electronic regulator circuit. In this case, to maintain its performance, it will be necessary to decrease the value of the resistor R1 in proportion to the input voltage.

Method 5. Allows you to automatically maintain the set temperature of the soldering iron with an accuracy of 1 ° C and is used for mounting microelectronic radio elements that are very critical to overheating. In this case, you will need to purchase a soldering iron with a thermocouple already installed inside.

Five Ways to Get the Right Soldering Iron Temperature
Heat stabilizer circuit diagram

The heat stabilizer circuit is made on one dual microcircuit DA1 (140UD20A) and a symmetrical thyristor (triac) VS1. On the DA1.1 element, a differential signal amplifier from a thermocouple is assembled, and on the DA1.2, an integrator that controls the operation of the pulse generator on a single junction transistor VT1. The pulses through the isolating transformer T1 are fed to the control of the switch VS1.

The use of an integrator in the circuit, instead of the commonly used comparator, makes it possible to provide a soft characteristic of the power change in the heater upon entering the thermal stabilization mode. This is done by changing the charge time of the capacitor C10, on which the frequency of the generator depends, and hence the initial opening angle of the triac. Until the voltage from the DA1 / 12 output exceeds the threshold value (on DA1 / 6) set by the resistors connected to the SA2 switch, the output of the DA1 / 10 microcircuit will be +12 V, which will ensure the operation of the generator (VT1) at the maximum frequency - triac will be fully open.

To power the device, you will need a transformer with two additional 18 V windings or one, but with a tap in the middle. The power of the transformer must match the power of the soldering iron (the electrical circuit of the control unit consumes no more than 15 mA).

The pulse transformer T2 has the same parameters as in the circuit in fig. 4.10. The rest of the parts can be used of any type. The DA1 chip can be replaced with two from the 140UD7 series, but this may reduce the accuracy of maintaining the temperature.

When setting up the thermostat to fully open the triac, it may be necessary to swap the leads on one of the windings of the T2 pulse transformer (phasing of the control pulse is important). The trimming resistors marked with "*" set the required temperature with the corresponding position of the switch SA1. More precisely, the desired temperature can be set using the resistor R15.

I almost forgot to point out another way, which may be an addition or a variation of the third. Instead of an additional quenching resistor, you can use a non-polar capacitor from the MBM series. It has a capacitance at a frequency of 50 Hz: Xc=1/314C. The larger the value of the capacitance, the lower its resistance.

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