Resistor soldering iron. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Ham Radio Technologies

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A soldering iron is the main “working tool” of a radio amateur. And given the widespread use of very “delicate” field-effect transistors and CMOS microcircuits, very stringent requirements are placed on it.

The most common heating element of a soldering iron is a nichrome spiral, insulated from the rod by a thin mica tube. Mica has a very high dielectric constant (it’s not for nothing that mica capacitors are considered the best), so all high-voltage interference entering the soldering iron coil via the power wires passes almost unhindered to its tip. If the soldering iron tip touches the track to which the field-effect transistor is soldered (which happens quite often), the “life” of this transistor is in great danger. Another drawback of such soldering irons is their low strength (even weak lateral forces when desoldering elements, not to mention impacts, can damage them).

Obviously, it is inconvenient to constantly work with such a soldering iron. Therefore, many radio amateurs resort to various tricks:

• power the soldering iron with reduced voltage (12...36 V). This voltage is safe for field-effect transistors, but the soldering iron requires its own source with the appropriate voltage;
• increase the thickness of the dielectric (mica), which impairs the heat transfer from the heating coil to the soldering iron tip;
• other materials are used as a heating element.

It was the latter path that I decided to take. Surely everyone has seen powerful domestic resistors of the PEV series. So, these are ready-made heating elements for a soldering iron with a power of 30...60 W! One can only wonder why descriptions of soldering irons based on them are rarely found in the literature. After all, powerful resistors are designed to withstand significant overheating. They can safely withstand heating up to 500...600°C, which is several times higher than the melting point of solder. This “non-standard” use of resistors is also facilitated by the fact that PEV-7,5 resistors have an internal hole with a diameter of 5 mm. those. the same diameter as the tip of a standard 40-watt soldering iron. The thickness of the ceramic dielectric of the resistor is about 3 mm, which cannot be compared with a mica layer 8 fractions of a millimeter thick.

As practice has shown, it is almost impossible to damage sensitive elements with such a soldering iron, even when powered from a 220 V network. In addition, by using a resistor, you can forget about dielectric breakdown (this happens quite often with “mica” soldering irons). Another advantage of a “resistor” soldering iron is a large range of resistor ratings (resistances), so choosing the right one is not difficult, and if the heater fails, you can simply change the resistor.

Industrial 40-watt soldering irons (Fig. 1) are excellent for reworking, although it is not difficult to prepare the case yourself. The only difficulty that may arise is that the diameter of the PEV-7,5 resistor (such a resistor can dissipate power up to 50 W for a long time, heating up to a temperature above 500°C) is slightly larger than the metal tip holder of a standard soldering iron. If it is made of a metal plate rolled into a tube, it will have to be slightly expanded (unfolded) from the tip side so that the resistor will “fit” into it (a solid tube will have to be cut to length). The resistor is held in the tube due to friction, and very reliably. The tube with the resistor must be turned so that the leads of the resistor stick up - then they do not interfere with the work so much.

Fig. 1

It makes no sense to solder wires to the terminals of the resistor - the terminals heat up to almost the same temperature as the resistor itself, i.e. above the melting point of the solder. It is best to take special plugs that are used in car radios, refrigerators and other household appliances where it is necessary to ensure reliable contacts without soldering. The wires from the resistor are inserted into the holes of the holder tube near the handle itself (the temperature there is not very high and is safe for insulating the wires), and then brought out through the handle, as usual.

For a 40 W soldering iron powered by a car battery, the resistor should be about 5,1 ohms (it will produce about 30 W of power). This takes into account the resistance of the wires (approximately 1 ohm). With this resistance, the soldering iron is normally heated up if the battery voltage is above 12 V and does not overheat at the maximum (14,4 V).

If the soldering iron is supposed to be connected through an automatic temperature regulator (with a thermocouple mounted on the tip), then the resistance of the resistor can be reduced to 3,6...4,7 Ohms. Then it will heat up faster - not 2...3 minutes, but only 40 seconds. And domestic PEVs are practically insensitive to current overloads. For other supply voltages, the resistor resistance should be different, as can be seen from the table. The thermostat, to increase efficiency and reduce heating of the control element, must operate in pulse mode. The thermal inertia of the soldering iron is very high, and the frequency of the current pulses can be less than 1 Hz. It is undesirable to make it too large (more than 1 kHz). Although the capacitance between the resistor coil and the soldering iron tip is negligible, as you know, as the frequency increases, the capacitance decreases, and it will be much more difficult to deal with high-frequency interference along the power wires.

Domestic resistors are coated with a special paint, which darkens when heated (from green to black). There is no need to be afraid of this; when it cools, it turns green again. The described design has been working for me for more than a year, and the appearance of the resistor has not suffered during this time. The soldering iron tip burns strongly to the resistor, but this drawback is also inherent in conventional soldering irons. In addition, it is easy to knock it out by inserting a suitable rod into the resistor. However, do not try too hard - the ceramic resistor body can easily be damaged by strong impacts.

The thermostat can be assembled according to the simplest scheme (Fig. 2).

Fig. 2

Of the temperature sensors available to most radio amateurs, it is best to use thermistors here. Semiconductor sensors cannot measure such high temperatures - after just a few hours of operation their characteristics deteriorate. Disk thermistors should also be abandoned - their leads are soldered with regular solder, and when the soldering iron heats up, they fall off. Tubular thermistors are good (the housing is like that of ordinary MLT-0,25 resistors, only twice as long), however, they are quite difficult to secure. The initial resistance of the thermistor can be almost anything. When heated, it decreases to tens of ohms for all resistors. Before attaching the thermistor to the soldering iron tip, it is advisable to wrap it (the tip) with asbestos threads or any other heat-resistant insulator.

The thermostat is assembled according to the classical scheme - a voltage comparator on the operational amplifier DA1.1 and a Schmitt trigger on DA1.2. A distinctive feature of the LM358 chip is its ability to compare voltages that are close in amplitude to the voltage at the negative power supply pin (pin 4). Most other inexpensive ICs go on strike in this mode. It can be replaced with the Ukrainian ICPA358P, or with the 4-element LM324 or KR1401UD2.

Trimmer resistor R1 regulates the temperature of the tip. As its resistance decreases, the temperature also decreases. It is advisable to connect a constant resistor with a resistance of about 1 kOhm in series with R1 - the microcircuit “does not like” more than 4/5 of the supply voltage to be supplied to its inputs.

While the tip temperature is low, the resistance of thermistor R4 is quite high, the voltage at the direct input DA1.1 is greater than the voltage at the inverse input, and the op-amp output is at a high level. At the output of DA1 2 - the same level, transistor VT1 is open and supplies voltage to the soldering iron. As the latter warms up, the resistance of the thermistor decreases, and soon the voltages at both inputs of DA1.1 will be equal. The amplifier will begin to switch chaotically (there is no feedback, and it is extremely difficult to introduce it, since feedback works normally only when the voltages at the inputs of the op-amp are close to half the supply voltage, and in our case they are only hundreds of millivolts above zero).

To combat high-frequency interference at the DA1.1 output, a Schmitt trigger was added to the circuit on the DA1.2 amplifier. It switches to the logical “0” state only after the constant component of the signal (of any shape and frequency) at the output of the DA1.1 amplifier becomes less than 1/4 of the supply voltage, i.e. after the soldering iron has warmed up to operating temperature. Then transistor VT1 also turns off. For some time, the temperature of the soldering iron tip increases due to thermal inertia, and the voltage at the output of DA1.1 decreases. Then the tip begins to cool, and the voltage at the output of DA1.1 increases. As soon as it (the constant component) exceeds 3/4 of the supply voltage, the DA1.2 trigger switches again and the soldering iron begins to heat up.

The supply voltage must be within 5...20 V, the voltage U2 (at the load resistor) can be any. But the resistor itself (resistance and power) and transistor VT1 must be designed for it. When using bipolar transistors between the output of DA1.2 and the base of the transistor you need a resistor with a resistance of 100...470 Ohms (the lower the voltage, the lower the resistance), the emitter of VT1 is connected to the common wire. Both voltages may be unstabilized. The current consumption in circuit U1 does not exceed tens of milliamps.

It is advisable to use field-effect transistors in the device, especially when the voltage U2 is less than 100 V. Then the transistor will be cold, and the entire circuit can be hidden in the handle of a soldering iron. A bipolar transistor at this voltage needs a small heatsink. For more reliable operation, it is advisable to increase the capacitance of capacitor C3. If it is impossible to set the required temperature with resistor R1, then the resistance of R3 must be reduced, or, better yet, thermistor R4 with a higher resistance must be selected.

Author: A.Koldunov, Grodno.

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