Soldering iron tip heat stabilizer. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Soldering iron tip heat stabilizer. 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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The author proposes a repeatable device for maintaining the optimum temperature of the soldering iron tip by measuring the resistance of the heater during short-term disconnections from the mains.

Various soldering iron tip temperature control devices have been repeatedly published on the pages of radio engineering magazines, using the soldering iron heater as a temperature sensor and maintaining it at a given level. Upon closer examination, it turns out that all these regulators are just stabilizers of the heat output of the heater. They, of course, give a certain effect: the tip burns out less and the soldering iron does not overheat so much while it lies on the stand. But this is still far from controlling the temperature of the sting.

Let us briefly consider the dynamics of thermal processes in a soldering iron. On fig. 1 shows graphs of the change in temperature of the heater and soldering iron tip from the moment the heater is turned off. The graphs show that in the first fractions of a second the temperature difference is so large and unstable that the temperature of the heater at this moment cannot be used to accurately determine the tip temperature, and this is exactly how all previously published controllers work, in which the heater is used as a temperature sensor. From fig. It can be seen from Fig. 1 that the curves of the dependence of the temperature of the tip and the heater on the time it is turned off only after two, and even more so three or four seconds, converge sufficiently in order to interpret the temperature of the heater as the temperature of the tip with sufficient accuracy. In addition, the temperature difference becomes not only small, but almost constant. According to the author, it is the regulator that measures the temperature of the heater after a certain time after it is turned off, which is able to more accurately control the temperature of the tip.

Soldering iron tip heat stabilizer
Rice. 1. Graphs of temperature change of the heater and soldering iron tip from the moment the heater is turned off

It is interesting to compare the advantages of such a regulator with a soldering station using a temperature sensor built into the soldering tip. In a soldering station, a change in the temperature of the soldering tip immediately causes a reaction in the control device, and the increase in the temperature of the heater is proportional to the change in the temperature of the tip. The temperature change wave reaches the soldering iron tip in 5...7 s. When the temperature of the tip of a conventional soldering iron changes, the wave of temperature change goes from the tip to the heater (with close thermodynamic parameters - 5 ... 7 s). Its control unit will operate in 1...7 s (depending on the set temperature threshold) and raise the temperature of the heater. The reverse wave of temperature change will reach the soldering iron tip in the same 5...7 s. It follows that the reaction time of a conventional soldering iron using a heater as a temperature sensor is 2...3 times longer than that of a soldering station soldering iron with a temperature sensor built into the tip.

Obviously, a soldering station has two main advantages over a soldering iron that uses a heater as a temperature sensor. The first (minor) is a digital temperature indicator. The second is a temperature sensor built into the sting. At first, the digital indicator is simply interesting, and then the regulation goes on anyway according to the principle "a little more, a little less".

A soldering iron using a heater as a temperature sensor has the following advantages over a soldering station:

- the control unit does not clutter up the space on the table, as it can be built into a small case in the form of a network adapter;
- lower cost;
- the control unit can be used with almost any household soldering iron;
- ease of repetition, feasible for a beginner radio amateur.

Consider the design features of soldering irons of different designs and capacities. The table shows the resistance values of the heaters of various soldering irons, where P_w - soldering iron power, W; R_K - resistance of the cold soldering iron heater, Ohm; R_r- - hot resistance after warming up for three minutes, Ohm. The difference between these temperatures shows that the TCS of the heaters can differ by a factor of 50. High TCR soldering irons have ceramic heaters, although there are exceptions. Soldering irons with a small TKS - an outdated design with nichrome heaters. It should be noted separately that in some soldering irons a diode can be built in - a temperature sensor, and I came across one very interesting soldering iron: in one polarity of turning on the TCS it was positive, and in the other it was negative. In this regard, the resistance of the soldering iron must first be measured in cold and hot states in order to connect it to the regulator in the correct polarity.

The controller circuit is shown in fig. 2. Duration of the switched on state of the heater is fixed and amounts to 4...6 s. The duration of the off state depends on the temperature of the heater, the design features of the soldering iron and is adjustable in the range of 0...30 s. It may be assumed that the temperature of the soldering tip is constantly "swinging" up and down. The measurements showed that the temperature change of the tip under the influence of control pulses does not exceed one degree, and this is explained by the significant thermal inertia of the soldering iron design.

Soldering iron tip heat stabilizer
Rice. 2. Regulator circuit

Consider the operation of the regulator. According to a well-known scheme on the rectifier bridge VD6, quenching capacitors C4, C5, zener diodes VD2, VD3 and smoothing capacitor C2, a control unit power supply is assembled. The node itself is assembled on two op-amps, included by comparators. To the non-inverting input (pin 3) of the op-amp DA1.2, an exemplary voltage was applied from the resistive divider R1R2. Its inverting input (pin 2) is energized from a divider, the upper arm of which consists of a resistive circuit R3-R5, and the lower arm of a heater connected to the input of the op-amp through a VD5 diode.

At the moment the power is turned on, the resistance of the heater is reduced and the voltage at the inverting input of the op-amp DA1.2 is less than the voltage at the non-inverting one. The output (pin 1) DA1.2 will be the maximum positive voltage. The DA1.2 output is loaded with a series circuit consisting of a limiting resistor R8, an HL1 LED and a emitting diode built into the optocoupler U1. The HL1 LED signals the heater is on, and the emitting diode of the optocoupler opens the built-in phototriac. The mains voltage of 7 V rectified by the VD220 bridge is supplied to the heater. Diode VD5 will be closed by this voltage. The high voltage level from the output DA1.2 through the capacitor C6 affects the inverting input (pin 1.1) of the OU dA 7. At its output (pin 1), a low voltage level occurs, which, through the diode VD6 and resistor R1.2, will reduce the voltage at the inverting input of the op-amp DA3 below the exemplary one. This will ensure that the voltage level at the output of this op amp is maintained high. This state remains stable for the time specified by the differentiating circuit C7RXNUMX.

As the capacitor C3 charges, the voltage across the resistor R7 of the circuit drops, and when it becomes below the exemplary one, at the output of the op-amp DA1. 1 low signal level will change to high. A high signal level will close the diode VD1, and the voltage at the inverting input DA1.2 will become higher than the exemplary one, which will lead to a change in the high signal level at the output of the op-amp DA1.2 to a low one and turn off the HL1 LED and optocoupler U1. A closed phototriac will disconnect the VD7 bridge and the soldering iron heater from the mains, and an open VD5 diode will connect it to the inverting input of the op-amp DA1.2. The extinguished HL1 LED indicates that the heater is turned off.

At the DA1.2 output, the low voltage level will be maintained until, as a result of the soldering iron heater cooling down, its resistance drops to the DA1.2 switching point, set, as mentioned above, by the exemplary voltage from the R1R2 divider. Capacitor C3 by that time will have time to discharge through the diode VD4. Further, after switching DA1.2, the optocoupler U1 will turn on again and the whole process will be repeated. The cooling time of the soldering iron heater will be the longer, the higher the temperature of the entire soldering iron and the lower the heat consumption for the soldering process. Capacitor C1 reduces interference and high-frequency interference from the network.

The 42x37 mm printed circuit board is made of one-sided foil-coated fiberglass. Its drawing and arrangement of elements are shown in fig. 3.

Soldering iron tip heat stabilizer
Rice. 3. Drawing of a printed circuit board and the location of elements on it

LED HL1, diodes VD1, VD4 - any low-power. Diode VD5 - any type for a voltage of at least 400 V. Zener diodes KS456A1 are replaceable by KS456A or one 12 V zener diode with a maximum allowable current of more than 100 mA. The oxide capacitor C3 must be checked for leakage. When checking the capacitor with an ohmmeter, its resistance must be greater than 2 MΩ. Capacitors C4, C5 - imported film for an alternating voltage of 250 V or domestic K73-17 for a voltage of 400 V. The LM358P chip is replaceable by the LM393P. directly to the output DA8 (pin 1). In this case, the VD1.2 diode can be omitted.

The resistance of the resistor R6 must be selected based on the available heater. It should be less than the resistance of the heater in the cold state by about 10%. The resistance of the tuning resistor R5 is chosen so that the temperature adjustment interval does not exceed 100 ^оC. To do this, calculate the difference in the resistances of a cold and well-heated soldering iron and multiply it by 3,5. The resulting value will be the resistance of the resistor R5 in ohms. Resistor type - any multi-turn.

The assembled block must be adjusted. A chain of resistors R3-R5 is temporarily replaced by two series-connected variable or trimmer resistances of 2,2 kOhm and 200 ... 300 Ohm. Next, the unit with the connected soldering iron is connected to the network. Having achieved the desired tip temperature with the engines of the temporary resistors, the device is disconnected from the network. Resistors are soldered and the total resistance of the introduced parts is measured. From the value obtained, subtract half of the previously calculated resistance R5. This will be the total resistance of the fixed resistors R3, R4, which are selected from those available at the closest to the total value. A switch can be placed in the break of this resistive circuit. When it is turned off, the soldering iron will switch to continuous heating.

For those who need a soldering iron for several soldering modes, I suggest putting a switch and several resistive circuits in different modes. For example, for soft solder and for normal solder. When the circuit is broken - forced mode. The power of the soldering iron used is limited by the current limit of the KTs407A rectifier bridge (0,5 A) and the MOS3063 optocoupler (1 A). Therefore, for soldering irons with a power of more than 100 W, it is necessary to install a more powerful rectifier bridge, and replace the optocoupler with an optoelectronic relay of the required power.

A comparison of the operation of different soldering irons together with the described device showed that soldering irons with a ceramic heater with a large TCR are most suitable. The appearance of one of the variants of the assembled block with the cover removed is shown in Fig. 4.

Soldering iron tip heat stabilizer
Rice. 4. Appearance of one of the variants of the assembled block with the cover removed

I remind you about safety. Be careful, especially when setting up: the unit does not have galvanic isolation from the supply voltage of 220 V!

Author: L. Elizarov

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