Soldering tip temperature stabilizer. Scheme, description

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Soldering Tip Temperature Stabilizer

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Encyclopedia of radio electronics and electrical engineering / Power regulators, thermometers, heat stabilizers

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The device described below is paired with a standard soldering iron with a power of 25 W for a voltage of 42 V (rms), powered by a 220 V mains through a step-down isolation transformer. The resistance of the soldering iron heater to direct current is approximately I 70 Ohm in a cold state (at a temperature of about 25 ° C).

Soldering Tip Temperature Stabilizer
Fig. 1

When soldering with ordinary solders, when the heater temperature reaches 250 ° C, its resistance is close to 71 ohms.

The heat stabilizer circuit is shown in fig. 1. The basis of the device is a clock generator assembled on the DA2.2 op-amp according to the standard scheme with separate charging (VD3R19) and discharging (VD2R18) circuits of the timing capacitor C4. The generator generates at the output a sequence of high-level rectangular pulses (12 V) with a duration of approximately 4 s, separated by one-second pauses (voltage close to zero). During the pulse, the heater of the soldering iron is turned on, during the pause, the temperature of the tip is measured.

A measuring bridge is assembled on precision resistors R1, R2, R4, R5, R7. One of the shoulders of the bridge is the heater EK1 of the soldering iron. With a selection of resistor R7, the bridge is precisely balanced with a cold soldering iron.

On the operational amplifier DA1.1, an amplifier with a gain of about 313 is assembled. Capacitor C2 smoothes out the residual voltage ripple of the useful signal and pickup on the body of the soldering iron. The comparator, made on the op-amp DA1.2, at the end of the measuring pause switches from a state with a high output voltage to a state with zero output voltage if the temperature of the soldering tip has exceeded the set value. The inverting input of the comparator receives voltage from the output of the amplifier, and the non-inverting input receives voltage from the exemplary voltage driver, which is assembled on the op-amp DA2.1. Resistors R15 and R16 provide a comparator switching voltage hysteresis to prevent bounce in its output voltage.

Capacitor C1 suppresses pickups on the input circuits of the op amp DA2.1. Resistor R10 determines the upper value of the operating temperature of the soldering iron.

Trigger DD1.1 stores the state of the comparator output at the moment of transition from the measuring pause to the operating heating pulse.

Switching transistors VT1, VT3 switch the operating current of the soldering iron heater, periodically connecting it to a 42 V voltage source. If by the end of the measuring pause the trigger switched to a single state, which indicates that the temperature of the soldering iron is insufficient for soldering, then the voltage at the direct output of the trigger DD1.1 close to its supply voltage, so the VD5 diode closes. At the same time, the HL1 LED turns on.

The voltage at the base of the transistor VT3 during the working pulse does not exceed 1,2 V (composite transistor). The diode VD5 remains closed, and the transistor VT3 opens during the working clock pulses. Together with it, the transistor VT1 opens, including the heater.

As soon as the VT1 transistor opens, the VD1 diode closes, preventing 42 V from entering the microcircuit power circuit. Within four seconds, the soldering iron is heated by the rated current. At this time, the opened transistor VT2 completely discharges the storage capacitor C3, which, together with the resistor R11, which protects the output of the op-amp DA1.1 from overload, forms a low-pass filter that suppresses voltage ripple at the inverting input of the comparator in the measuring pause.

At the end of the working pulse, all transistors close, the VD1 diode opens. Another measuring pause begins. At the same time, the resistance of the heater RK1 increased, the output voltage of the amplifier DA1.1 increased, the capacitor C3 charged to a higher level by the end of the pause. Such heating-measurement cycles will occur until, by the end of the next measuring pause, the voltage at the inverting input of the DA1.2 comparator is greater than at the non-inverting one.

Then the comparator will switch, the voltage at its output will decrease almost to zero, and with a positive voltage drop at the input C of the trigger DD1.1, it will switch to the zero state. The HL1 LED will go out, the VD5 diode will open, due to which the voltage at the base of the VT3 transistor will not exceed 0,6 V, and it will remain closed. The transistor VT1 will not open either - the soldering iron heater will start to cool.

As soon as the soldering iron cools down so much that the voltage at the inverting input of the comparator becomes less than at the non-inverting one, the comparator will switch back and the trigger will follow it - the process of heating the soldering iron will begin again. By changing the voltage at the non-inverting input of the DA1.2 comparator with a variable resistor R8, you can adjust the maximum temperature of the soldering tip.

Resistors R17 and R21 limit the base current of transistors VT2 and VT3, respectively, preventing overloading the output of the op-amp DA2.2.

The microcircuits of the device are powered by the DA3 voltage regulator, and the soldering iron heater is powered directly from the VD4 rectifier with a smoothing capacitor C7. The construction of the rectifier used made it possible to get by with one secondary winding (with a tap) on the network transformer T1 to obtain two sources of constant voltage. It must be borne in mind that the operation of the stabilizer with the capacitor C7 disconnected can lead to the failure of the transistor VT1.

In the stabilizer, you can use, in addition to those indicated in the diagram, LM358 operational amplifiers or microcircuits containing an op-amp paired with a comparator - KR1401UD6, LM392 - and the like, allowing operation when powered by a voltage of 12 V, and through signal inputs - down to zero. IC panels are not recommended. Any voltage stabilizer is suitable for an output voltage of 12 V and a current of at least 0,2 A. It must be equipped with a heat sink capable of dissipating power of at least 2 watts.

Transformer T1 - any network power of at least 30 W with a secondary winding for a voltage of approximately 2x16 V and at a load current of at least 0,75 A. Diode bridge VD4 - any for a voltage of 50 V and a current of 1 A. It is better to use a variable resistor R8 of group A and not small-sized (suitable, for example, SP-1), otherwise it will be difficult to accurately set the operating temperature of the soldering iron.

Transistor VT2 can be replaced by any low-power silicon npn structure. Any diode VD1 is suitable with a maximum current of more than 0,2 A and a reverse voltage of at least 50 V.

Precision resistors - C2-29 V.

Soldering Tip Temperature Stabilizer
Fig. 2

All parts of the stabilizer, except for the transformer T1 and the variable resistor R8, are placed on a printed circuit board made of foil fiberglass 1 mm thick. The drawing of the board is shown in fig. 2. Resistor R8 is installed on the front panel of the instrument case and is provided with a simple scale calibrated in temperature units.

With a properly balanced bridge, the output voltage of the DA1.1 amplifier should change from +0,2 to +0,9 V when the temperature of the soldering iron changes from room to working when soldering.

In conclusion, it should be noted that all connections and contacts that make up the measuring bridge must have a minimum and stable resistance. It is advisable to compress the connections of the soldering iron heater spiral with the lead wires with pliers in bushings made of thin tin. The wires must have a copper cross section of at least 0,5 mm2. It is best to connect the soldering iron cord to the stabilizer by soldering, twisting and detachable contacts are not allowed.

Author: A. Matytsyn, Voronezh; Publication: radioradar.net

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