Preheat soldering iron


noninvasive improvement of my economic Station 936 built and marketed under several brands, mine in particular is marked ATTEN but you can find several on the bay 30 € or less.

This soldering station to have chosen for its price very affordable for all but, in addition to this non-negligible factor in the ease in availability of spare parts at low prices, it is easy to find either the entire stylus that individual tips. Despite its cheapness it presents all the devices that make it a tool to work in peace.

The metal part of the stylus is connected to the ground to avoid the accumulation of electrostatic charges, the power of 50W resistance makes it practically universal, then it is fed with a transformer isolates it from the network. It has a great temperature regulation, also being the small mass that the resistance must heat make it quick to reach the set temperature and light so as not to tire during use.

So far so good, but some defect will still have it!

In fact, unlike the more expensive stations it does not have the pre-ignition which leads the stylus at a lower temperature in the moments of pause and then quickly return to the set temperature as soon as the stylus moves. This arrangement extends the life of both the tip of the stylus.

This feature could be useful if the welder is on continuously available for several hours if not used.

Wanting to implement this feature on my station I had to face the fact that the stylus has is the thermocouple for detecting the temperature of the tip but did not have anything that indicates whether the stylus is placed on its stand waiting to use or I'm using.

The idea of ​​placing a sensor for movement for a moment he touched me, But only for a moment, in the case of stylus replacement I would to the starting point and would not solve anything, better an external circuit to the station that the pressure of a bring me the station button for a time determined by the lowest temperature at normal operating condition, past this time interval returns to the preheating condition.

Since I did not want to do something invasive stylus I tried to do the same for the station.

By studying the internal schema of the setting I can not publish because it does not possess the rights, It notes a triac applied to the secondary of the transformer which regulates the direct power at 27V alternating stylus.

From the same secondary via a diode and a limiting resistor it feeds a TL431 which stabilizes the voltage of 5V used for the control circuit in my version based on a double operational LM358 also this is a component in line with the price range of the station. An electrolytic serves to filter the power supply which in any case has absorptions minimum order of tens of mA.

Given the circuit the simplest idea is to put a diode in series to 230V to cut a half-wave and halve the power as in the past made the PHILIPS in its mythical welder 25 / 50W, which is now only a relic for nostalgic but which was given in provided in vocational schools in the 80.

Without the flying test the situation has not been successful for two reasons, the first is that since the secondary of the transformer uses a diode to get the power of the electronic control circuit can, depending on how insert the plug block the positive half-wave on the secondary with the result of not feeding the same circuit.

Once you guessed the direction of insertion of the plug, however, I am using the stylus that is able to heat up, however, the set temperature and the transformer that buzzes because of the strongly distorted input voltage compared to that for which it was designed.

I removed this attempt does not remain me that reduce to 230V input with a power resistor suitably calculated to create a voltage drop such as to render the circuit not efficient from the point of view of temperature but which, however, leaves the sinusoid input intact. Immediately after thought, I realize that the situation is impractical because dissiperei unnecessarily into heat power.

What can give a voltage drop without dissipating much heat? The next day comes the idea decisive, a capacitor in series with its reactance causes a voltage drop such as to enable the preignition but not sufficient to bring the stylus at a temperature such as to melt tin, once the capacitor short-circuited the situation would return to normal.

From the tests carried out the ability to 1uF it is sufficient to feed the station with 80V alternated, that the transformer output voltage is transformed into alternating 9V sufficient to create the control circuit 5V power supply that does not notice the difference but, that was unable to put the stylus in temperature in spite of the triac ports to the resistance across the available voltage.

Now that less is done!!! It remains to create a circuit that drivers relay that shorts the capacitor but after some time I set at approximately 15 minutes automatically turn off the relay, other fundamental function is to know under what conditions is the circuit and have a visual indication of the time remaining before the break in return.

I finally found an elegant solution to small and especially low power consumption, the dissipation of the control circuit settles on 250mW in addition to the consumption of the desoldering station, pictured before the first prototype of the circuit modifications made on the field after testing

The whole is enclosed in a small plastic box 8,5 x 6 x 2cm but if you want you can do better given that a quarter of the matrix board is not used.

Now we come to the diagram


On the right is the network input and output for the soldering station, high on a phase of 230V R1 are used as initial limiter and as a fuse in case of breakages, isolating prevents any other risk in case of breakages. The other phase has in the C1 series that serves to create a power supply to capacitive reactance together with the bridge D1, DZ1 and C2 stabilizes the voltage to 12V with a deliverable current of around 20mA without components that warm.

R2 is used to download C1 when power is disconnected.

R3 and C3 determine the working time of the soldering station, while R4, R5, R6, R7 and R8 determine the elapsed time display, together with the LM324 quad operational amplifier which drives the LED indicator with their limiting resistors.

Initially C3 is discharged and the non-inverting inputs of the operational are at a voltage level higher than those inverting and consequently the outputs will be high. These outputs illuminate the 4 led.

Man mamo that we C3 charge off in sequence of the LEDs L4, L3, L2 and L1 last.

When this will turn off the relay disable which until now had shorted C4 and C5 allowing the station to work at the maximum power.

To save mA the same current that lights L1 serves to pilotareT1 that controls the relay, a separate consideration must be made for the relay that will be chosen in a low holding current, in my case, I found a relay which satisfies itself with only 13mA to remain active while the remaining circuit is content to 4mA LED including. If you are unable to find a low-current relay touches increase the value of C1. In the scheme it lacks to describe R13 adding ultimately to create a small hysteresis and give a reliable and stable switching to the relay, In fact, when you turn off the relay fails the vast majority of absorption and the supply voltage rises a few mV doing reignite the relay, adding the hysteresis has eliminated the inconvenience.

The P1 button is used to reset the counter and start in the event that the station was in pause, C3 shorting the cycle starts again whatever state the state of charge of the capacitor, in fact it serves to lengthen the operating time if we realize that the remaining time is not enough to finish the work in progress.

Amilcare Greetings

1 reply
  1. Avatar
    theremino says:

    great idea!
    When I get a little’ time I add it to my station. But I have a WEP Welleman-20 and I'll definitely make some changes, soon as possible to let you know how it went. I'm also thinking it would be nice to delete the button and feel somehow that the stylus is no longer on its support. Perhaps you could hear it with a HALL element coupled to a weak magnet. So when the metal stylus nears (the base where the stylus is not too hot), the magnetic field decreases and the lobby snaps. Or you could use a real sensor than working with an alternating magnetic field, They are small and do not cost much.


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