Hello all,

I have received for free one of these soldering stations that is sold under the names of Fixpoint AP2, Velleman VTSS4N, ProK Power Solder 2/3/4 (here is a review by a Dutch guy) as it was defective and it had the following problem: when rotating the potentiometer, it would essentially not heat up the iron or the tip unless the potentiometer was set to a position of maximum power/heat. I could also hear sparks (as from an inductive/capacitive current) for a very narrow range of potentiometer positions/angles (transition between no power consumption to significant or maximum power consumption). Since I had already had experience with this soldering station before (I bought one for myself somewhere in 2013-2015), I suspected the culprit was the heating element, since this was the problem I had had with the soldering station I bought, and the problem had been solved by replacing the heating element (in 2023).

I bought a replacement heating element and installed it (the older heating element had a resistance of about 1.6 kOhm and was somewhat falling apart, while the new one had a resistance of about 1.2 kOhm). The problem didn't get solved. I therefore opened the soldering station and started doing some basic tests (mostly measuring resistance values of components, to check for open and short circuits). I realized something weird about the potentiometer terminals: the resistance wouldn't change for pretty much the whole rotating range of the potentiometer (about 326 kOhm), and it would then suddenly drop to essentially zero (less than 5 Ohm) for a very narrow range (maybe the last 15-20 degrees). Note that the potentiometer is in parallel with another potentiometer (which explains the 326 kOhm value) and that two of its pins are shorted (which is okay).

I then desoldered the potentiometer (ironically, with a soldering station of the same model, that I found and bought after getting the defective one; I cannot use the one I bought in 2013-2015 since it is far away, i.e., in my home country). The potentiometer was indeed faulty, and electrically had a behavior very much like an open circuit for almost the whole rotating range and a few Ohms for the narrow range. It was also the source of the sparks, so I soldered two wires to the PCB and to the potentiometer and recorded such sparks while rotating it (video link; yes, "do not try this at home"... also, the potentiometer stopped working completely not long after recording this video). I opened the potentiometer, and what I remember seeing was that, for that narrow range, the resistive material was not on the PCB material/substrate anymore but on the tiny "hooks" of the slider, as if the slider had scrubbed it off.

This led me to think that maybe the new potentiometer could fail in the same way. I therefore spent some time reverse engineering the circuit (PCB photo below) and simulate it using circuitjs (here, or upload the attached file to the circuitjs website). The TRIAC is MAC97A6, so I adjusted the parameters of the TRIAC more or less according to its datasheet and the DIAC to have a breakover voltage of 30-40V and to have a representative power consumption by the load for different positions of the potentiometer (minimum around 7W, middle around 20W, maximum around 46W). According to the simulation results, the potentiometer seems to dissipate excessive power, exactly within a quite narrow range (the range in which the sparks/worn out resistive material would occur). For example, when the potentiometer is set to 22.3 kOhm, average power dissipation is 72 mW; assuming the nominal power for the whole resistive material is 250 mW, then proportionally when set to 22.3 kOhm (22.3/500=0.0446 or about 4.5%), the potentiometer should only be dissipating about 4.5% of 250mW, i.e., about 11 mW. That means it's dissipating 6.5 times what it should be dissipating!


I also have another problem: there is coil whine (from the black inductor, I assume, from the current through it being switched at 2*50=100 Hz — I live in Europe) for almost the whole range of the potentiometer except close to the very end (I think that, because the chopped voltage is very close to the mains sine wave, the result is that the current doesn't increase sharply and therefore noise is much lower).

I also have a photo of the PCB of the soldering station (model ZD-98) I used to replace the faulty potentiometer. The design is very similar, it is just missing the EMI/RFI filter (transformer, X2 capacitor, inductor) and the RC snubber of the TRIAC.



The reasons I posted this:
1) how could the circuit be improved to prevent the potentiometer from overheating/wearing out prematurely?
2) how could I alleviate or completely suppress the coil whine, while keeping the filtering of the inductor (I could just short it by soldering a wire to its terminals, sure, but then the filtering it provides is gone)?


Thank you in advance

 

I suspect it could be improved by discarding the original control circuit and substituting a TRIAC light dimmer. Or discard everything and get a soldering station kit that uses integrated tips like the T12. You get temperature regulation, automatic sleep mode which extends tip life and saves power, and portability since you can run them off a battery.

 

@bassbindevil thank you for your input. I might consider another soldering iron/station in the future, but for now, given that I seldom need to solder anything at all (in a whole year, I do it in maybe 5 sessions) and don't need anything "professional grade", I think I'll stick with replacing the heating element and/or the potentiometer once every 10 years or so.

 

1. if I have this correct you want a way to limit the amount of current to the potentiometer?
2. maybe encapsulating the transformer with epoxy would help with the whine.

 

@Rockettfuel
1. yes, but how, preferably with minimal changes to the existing circuit? if the current through the potentiometer would need to decrease by a factor of around 10, that means there should be a way to "amplify" the then lower current (I am thinking of the Darlington pair and what the analogous in AC/a TRIAC circuit would be)
2. ah, you think it's the transformer; I assumed it was the inductor, but I actually don't know since I didn't exactly put my hear next to both to identify the origin of the noise; I don't feel like getting physically close to 230V AC without some electrical insulation, like the plastic of the soldering station

 

this is a classic light dimmer. i used to make exact same circuit and never had a problem. not sure what is your problem with it... you can adjust potentiometer to any valuer you like sn you will see that dissipated power is less than 1/2 of your potentiometer rating. one major issue with this circuit is lack of feedback. it is an open loop regulator. that means soldering iron responses are sluggish. but that is what you paid for...

 

not sure what is your problem with it... you can adjust potentiometer to any valuer you like sn you will see that dissipated power is less than 1/2 of your potentiometer rating.

As far as I understand, if only a fraction of the potentiometer resistance is being used, then the power rating of that fraction is the same fraction of the potentiometer rating. In the simulation linked above, 22.3 kOhm corresponds to 22.3/500*100=4.5% of the total resistance, so the power rating of that resistance fraction would be 4.5%*1/4 W=11 mW (milliwatt), assuming the potentiometer power rating is 1/4 W or 250 mW, while the average power measured by the wattmeter (penultimate scope on the bottom of the simulation window) is about 72 mW, which is way above 11 mW, which in turn makes me wonder if overheating was the cause of failure of the previous (original) potentiometer.

one major issue with this circuit is lack of feedback. it is an open loop regulator. that means soldering iron responses are sluggish. but that is what you paid for...

Yes, you can see that in the review I linked before: 2.5 minutes to reach 250 ºC, 5 minutes to reach 350 ºC, 10 minutes to reach the final temperature value. But since I only use it for a couple of repairs/some tinkering with electronics that doesn't require professional grade equipment, I'm okay with it. I can wait a few minutes until I can start soldering.

(...) I didn't exactly put my hear next to both (...)

*ear

 

This thread has been really helpful.

Based on what I’ve seen here, the biggest issue looks like the "potentiometer handling too much power in a very narrow range". That would explain why it heats up so quickly. A straightforward fix would be replacing it with a higher-rated pot or using a type with a more robust construction. Adding a small series resistor to limit the maximum power through that section could also reduce the stress on the pot.

The comments about the open-loop control also make sense. Since the station doesn’t use temperature feedback, regulation will always feel a bit inconsistent. If you want a more stable setup later, adding a simple thermistor or thermocouple feedback circuit with a TRIAC driver would give much tighter temperature control, though that’s a bigger mod.

As for the coil whine, it’s probably coming from the transformer reacting to the chopped AC waveform. Potting the transformer or improving the snubber network could help damp the vibration and reduce the noise.

For now, I’d start with the pot replacement since it’s the easiest and most direct improvement. Once that’s sorted, you can decide if you want to take the station further with feedback control.

Thanks again for sharing the findings.

 

@dan.techlab I'm glad you found it helpful (well, that's why I've created the thread). I have more input on this, I just have not had the time and mind to post it. Also, although I may not reply quickly, I receive e-mail notifications from the thread.

Soon, I hope.

 

@dan.techlab I'm glad you found it helpful (well, that's why I've created the thread). I have more input on this, I just have not had the time and mind to post it. Also, although I may not reply quickly, I receive e-mail notifications from the thread.

Soon, I hope.

Thanks so much for the update! No worries at all about the timing — I appreciate any input you’re able to share whenever you have the chance. Looking forward to your next insights!

 

about the case when potentiometer is adjusted to a lower value (1/10 of a turn for example), the capacitor gets charged faster - while AC is closer to zero. this triggers triac sooner (withing AC half-wave form). then triac remains practically short circuit until next zero cross. so portion of potentiometer that is used in this case is rated to 1/10 of the potentiometer power, but most of the time, voltage seen by potentiometer is about zero, even the peak voltage is low. the larger potentiometer value, the smaller the current.

while using potentiometer with higher rating does not hurt, i would not worry about it. i have seen this used in numerous instances using standard carbon pot, rated 1/4W. if curious, one can simulate this and see the actual power - that would be a good exercise.