Cunningly disguising your thermocouple as a heating element might seem undesirable but perhaps not quite for the reasons you might have thought. As long as you can afford to shut off the heating current for long enough to sample the thermocouple voltage (the 8 ohm resistance in one leg of the circuit won't matter when detecting a voltage with a relatively high impedance circuit - in this case 10KR or higher should be high enough), say a millisecond or two every 100 to 200 or more milliseconds, there shouldn't be any problem monitoring heater/tip temperature this way other than for the inconvenient fact that it is the temperature of the heating element rather than that of the actual business end of the tip that's being monitored in the pious hope that, through the high thermal conductivity of the metal used to conduct this heat the actual tip temperature won't be lagging too far behind.
But there is a problem. When the heater is on, a temp gradient will develop between the heater/sensor and the copper slug at the base of the tip. The heater will reach higher temperatures than the surrounding copper ever will; and the sensor is directly coupled to the heater. So when the power is on full blast, and it is temporarily cut to take a reading, it will measure higher than when the heater has been off for longer and the temp of the sensor is actually closer to that of the surrounding copper. Despite the copper could be the same exact temp in either case.
In the 888, the sensor is thermally insulated from the heater. The sensor is measuring the temperature radiating from the inside of the tip. For the heater to affect the sensor, it has to heat the tip; then the tip heats the sensor.
that once a stabilised tip has been set to the maximum of 480 degrees, the percentage of power, after the initial burst of accelerated heating at 100% for the first second or so, it soon drops down long before it gets to within 50 degrees of the target temperature to just 15%.
Both my clones exhibit this behavior at all set temps. They throttle down well before reaching set temp and only slowly coast to full set temp WHEN UNDER NO THERMAL LOAD. When under load, they sag, significantly.* I believe it to be the natural side effect of the thermal coupling between sensor and heater. Maybe there are clones with more sophisticated sensor compensation.
*Yes, all irons exhibit temperature drop/gradient between the base of the tip where the sensor is and the point that is on the joint. But apples to apples, similar 3mm bevel tip on either, the T12 clones I have exhibit an additional sag compared to the lower power 888, which I suspect is due to the sensor/heater thermal coupling.
The "less overshoot" is bullshit on these clones. Any overshoot on an 888 is minimal. And when it does happen, it's temporary. The T12 clones have to be set to a significantly higher set temp to overcome the thermal sag, and they spend a lot of time at this significantly higher set temp while doing production type soldering. The overshoot of an 888 is much preferable to this situation. These T12 clones basically have way more "undershoot" than the overshoot of an 888.
The T12 clones (which I have) are excellent for a hobbyist that wants a fast warm up time and for anyone who needs to run such high temps anyhow that standby is essential. And cartridge tips are excellent for sustaining high load/output without warming up the handpiece. But they do not regulate better/tighter.
The heating element is made with nichrome (chromel)
Yeah, if you say so, I'll roll with it. W/e it is, it's just one of many stainless steel alloys that is used for heating wire. Like other stainless steels it is a poor conductor of heat compared to non-stainless steels/irons. And even cast iron is a poor conductor of heat compared to copper. Add the fact that the heater wire must be electrically insulated from the copper, and that it's tightly coiled up in a relatively small volume rather than spread throughout the copper,*** and that is why a rather significant temp gradient will occur when the heater is active. Heater gets hotter than the copper; sensor is directly connected to the heater; hence what the sensor measures is affected by other variable than just the temp of the copper at the base of the tip. Your signal has a skew to it, which is an interesting challenge to fix through software. It is especially interesting since the user of the iron represents an unknown, unpredictable variable to the thermal load on the tip, and that load will also increase the amount of sag in this loop. In addition to the sensor being connected/coupled to the heater and being affected by heater activity, the T12 heater needs to have a certain amount of mass to do its job, and the thermocouple needs a certain minimum mass to transmit the power without heating itself up. The ideal sensor for this, say like the one in an 888, would have a super high surface area to mass ratio, with very minimal mass, since it only needs to be a variable resistor to a very low voltage and current signal.
***I imagine the heater is a relatively small and compact length of coiled nichrome that is tucked into a hole in the back of the copper tip-slug with a dab of thermal cement, based on the price point these are made to.
I'm a little ashamed to say that the posting you're responding to contained a major misconception on my part. I've since edited that post to correct this but I'll paste that edit here to clarify the situation.
[EDIT of Reply #52 13:25 BST 15 Sep 2019]
Inspired by that wikipedia article and the exorbitant price of even the cheapest of replacement thermocouples for these soldering iron tip thermometers (they're a 'consumable' where 'cheapness rises to the top of the list of desirable 'properties'), I started looking for "DIY Thermocouple" articles during the reading of which, I suddenly had an epithany.
Whilst it is true that the voltage produced between the hot and cold junctions is a consequence of the Seebeck effect over the whole length of the thermocouple wires joining the hot and cold junctions, this doesn't alter the fact that the voltage produced is still due to the difference in temperature between each of the junctions even when the conductors pass through a region at a higher temperature than the 'hot junction'. The reason of course, being that it's only the net temperature difference between the hot and the cold reference junctions that matters.
Excursions of temperature along the wires joining the two junctions beyond that of each junction simply cancel themselves out in the net temperature difference equation that informs us of the temperature at the hot end junction compared to that at the cold end junction.
What this means is that it remains possible to place our hot junction close to the 'business end' (perhaps right at the 'business end' by drilling a short gallery into the core of the copper bit into which the hot junction can be placed) and essentially ignore the higher temperature on the thermocouple wires obliged to pass through the much higher temperature region of the heating element.
Of course, practical limitations in the placement of the hot junction means the effect of the thermal gradient between the heater and the tip will compromise the goal of sensing the tip's actual temperature to a greater or lesser extent depending on the details of the design of our self sensing direct drive cartridge tip. It's not too difficult to see where clone tips might fall short of the originals.
[/EDIT]
You seemed in doubt about the use of nichrome (chromel which is one of the alloys used both in heating elements as well as one half of a K type thermocouple) for the heating element in the Hakko (and cloned) T12 cartridge tips but it makes perfect sense to choose this particular alloy since it then becomes a simple matter to convert it into a K type thermocouple by welding the top end connection close to or within the copper tip to its partner alloy, alumel, for the return conductor.
Rather neatly, Hakko have created a dual function all in one heating element/K type thermocouple, where each function is just a matter of time sharing with a PID controller. My misconception of the thermocouple so formed only being able to read the heater element temperature simply doesn't apply. It is no less capable of sensing the hot end temperature than a thermocouple connected via a separate dedicated circuit to the controller (other than for some slight heat leakage along the chromel heating wire leading from the coiled chromel wire which makes up the bulk of the heating element - a factor also present to a lesser extent in the separate thermocouple case since its wires are likewise forced to pass through the heated region).
No doubt, the PID control algorithms can be fine tuned in both cases to take account of this effect, leaving little to choose between the dedicated and dual function sensing options. In the end, it all comes down to the details of the cartridge tip design and that of the controllers and the sophistication of their PID control algorithms which, going by my own experience with the KSGER version 2.10 firmware, still has some way to go in its development.
As for my idea of reducing the drive voltage by a few volts to tame the KSGER T12 controller's behaviour, I'm having second thoughts about expending any further time and energy in pursuing such a strategy. As I've already stated, I can't see much benefit in using temperatures above 350 deg C for soldering in general, particularly as I don't intend to use lead free solder in any of my projects.
Since stability and performance seem perfectly ok in the more sane soldering temperature ranges below 350 deg, there seems little need to try a reduced voltage option. In any case, on those rare occasions where I might need more heat output to solder sheet metalwork such as creating shielding boxes from tin plate or copper clad board, I'm sure the controller will respond much better in those cases of heat demand when the tip has to be kept in contact with such heavy duty thermal sinks.
Since I've never had experience with any soldering tools other than the half century old sleeve bit technological innovation made by Antex (now refined in the Hakko T12 cartridge tips) and just lately, a brief experience with the KSGER T12 soldering station, I see no point in arguing against your case for the Hakko 888 soldering station kit's superiority over that of the FX-951 and these cheap Chinese upstarts designed for use with T12 tips.
However, we do seem to be in agreement regarding these KSGER T12 soldering stations being a very cost effective upgrade from the basic soldering iron when it comes to hobbyist use (ignoring the issues of electrical safety and poor workmanship in the assembly of the handles).
Quite frankly, these cheap soldering stations based on the T12 tip technology represent a leap forward on a par with that of Antex's electrically heated sleeve bit technology over that of the old fashioned gas heated poker irons that preceded even the crappy electrically heated versions of those old fashioned poker irons (Solon Irons, anyone?).
They may lack the level of power and temperature stability of professional kit designed for production soldering such as Pace and JBC but they more than make up for this with a rich feature set more suited to the hobbyist's typical 'hit and run' pattern of use, even outdoing the Hakko FX-951 soldering station in this respect (I watched the following review video to refresh my memory of the FX-951)
It was a competently done review but I did notice a serious error in regard of the T15 (T12 tips for most of us purchasing via Ebay and Amazon) when showing the circuit of the tip connections. The two ring connections are the only connection to the heater/thermocouple circuit. The sleeve contact, which was shown as a common connection point for separate heater and thermocouple circuits, is totally isolated from the heater circuit, the function of which is solely to provide an ESD safety earth contact.
After my experience of the KSGER soldering station, what struck me most about the FX-951 setup was it's needless complexity in sensing the idle time through the use of a 'special' soldering iron holder with its gravity operated micro-switch requiring all the paraphernalia of a stereo cabled link using 3.5mm jack and sockets at each end and a feature poor user interface requiring a pathetic plastic key to minimise interference with its "key settings" (pun unavoidable) by cow-orkers in a factory floor setting.
I did notice that the startup timings from cold (and from the needlessly high 200 deg C standby temperature), though still respectfully fast, were a little bit more protracted than the KSGER unit's timings (but we all know the price of such speed - stability issues at temperatures above 350 deg C and the need to "deflower" virgin tips).
Referring back to the question posed by this thread's title, I'm very much of the opinion that the KSGER T12 soldering station is a better alternative than going for an all original Hakko setup with it's needless complication, cumbersome UI and higher price even when purchased secondhand. This cheap Chinese alternative does have the virtue of being based on the Hakko T12 (T15) cartridge tips (whether fakes, KSGER clones or original Hakko) which provides a range of 85 or more tip shapes to suit every conceivable need (perceived or otherwise).
The only major downsides that I can see with these cheap Chinese made units are the safety issues in the 24v smpsu boards used in most variants of this T12 soldering station and the quality of the workmanship in the handles, all of which are amenable to uncomplicated DIY re-working by almost every hobbyist blessed (or cursed) with even the most rudimentary of soldering skills. As such, I certainly couldn't recommend them as a first time soldering iron purchase to a novice electronics enthusiast going beyond the solderless breadboarding stage of their hobby for the very first time.
However, I would recommend these soldering stations to anyone running a class in basic practical electronics as an excellent repair or (for the kit versions) build project for such novices to take away at the completion of the course. It could either be in the form of basic coursework materials stock paid for out of tuition fees or else at the top of a "recommended buys" list of self provided materials not to be plugged into a mains socket without the supervision of the tutor to verify that it would safe to do so.
I know we should be discouraging such shoddy disregard for electrical safety (and to a less urgent extent, quality of assembly) by refusing such cheap Chinese death tr... bargains but this is the real world and there seems to be an inexhaustible supply of cash strapped hobbyists lacking the essential life skill often referred to as "Cynicism" (millennials) all too willing to part with their hard earned 'at any price', so that laudable "Refuse to buy" action just ain't gonna fly.
The way I see it, such Chinese cheapies represent a way for me to monetise (in a round about way) my electronics skills by converting such danger ridden cheap and shoddy kit into cheap kit that I could otherwise not afford to own. In times past, I've used my skills merely to convert shoddy expensive kit into expensive kit worthy of its expense.
This time round, the difference now, courtesy of China's disregard for electrical safety, is the added sense of danger you get when receiving such goods from the likes of Banggood and chums. Plugging such Chinese cheapies straight into a wall socket unchecked after unboxing them is a game I call "Chinese Roulette". It's similar to Russian Roulette only without the loud bang and smoke of a revolver and with much longer odds than the one in six chance of being the loser in the traditional game.
Whilst the extruded aluminium cased KSGER T12 soldering stations all seem to be cursed by the dangerous smpsu board issue (the tatty plastic cased units seem to use a safer design of 24v 3A smpsu board) and the build quality of the handles supplied/available for this series of soldering stations is rather questionable, I don't see why this alone should deter anyone in this group, looking to satisfy a hobbyist usage requirement, from buying one.
The faults are trivial to remedy by anyone with a modicum of DIY electronics repair skill in possession of a basic 25 watt soldering iron and a 180 watt or so rated soldering gun or equivalent (requirements which I believe should easily be met by almost everyone reading this sage advice of mine here
). Suggesting that such defects are reason alone to recommend against its purchase to the membership of the eevblog forums would, I feel, be an insult to the group's intelligence, especially so for the ever resourceful hobbyist looking for a cost effective way to upgrade and expand their existing soldering toolkit.
Mention of the "Cash strapped Hobbyist" case, it's possible to build one from the many parts available for even less money than I paid for my ready built unit. If you already have a 20 to 24 volt dc 3A rated power supply or a 4S to 6S liPo battery pack (with charger) to hand and a suitable enclosure to build it into, you can buy the 3 digit LED versions for as little as six or seven quid and save a ton of cash. I think the OLED versions were around the 18 quid mark the last time I looked
However, having checked out Banggood, Ebay and Amazon, they don't appear to be quite so prolific as they once were and looking at what
is available, I get the impression that this is older stock with all the "Good Stuff" having been hoovered up. I believe there were issues with these early versions burning out their analogue 3.3v regulator when powered from 24 volt supplies (the later models used a dc-dc converter to get round this issue) so this might not be quite as good an idea after all. In the end, it might be better simply to buy a ready built unit like this one
https://www.banggood.com/KSGER-V2_1S-T12-Digital-Temperature-Controller-Soldering-Station-Soldering-Iron-Tips-T12-K-p-1338117.html?cur_warehouse=CN for which I paid £39.05 last month.
It's currently at £38.39 right now (it had been dropped to just under 30 quid just over a week ago for maybe a whole week) so what its price might be in a few days time is anyone's guess. Internet shopping has never been easy at the best of times IME and Banggood's trampoline pricing doesn't make this task any easier. As far as I can see, that particular item still represents the best bang for the buck as well as the best combination of accessories, not just on Banggood but also everywhere else I've searched.
These
can be bought in kit form but they're often way more expensive this way. I even saw a kit of this model for 37 quid and some change that included everything
except a PSU board! Also, if you're new to buying from Banggood make sure you don't pick the dearer warehouse option by force of the habit that the CN option is always cheaper. This isn't always the case as I found out to my cost when I ordered a Mustool G600 Digital 1-600X 3.6MP 4.3inch HD LCD Display microscope for a couple of quid more than the UK warehouse option price which would have also allowed me to take earlier delivery as opposed to my still having to wait for it to arrive a week after placing the order.
If anyone is tempted to try out one these cheap soldering stations, good luck in securing one at the right price and don't forget to check out all those youtube review and teardown videos if you haven't already bingewatched them like a good boy scout (always be prepared!).
JBG