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'[EE]: OpAmp Max differential input voltage GL358'
2020\09\12@045735 by Justin Richards

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Experimenting with using a ESP8266 as a soldering iron PID controller
driving the ADC with the output of an opamp.

The GL358 opamp datasheet specifies max differential input voltage as 32V.

I would like to know if that is as it sounds and I can force the inputs to
have a max of 32v across them with a single supply to the Opamp of +5V.  I
am still learning about opamps and thought the inputs were to be considered
as a virtual ground and they will effectively be at the same potential but
here i would be brute forcing a voltage across them and concerned it may be
damaging and bad practice.

The GL358 (was in the junk box) is configured as a dual amp to amplify the
thermocouple (integral part of soldering tip) output of approx 2mV - 8mV
(~30oC - 400oC)  .to approx 40mV - 1500mV.

One side of the thermocouple is grounded and connected via a resistor to
the inverting input while the other side is connected via another
resistor to the non-inverting input.

I plan to measure temperature then apply 24V across the thermocouple. which
will drive 24V across the two 1k ohm opamp input resistors.

As a caution I plan to have back to back diodes at the inputs to limit the
voltage across them to approx 0.7v.

But back to the original question, are the protection diodes needed given a
maximum differential voltage of 32 volts or is that only if the supply is
also at 32V.

I found a design that disconnects the thermocouple from the opamp during
heating but not sure if that is actually required and obviously simplifies
the design if not.

Regards
Justin
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2020\09\12@052645 by Clint Jay

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I think perhaps you've got a few misconceptions here.

Thermocouple usually refers to a dissimilar metal junction sensor, applying
heat to it creates a voltage across the two wire ends*, you don't feed
power to a thermocouple.

Most chip datasheets specify the maximum input voltage in relation to VCC,
your GL358 (if I'm reading the datasheet right) will be able to cope with
VCC +/-1.5V, so for 5V VCC you could safely input +6.5V or -1.5V, 32V will
probably release the magic smoke.

*Thermocouples are *really* useful, put enough of them in series and apply
fire, you can charge a car battery!!!

http://www.douglas-self.com/MUSEUM/POWER/thermoelectric/thermoelectric.htm#g



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2020\09\12@071238 by Justin Richards

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Hi Clint,

yes, thermocouples are as you say but with the T12 style soldering tips the
thermocouple is also used as the heating element as strange as that
sounds.  There are only 2 connections to the tip and it provides both
heating and temperature measurement.

I am testing on my bench and applying 12v getting it hot then connecting to
the op-amp and reading the output and has survived many cycles.  And
convinced its a thermocouple as reversing the connections to the op amp has
the expected outcome.  I am so tempted to connect the 12v while connected
to the opamp  ( I found a spare) as it is protected by the 2 x 1k ohm input
resistors but I will heed your advice.

This chap https://youtu.be/vudfIq6PQMw?t=1290 has reversed engineered the
same station I have.  On closer inspection I don't see the disconnect when
heating.  I was sure that schematic had a disconnect transistor to protect
the opamp, I will have to watch again.

That station is ok, except I want a web page to tweak all the PID constants
and a rotary encoder with OLED display for fast temperature adjustment.  It
currently has a weird insert card and clunky non-intuitive buttons for temp
adjust.

All is tested ok, my code supports MAX6675, DHT, thermistors and 4diodes in
series and is used at home to make black garlic where the humidity is
maintained with a peristaltic pump etc among other things

A very early version can be seen here
https://hackaday.io/project/167840-slowcooker-esp8266-based-pid-controller

I now want to leverage of all that development and control a soldering iron
the way I want it.  Fast heating, setable standby temp, easy adjust, just
need to do the last bit.

Cheers Justin

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2020\09\12@075358 by Justin Richards

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>
>
>
> http://www.douglas-self.com/MUSEUM/POWER/thermoelectric/thermoelectric.htm#g


That made for an interesting read, some clever designs. I did not know
RTG's used heat this way.
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2020\09\12@080916 by madscientistatlarge n/a

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A thermistor is not a thermocouple.  You could never put a thermocouple across the heating element, thermocouples have very low resistance, it would pop like a fuse if you put voltage on it, they are normally connected to a very high resistance.  Thermocouples produce mV of output, the op amp you want to use has a high input offset and high input offset drift, it would completely obscure the the actual input voltage.

I think that station monitors the resistance of the element, after briefly disconnecting it from the power supply. Something like this <https://circuitcellar.com/resources/ee-tips/how-to-measure-temperature-with-a-soldering-iron/>


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2020\09\12@081611 by madscientistatlarge n/a

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Is that link broken?  I get a 404 error.

On power from thermocouple stacks, I lived in a house for awhile with a furnace that used a thrermopile (lots of thermocouples in series) to power/drive the gas valve through the thermostat.


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2020\09\12@084300 by madscientistatlarge n/a

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Couldn't find the schematic, but the manual for that iron does have the iron pinout.  Normally it has a built in thermistor for temperature sensing (the 5 pin connector has 2 pins for the thermistor, 2 for heater, and one for ground  see this <https://runawaybrainz.blogspot.com/2014/08/hakko-fx888d-din-connector-pinouts.html> or the manual.  It's possible to only use 2 wires and use the heating element itself as a thermistor, as in the article for Steve Circa I mentioned earlier.  I can't find a readable schematic for that station.


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2020\09\12@085150 by Justin Richards

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I am reasonably certain these are thermocouples.  If I heat it up and
connect it to my multimeter it reads about 8mV and drifts down to about
2mV.  I dont think a thermistor would do that.  It reads about 8 ohms when
measuring resistance.

I suspect built into the tip is heating elements that are in series
with the thermocouple and make up each arm of the thermocouple.

There was a x-ray of a soldering iron tip but I am having trouble finding
it.



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2020\09\12@093850 by madscientistatlarge n/a

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Please see the manual <https://doc.hakko.com/download.php?_gs=on&l=en&kp=fx888d&d=5680>  It shows the pinout of the iron, and very, very clearly shows the temperature sensor does not share any pins with the heater.  It also shows the resistance of the element and the sensor.  It is a thermistor.  If it were a thermocouple the voltage wouldn't drop when the iron heated.  I suspect you measured poorly/got interferance picked up.  mv signals in that range, when anywhere near power line connections/transformers can be from capacitive coupling.  Also what meter did you use?  I believe you are on a snipe hunt, having been misled.


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2020\09\12@094331 by Justin Richards

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The station you link to uses a different style of tip T18 series where the
element is not part of the tip but rather the heating element is the
traditional setup with what I suspect is a separate element and separate
thermistor which are slow to respond and suffer more from over shoot.

The FX-951 uses the T12 tips that incorp the thermocouple and element into
the tip.  They cant be separated, directly connected and integral to the
tip and therefore can be more tightly controlled. There are only 2 possible
electrical connections.  The outer shield is not in any way connected to
the other two connections.  It reads open circuit.

There are many projects that treat the T12 tips as thermocouples.  There is
some debate regarding the type.  Most have assumed type K but some believe
these tips better fit a type C profile.  If I plug my tip into the Fluke 87
in temperature mode, it seems to read a bit low.  I initially dismissed
this as bogus connections but it seems to fit the type C thermocouple
tables.  eg at 1000oC K is about 42mV wheres type C is about 18mV,  Type C
also fits with my approx value of 8mV for 400oC

Back to the original question, i found some datasheets that state a max
input current of 50mA, so if i size the input resistors correctly and use
diodes to clamp the input, I think it should be possible to minimise the
input current and protect the opamp. 6k ohms on each input should limit the
current to around 5mA assuming a max driving voltage of 30v.  I intend to
use 24V, I see someone has direct connected to a solar panel. that would
nominally have Voc of 21V.


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2020\09\12@094600 by madscientistatlarge n/a

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That resistance is wrong according to the manual which says  ohms for the sensor and 2 ohms or less for the heater.  Have you connected any of the wires together?  If you directly put a power supply across the sensor it was likely damaged.  It has a 6 pin plug on it originally, Yes?  Thermocouples are very unlikely to share any wire/pin with the heater, it would make the measurement very noisy etc.  You could email Hakko and ask them what kind of sensor it is.  They apparently have removed the schematics from other websites but are likely willing to tell you what kind of sensor it is and what pins it's normally connected to.  The sensor should be between the two blue wires and it likely is a thermocouple, but somethings gone wrong.  Again, it's all in the instruction manual.


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2020\09\12@095016 by Justin Richards

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> were a thermocouple the voltage wouldn't drop when the iron heated.  I
> suspect you measured poorly/got interferance picked up.  mv signals in that
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It was my intention to state that when heated and measured it starts out
reading 8mV then drops to around 2mV as it cools.
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2020\09\12@095433 by madscientistatlarge n/a

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Do the tips look like this <https://www.amazon.com/Soldering-Station-Solder-Replacement-FX-951/dp/B07FYV7ZV8>?  In which case there are 3 connections, 2 would be for the heater and one would be for a thermisor with one wire shared.  This would have to be a thermistor, a thermocouple would work very, very poorly in this case.


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2020\09\12@100011 by Justin Richards

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We are talking about completely different irons and stations.  If you
google images of the T12 tip you will see that there just isnt the number
of connections you are suggesting,

In my hakko there is only 2 wires going from the circuit board to the
plug.  If you had a close look of the FX-951 tear down and reverse
engineering that I originally linked to you would see this.  There is no
measurable connection between the outer cas and the two ring-contacts.

This describes the T12 and indicates the sensor is infact a thermocouple
http://redlightgreen.org/2019/09/obzor-nabora-vysokokachestvennyh-pajalnyh-zhal-t12/


I had to google snip hunt.

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2020\09\12@100202 by madscientistatlarge n/a

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here's a link for a home made controller for the t12.  <https://www.hackster.io/sfrwmaker/soldering-iron-controller-for-hakko-t12-tips-on-arduino-f7a888>  Apparently it is a thermocouple and shares the ground line, this is a very bad design.  Hacko discontinued it a long time ago though I know clones are still being made.


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2020\09\12@100555 by madscientistatlarge n/a

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Problem with that measurement is that you have unwanted thermocouples every where wires with different metal connect.  In any case, 32V will absolutely let the smoke out if you are supply the op amp with <30V.  Please send a schematic of your' test setup, or call hako and ask them.  It looks like some use a temperature sensor and read switch in the hande.  A photo of the disassembled handle would really help.


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2020\09\12@103135 by Justin Richards

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In my opinion it is a superior design as it can be tightly controlled and
recover quicker when soldering. The FX-951 that connects to a FM-2808
handle that supports the T12 tips does not look discontinued, at least
there appears to be no hint on the Hakko site that I could find.

Yes, there are unwanted thermocouples everywhere and I spent some time
eliminating these in a previous design until it became apparent that if
there are an equal number of the same type in each leg then they can
effectively cancel out.

My tests with a CRO seem to indicate that it is relatively noise free,
repeatable and I am using leads that would definitely have many dissimilar
metals making thermocouples and it was this I assumed was the cause of
lower readings relative to a K type until I discovered it is most likely a
C type.

I believe the TS100 and TS80 use a similar design which work very well
according to some reviews.

It may have been a TS100 tip that was x-ray'ed.  Still looking...

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2020\09\12@103446 by madscientistatlarge n/a

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Ok, here's the manual for the 951 <https://www.manualslib.com/manual/1388136/Hakko-Electronics-951.html?page=3#manual>  from the specified resistance you no it's some type of RTD, thermocouples usally measure as a low resistance.  The manual says 43-58 ohms for the sensor, which means it's some type of RTD/thermistor.  Also note the sensor has 2 wires that aren't shared.  See the pinout on page 2.  It uses a 5 pin connector, 2 for power, two for sensor, and one for tip ground.  Also the t12 and t15 are the same with the difference being what country they are sold to, it's more marketing scam.


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2020\09\12@110012 by Justin Richards

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Thanks for the link but that is the 951 952 soldering gun.  A completely
different beast from the FX-951 soldering station that connects to an
FM2025 handle.that uses T12 tips.

Mine is more like
https://doc.hakko.com/download.php?_gs=on&l=en&kp=fx+951&d=401 and
indicates the heater and sensor resistance should be 8 ohms which is what I
get.  It is only taken from two point as shown in the diagram.

Thanks for all your input




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2020\09\12@115125 by madscientistatlarge n/a

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Here's the service manual, still no schematic which is just silly.  These are expensive and not hard to reverse engineer.  I've found most things aren't that hard to reverse engineer, especially something so simple.  The software may be protected, but that's pretty easy to emulate as well.


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2020\09\12@122335 by madscientistatlarge n/a

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After more looking around I do think the sensor is between the + side of heater and a sense wire.  This makes it a bit trickier to read but certainly doable.  If it's rtd I'd drive the sensor/heater line with low current AC, and connect a resistor from the sense wire to ground through a resistor.  By using AC you could capacitively couple this to block the DC on the heater and just measure the voltage on the resistor connected to ground.  IF it's a thermocouple you'd need an op-amp with excellent common mode rejection.  I do like the specs of these irons, but the integrated heater/tip does make them expensive to maintain or have a wide selection of tips for.


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2020\09\12@123130 by madscientistatlarge n/a

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A lot of interesting info here <http://dangerousprototypes.com/forum/index.php?topic=7218.0>  I'll be reading it all later.  Apparently it is a Thermocouple (your voltages do make sense for that), the trick is to peiodicaly cut the heater power and wait for the TC voltage to settle.  Makes the code a bit harder but would work nicely other than not having a 100% possible heat output.  I was planning to use this style iron, guess I'll have to get one and measure it out.  Probably best to measure at the rings on the tip to determine what kind of thermocouple etc.  I've also found that some irons that are "24V" need 12-15 V for their rated power, meaning at 24V you'd want to limit the duty cycle (or use a lower voltage).  These would be the cheap replacement heads at electronics gold mine.


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2020\09\12@123249 by madscientistatlarge n/a

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This page also has some other good links I'll have to go through <https://hackaday.io/project/162676-dual-t-12-station>


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2020\09\12@125252 by Bob Blick

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As long as you have a resistor in series along with diode/zener to clamp the opamp input, you should be ok.

If you use the 358, you'll need the power switching element on the positive side of the heater, because the 358 only has common mode up to Vcc minus about 1.2 volts. Interesting challenges await! Thermocouples are always "fun" to deal with.

Best regards, Bob

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From: @spam@piclist-bouncesspamspamKILLspammit.edu <spamBeGonepiclist-bouncesRemoveMEspamEraseMEmit.edu> on behalf of Justin Richards Sent: Saturday, September 12, 2020 1:57 AM
To: Microcontroller discussion list - Public.
Subject: [EE]: OpAmp Max differential input voltage GL358

Experimenting with using a ESP8266 as a soldering iron PID controller
driving the ADC with the output of an opamp.

The GL358 opamp datasheet specifies max differential input voltage as 32V.

I would like to know if that is as it sounds and I can force the inputs to
have a max of 32v across them with a single supply to the Opamp of +5V.  I
am still learning about opamps and thought the inputs were to be considered
as a virtual ground and they will effectively be at the same potential but
here i would be brute forcing a voltage across them and concerned it may be
damaging and bad practice.

The GL358 (was in the junk box) is configured as a dual amp to amplify the
thermocouple (integral part of soldering tip) output of approx 2mV - 8mV
(~30oC - 400oC)  .to approx 40mV - 1500mV.

One side of the thermocouple is grounded and connected via a resistor to
the inverting input while the other side is connected via another
resistor to the non-inverting input.

I plan to measure temperature then apply 24V across the thermocouple. which
will drive 24V across the two 1k ohm opamp input resistors.

As a caution I plan to have back to back diodes at the inputs to limit the
voltage across them to approx 0.7v.

But back to the original question, are the protection diodes needed given a
maximum differential voltage of 32 volts or is that only if the supply is
also at 32V.

I found a design that disconnects the thermocouple from the opamp during
heating but not sure if that is actually required and obviously simplifies
the design if not.

Regards
Justin
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2020\09\12@133352 by madscientistatlarge n/a

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Hakko does have an array of similar solder stations.  It's a lot like all injet cartridges being incompatible with other machines, i.e. a money grab.


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2020\09\12@140947 by madscientistatlarge n/a

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Thank you for posting, It's led me to search for other designs and specs!  I'll be spending a lot of time searching for homemade soldering stations.  Given the resistances in the Hakko manuals I believe it does share A pin between heater and sensor.  Not so bad if you know, clearly an attempt at obfuscaton!  There are a lot of articles about  homebrew stations with the t13/t15.  The difference between those to model series is either nothing, or possibly different materials in construction do to various ROHS statutes which are a lot tougher in Europe than the U.S.  Aliaexpress also has reasonable prices for smoke suckers and filters, the brand name ones are ridiculously expensive both for the unit and filters, and they aren't mill spec or nasa spec which would be the only legit nongread based reason for the prices.  I don't mind companies making a reasonable profit but I do object to being out right ripped off!  Seriously, a muffin fan and some carbon filter material are dirt cheap if you buy them for anything else, like furnace filters for instance.  Grainger has pretty low pricing on the carbon filter material if one is willing to cut it to size.


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On Saturday, September 12, 2020 10:52 AM, Bob Blick <EraseMEbobblick.....spamKILLspamoutlook.com> wrote:

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2020\09\12@143109 by madscientistatlarge n/a

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Does any one know of any temp controlled irons between 100-250W?  I solder heavy stuff sometimes and my ancient weller gun is heavy and the transformer soon gets unpleasantly hot.  Not to mention you have to keep waiting for it to heat up if you put it down, and it gets way too hot if you do a bunch of connections in one go.


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On Saturday, September 12, 2020 8:59 AM, Justin Richards <justin.richardsSTOPspamspamKILLspamgmail.com> wrote:

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2020\09\12@145153 by Manu Abraham

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On Sat, Sep 12, 2020 at 4:45 PM Justin Richards
<EraseMEjustin.richardsspamBeGonespamKILLspamgmail.com> wrote:
>
> Hi Clint,
>
> yes, thermocouples are as you say but with the T12 style soldering tips the
> thermocouple is also used as the heating element as strange as that
> sounds.  There are only 2 connections to the tip and it provides both
> heating and temperature measurement.




Is it *really*, the thermocouple is used as the heater ?

I've heard a few people state that those tips have the thermocouple in series
with the heater; A pain to do actual measurements when the heater is used.
ie, the heater needs to be actually switched off to do a measurement.


Cheers,

Manu
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2020\09\12@160845 by Justin Richards

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Hi Manu,

I think it makes more sense to assume that the thermocouple is in series
with the heater.

Cheers Justin

On Sun, Sep 13, 2020 at 2:54 AM Manu Abraham <RemoveMEabraham.manuspamBeGonespamspamgmail.com> wrote:

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2020\09\12@164938 by madscientistatlarge n/a

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Agreed, I've just looked at a lot of the manuals and that seems to be the case as there's only one resistance to measure between connector pins.  The TC might or might not be part of the heater, but functionally it's in series with the heater, they just have to use a heavier TC to tolerate the current.  It is sneeky but not terribly effective obfuscation.  There's no functional advantage to doing it that way other than maybe, maybe slightly tighter thermal coupling.  Then again, you really want to know the tip temperature, not the heaters temp.  Ideally there'd be an offset programed for each tip so it reads more true to what you care about, though in that case you really need to check/adjust the temperature setting while monitoring with an external TC.  It would be worth checking if any of the bands on the heater connect to the junction between TC and heater, you'd get better reading.  There seems to be a number of ways different stations are wired to the handle.  Some have a reed switch so you can tell when it's in the stand (which must have a magnet), apparently some of the handles include a tilt sensor of one kind or another to do the same thing.  Truly endless silly variation, many people on this list could easily reverse engineer the whole thing, it's silly "SPY VS. SPY" stuff if your' familiar with mad magazine.  Obviously it hasn't prevented people cloning the whole station.  I've even seen them on alibaba marked as "Hakko", but the description said it wasn't (there are dumb sellers everywhere, admitting it is fake in the listing is not smart).

If I were doing a lot of lead free work I'd probably go for the nitrogen blanket variation, though a bit more flux to be cleaned later does about the same thing (though many of the led free fluxes will cause corrosion if not rinsed, I've seen nickel plating come off a board overnight!).


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2020\09\12@212408 by Spehro

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LM358 is okay with either input at up to 32VDC or as low as -0.3V (Abs max, so it’s best to stay away from the limits a bit) regardless of the single supply voltage .

It won’t fully function, of course, unless both inputs are within the common mode range, which extends to about Vcc-2V.


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2020\09\12@220859 by madscientistatlarge n/a

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Here's a working design, it may have been the one you mentioned, in any case some good information and a working interface for t12 heaters/tips <https://www.hackster.ioh/sfrwmaker/soldering-iron-controller-for-hakko-t12-tips-on-arduino-f7a888>  IIRC this one has a better sclematic, IIRC the one you linked to was hard to see.


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2020\09\13@081131 by Justin Richards

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Thanks Spehro,

I am guessing that GL358 and and LM358 are one and the same.

Justin

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2020\09\13@084252 by Justin Richards

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Update:
The system as proposed appears to be functioning as expected.

The ADC readings are plagued with low level noise but this is generated
from the ESP8266.  I think this is while it is WIFI'ing, but it is so
minimal I don't feel the need to chase it down.

There was a minor gotcha.  The OFF time before sampling needed to be
increased to 1/50 * 1/2 +abit or slightly over 10mS worst case.  This is
because the TRIAC latches until the next zero crossing.  At 50Hz this could
remain latched for up to 10mS from when the ESP8266 output pin is
tri-stated.  Obviously a ADC sample while heating yields bogus readings so
needed to be avoided.

In reality, the system regulated ok without any concern with 'OFF time
before sample' as the two events (i.e sampling and heating) rarely and
randomly coincide and therefore the undesirable effect of sampling while
heating averaged out.

There are other approaches that could be taken to ensure that the sampling
never coincides with heating but it does not seem to warrant the effort.

Surprisingly, the random selection of feedback resistors, resistor dividers
and the circuit in general produced ADC readings where  ADC/2 was fairly
(scarily) close to the actual temperature in degrees C at or around the
normal solder melting temperature of 183degC.  Was not expecting that.

Thanks for all the input.

Regards justin
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