From: Justin Richards <justin.richards@gmail.com>
To: Microcontroller discussion list - Public. <piclist@mit.edu>
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Date: Sat, 12 Sep 2020 04:12:19 -0700
Subject: Re: [EE]: OpAmp Max differential input voltage GL358
Thread-Topic: [EE]: OpAmp Max differential input voltage GL358
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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=3D1290 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

On Sat, Sep 12, 2020 at 5:29 PM Clint Jay <cjaysharp@gmail.com> wrote:

> I think perhaps you've got a few misconceptions here.
>
> Thermocouple usually refers to a dissimilar metal junction sensor, applyi=
ng
> 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 wil=
l
> probably release the magic smoke.
>
> *Thermocouples are *really* useful, put enough of them in series and appl=
y
> fire, you can charge a car battery!!!
>
>
> http://www.douglas-self.com/MUSEUM/POWER/thermoelectric/thermoelectric.ht=
m#g
>
>
>
> On Sat, 12 Sep 2020 at 10:00, Justin Richards <justin.richards@gmail.com>
> wrote:
>
> > 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 ma=
y
> 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 - 8m=
V
> > (~30oC - 400oC)  .to approx 40mV - 1500mV.
> >
> > One side of the thermocouple is grounded and connected via a resistor t=
o
> > 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 durin=
g
> > heating but not sure if that is actually required and obviously
> simplifies
> > the design if not.
> >
> > Regards
> > Justin
> > --
> > http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive
> > View/change your membership options at
> > http://mailman.mit.edu/mailman/listinfo/piclist
> >
>
>
> --
> Clint. M0UAW IO83
>
> *No trees were harmed in the sending of this mail. However, a large numbe=
r
> of electrons were greatly inconvenienced.*
> --
> http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive
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