MAX6675 vs op-amp

[shobo]

Hi all,

as one of my first projects i want to build myself a soldering station.however i haven't decided to use a MAX6675 module/ic to sense the temperature or use an op-amp.

The iron i have is an Atten clone on a hakko.

what would be the cons and pros of each method?

[David Hess]

If you use an operational amplifier, then you will have to add external cold junction compensation in some form or another to use the thermocouple but this is easy enough.  The MAX6675 has built in cold junction compensation.

If your control loop is going to be analog, then the digital output from the MAX6675 will be inconvenient and something like an LT1025 would be easier to use.

[shobo]

If you use an operational amplifier, then you will have to add external cold junction compensation in some form or another to use the thermocouple but this is easy enough.  The MAX6675 has built in cold junction compensation.

If your control loop is going to be analog, then the digital output from the MAX6675 will be inconvenient and something like an LT1025 would be easier to use.

The control of the will be digital,by using an arduino (arduino mini on perfboard) or ar atmega with the arduino bootloader.

The modules are cheap enough to either stuff one in or salvage the IC if i get to etch my own board.

[David Hess]

The control of the will be digital,by using an arduino (arduino mini on perfboard) or ar atmega with the arduino bootloader.

The modules are cheap enough to either stuff one in or salvage the IC if i get to etch my own board.

I do not disagree; I was just pointing out that the MAX6675 is only going to be useful with a digital control loop while an analog implementation can work with either.

If you are using a microcontroller for the control loop, then another alternative is to implement the cold junction compensation digitally using any temperature sensor.  Then you can use any precision operational amplifier.  That is what the MAX6675 handles internally for you.

So for instance the thermocouple output is amplified and digitized, and an inexpensive silicon temperature sensor like an LM86 located near the thermocouple interface is also digitized to provide the cold junction temperature.  Using a thermocouple requires *two* temperature measurements.

[kazan417]

Actually lm86 are not so cheap, you shoud find cheaper with at least 1 degree precition, to outperform price of chinese produced max6675. Morover it is better to use some cortex microcontrolleer with integrated 12 bit adc instead of 10 bit interated into avr.
This way with good precition opamp for about 1 dollar you save money and not loose precition.

[Kleinstein]

The 12 Bit ADCs inside the ARM based µC are often not much more accurate than the 10 Bit ADC inside the AVRs. For a soldering station 10 bit resolution should be good enough - as there will be some added resolution from oversampling.

The probably cheapest sensor for cold junction compensation would be a diode, read out from another ADC channel.

[JS]

The error sources in tip temperature are usually more because of thermal model error than temp sensor, you need some cold junction compensation but then the errors due to the temperature sensor or amplifier can be corrected in the firmware, to later apply them to a good model to get a nice controlled temperature. Depending on the used tips the model might change a little but there's the key in accurate tip temps.

JS

[kazan417]

JS absolytely right. So for this project you shoud digging into PID regulator theory and it took some time.
So better to save time by using good solution for temp sensing. Moreover you can not beat by the price point new max31855 chip.
It is cheaper to buy max31855 instead of precition opamp + temp sensor + precition reference voltage. But this all for thermocouple.

Soldering tip have integrated RTD not thermocouple! So here we can use different solution, maybe general purpose opamp and it will be cheaper.
We dont need cold junction comprnsation for RTD. max 6675 and 31855 in not for this project at all. Here better to use general purpose opamp.

[JS]

JS absolytely right. So for this project you shoud digging into PID regulator theory and it took some time.
So better to save time by using good solution for temp sensing. Moreover you can not beat by the price point new max31855 chip.
It is cheaper to buy max31855 instead of precition opamp + temp sensor + precition reference voltage. But this all for thermocouple.

Soldering tip have integrated RTD not thermocouple! So here we can use different solution, maybe general purpose opamp and it will be cheaper.
We dont need cold junction comprnsation for RTD. max 6675 and 31855 in not for this project at all. Here better to use general purpose opamp.

  I think a PID isn't enough by itself, I think you need to do some math with the temp before getting into the loop, to estimate the tip temperature using the thermocouple value, applied power and what you know from the tip. The tip thermal model should remain pretty stable but some parameters will change, mainly tip thermal capacity and thermal resistance between thermocouple and tip. Then you select a tip and you have different models for each of them, not just changing an offset or gain for each but this parameters too, so your model is always agrees with the nature of the tip. I guess just tweaking 3 or 4 parameters for each tip should be enough, heaters and thermocouples should be pretty much the same between tips, but from the thermocouple to the tip things might change quite a bit. There are some videos talking about this, I could look for some if you are interested going into that rabbit hole for getting a real control on tip temp. The difference is seen in Dave's videos testing different stations, why some apply enough power with big thermal capacity pads and some doesn't even if they have more power available.

  Is that or you change the temp all the time depending on what pad you are soldering, I got a cheap soldering station with T12 type tips and it does a decent job but I find my self changing the temp for big pads. Even more when reworking when you want the solder to remain liquid for a bit so I can suck it out.

JS

[kazan417]

OOO yes, i saw this Dave's videos comparing different soldering station and their thermal capacity.
But i thought and i sure that PID is model type exectly for those types of situations.
Of cource you need some math. But i think by knowing  heating curve of soldering tip with no load we can calculate
proportional, integral and differential parts of heating element and it will be all we need - 3 coefficient for the model.
After fuguring out this 3 coeffitient pid algorithm take care about applying more power if we trying to solder big polygon. PID was developed exactly for that.
Of course if soldering tip have no enough thermal capacity it can not reach defined temperature, but PID will be try to do that anyway.
Am i wrong? Then of cource i would like to see those videos you talking about.

[JS]

  If you have a low thermal mass tip and, most important, with high thermal resistance from the thermocouple to the tip (like a J type), on a big pad you will have much lower temperature at the tip than at the thermocouple, a 4mm D or C tips in a small pad will have much lower errors from this, as the thermal resistance from thermocouple to tip is much lower, think it as a resistive divider from ambient (the pad) to the tip and from the tip to the thermocouple, if you want 320ºC at the tip and you have a low thermal resistance from tip to ambient (big pad) and a big thermal resistance from tip to thermocouple (J type tip) you have much lower temp than expected at the tip. I'm not saying you are going to solder a huge ground plane with a J type, big and small are relative to the tip size, but sometimes you can't get in there with a 3mm tip, you need a 0.8mm tip and you are screwed.

  A mathematical model using the thermocouple reading and the applied power will allow you to correct for that error, allowing the thermocouple to read 350ºC when you have 320º at the tip and as the set temp. This could lead to overshoot once you remove the tip from the pad, but to be much more responsive and precise keeping the temperature while at the pad.

JS

PS, here is a VID explaining this with images and measurements... I found it some time ago, I expected to see exactly what he talks about, it's not rocket science but actually doing it right might have some traps. I would expend quite some time in the code if I were to do something like this, if you use the model and you just trust that you could end up pumping way too much power, so I would trust in the model up to a limit and then let the tip go colder than it should when it just can't keep it, so let's say on my example, with a 320º set temp, I would go to 350º at the thermocouple if needed but wouldn't let it go to 400ºC if it reads it needs to do so. It might also be a way to smartly detect a few pre-set different tips, like measuring the initial heating rate, so the soldering iron recognizes what tip you are using. If you use two very similar ones it might get confused but it might not care if the models are close enough. Also, the dispersion between the resistance of different heater element might help to pick the right one too.
https://www.youtube.com/watch?v=EP_Ff2YvkSA&t=0s&list=WL&index=55&frags=pl%2Cwn