Soldering station (CA3130 and LM393)

[mike_mike]

Hi! I just found on the internet the attached schematic but I did not found the CA3130 at the electronic parts shop, so I replaced it by 1/2 of LM358. But initially I tested the schematic using a TL071 which didn't work.
Then I tested the schematic using 1/2 of LM358 instead of TL071. It seems to work, but I need your advice. Now the red led is ON until the soldering iron temperature is rising to the set temperature then the red led flashes with a period of about 1 sec. Also sometimes the the pulse is very short.

Some info about the original_schematic.jpg: The opamp from the bottom left (CA3130) is a thermocouple amplifier (I know that there is no thermocouple junction compensation), 1/2 of LM393 from the left side is a signal generator which generates rectangular pulses, and the 1/2 of LM393 from the right side adds the signal coming from the amplifier and from the signal generator.

Please have a look at the schematic (ss_lm358_v1) and tell me if I need to add or remove any other components ?
What tests do I need to do ?

[Kleinstein]

The regulator is working with PWM. So the flashing of the LED is normal and indicating the circuit is working.
The circuit needs a single supply OP. The CA3130 was not a good choice to start with as it is relatively high drift. For a soldering station it may still be good enough. The LM358 is similar in this respect, just cheaper.

The capacitors C1 and C3 are somewhat odd - I would leave them out.
I may be better to have a capacitor in parallel to R11. There is no real need to have R11 a fixed resistor should be OK.
The regulator loop gain is kind of fixed, so there is no good way to change the regulation loop gain. Adjusting R11 would effect both the set range and the loop gain and is thus not a good way.

U1B should create an approximate triangle wave.
For a test one could record the measured temperature (output of U3B) after turn on to see how fast the settling is.

[mike_mike]

I added a 1M resistor from the 12V rail to pin 5 of LM358 which is used when the thermocouple is accidentally disconnected. This makes the output of the LM393 to turn OFF the NPN transistor. Is this correct ?
Also, I added a 100nF capacitor in parallel with the 1M trimmer.
Please find attached the screenshot with the output of LM358 (pin 1).

[Benta]

There's a reason for using CA3130: it's a CMOS opamp with almost infinite input impedance (~1.5 Tohms), and it's used as a thermocouple amplifier.
Do an internet search yourself for what that means.
And no, C1 and C3 are not "odd" but placed before two comparators responsible for the triac trigger circuit. Also not unnormal.

[free_electron]

CA3130 and CA3140 are special. you can't just substitute those. They have the ability to have their common mode below the rail. Very few opamps can do that ( while keep on working correctly ) Even input rail to rail opamps can't do it.
There are many opamps that have common mode that includes ground, but very few that can go below ground.
The trick is in using P-Mos transistors as input.

TLC27M2 can do 0.3 volt below ground. The CA3130 can do 0.5 volt below ground. Maintains Teraohm input impedance.
Be careful with the part numbers on these things. The root number is TLC27. the M2 L2 M7 L7 V2 and other variant suffixes indicate the offset classification ( ranges from 10millivolt to 500microvolts. ) . You can't willy-nilly swap these things !

There are some others like OPA2237 but they have lower input impedance. (5 meg diff)

[mike_mike]

CA3130 and CA3140 are special. you can't just substitute those.

I didn't found CA3140 or CA3130 (out of stock) at the local shop/ internet shop.
I found on the internet a lot of schematics which uses the Lm358 as thermocouple amplifier, and this is why I thought that Lm358 could be used. I know (and I am aware) that the Lm358 doesn't have cold junction compensation.

LE: For example, why in this schematic, is used Lm358 as thermocouple amplifier ? https://arduinoplusplus.wordpress.com/2016/08/15/diy-soldering-station-with-hakko-fx-888-iron/
In the schematic from the link, the LM358 is doing the same job as in my schematic ? If yes, why in my schematic isn't ok while in the Arduino schematic from the link is OK ?

[free_electron]

TL071 cannot measure down to ground. common mode is half a volt off ground.
358 can almost go to ground.

None of these devices have cold junction compensation. they are just opamps. not thermocouple specific devices.

[mike_mike]

I placed the 1M resistor from the 12V rail to non inverting input of 358. The role of this resistor is to turn off the 2n2222 when the thermocouple is disconnected or is not working correct. Does it is correctly connected ?

LE: regarding the Lm358, I know that it doesn't have cold junction compensation. I used it in a few soldering stations, and it was ok for me.

[mike_mike]

Despite the fact that LM358 doesn't have cold junction compensation and doesn't have the same performance as CA3130 how can I check if the schematic which uses the LM358 is good and it is correctly working ?

[Kleinstein]

The CA3130 is a nice high speed, low bias OP, but not really the best choice for a thermocouple. For the TC the LM358 is about as good. If worried about accuracy one would need cold junction compensation and a better OP (e.g. LT1013 or a low cost AZ OP like MCP6V66).
The regular is a simple proportional control and the accuracy is limited anyway. Still good enough for a soldering station in most cases. An output to read the temperature (e.g. 10 mV/C : gain of 250 from the TC) may be a possible useful addition.

To check if the circuit is working, one could check the amplifiers output at 100 C (boiling water) when using a fixed gain.
The other point to check is the "triangle wave form" and the ouput PWM with a scope.
The simple test for the control loop is to record the temperature reading (output of LM358) after turn on and see how fast the temperature is reached and if there is significant overshoot in the temperature.

[mike_mike]

I tested using the scope. I can't do the test with the boiling water right now.
1. Triangle waveform and the PWM output of the LM358: DS0093, yellow trace is the output of the PWM generator pin 7, and blue trace is the pin 6 of LM393
2. The output of the thermocouple amplifier: DS0092

Please have a look at the screenshots and tell me what you think.

[Kleinstein]

The scope traces look good, as expected.

[mike_mike]

It should be a good thing to install the soldering station into the case, or more tests are needed ?
I tried the test with the boiling water, but even if I introduced only the tip of the soldering iron into the water then water entered into the soldering iron, and I stopped the process, to protect the soldering iron.

[Doctorandus_P]

I do not see a good reason to use an opap with an exremely high input impedance for the thermocouple.
Thermocouples generate small voltages, but their output impedance is generally quite low.
In some applications, even a stack of thermocouples (called a Thermopile) are used as an electricity source, and they can generate kilowats of power.

I do find the circuit odd.
Some things I do not understand is the generation of a separate power supply with Zener D1 and Series resistor R14.
You've also got an opamp left over.
If you buffer the zener voltage with the opamp, then you have a signal you could use as a "Zero" reference for the thermocouple. If you do that, you can use the ubiquitous TL072.

How much do you understand of how this circuit works?
Can you describe the function of each opamp?
I do like this circuit, as it's a very simple circuit and fit for beginners who are learning electonics by doing and want to build something useful.

I also find the circuit around the Triac and MOC3041 overly complicated.
Also, using 24Vac on a LED is not nice. LED's do not like reverse voltages.
That whole part can be exchanged with a decent MOSfet to switch the soldering Iron from the DC bus, but this does require that your bridge rectifier can handle the current of your soldering Iron.

Because of the simplicity and there is no cold junction compensation, your "room temperature" is your reference. Make sure to keep the cold junction away from any part that can get hot. (Even your hands).

A good calibration point is to adjust the circuit to the melting point of your solder. (Different types have different melting points, so look at their specification).

[magic]

The cold junction in many cheap irons is inside the handle. The cables going from there to the station are all copper. Yes, this means that temperature creeps up as the handle gets warm. Tough luck.

Bias current is not a major problem. Something like 100nA flowing through 100Ω is 10µV, equivalent to 0.25°C. LM358 has less bias than 100nA and those soldering iron sensors should have less resistance than 100Ω. There is zero reason to bother with CMOS or JFET in such noncritical application.

[mike_mike]

How much do you understand of how this circuit works?
Can you describe the function of each opamp?
I do like this circuit, as it's a very simple circuit and fit for beginners who are learning electonics by doing and want to build something useful.

Also, using 24Vac on a LED is not nice. LED's do not like reverse voltages.

I understood that the 1/2 of LM358 is used as thermocouple amplifier, which amplifies the very small voltage coming from the thermocouple.
The left side of LM393 is a signal generator, which generates square signal at pin 7 and a sawtooth signal at pin 6. Then the signal goes into the LM393 right side op amp which is going on and off by comparing the input voltages (pin 2 and pin 3) turning on and off the 2N2222, and the 2N2222 is turning on and off the optotriac (MOC3041) then the triac.
The 10k pot is used to set the temperature higher or lower. The 1M trimmer is used to set the maximum temperature of the iron.
I will move the red led in series with the MOC3041 emitter (led).

[Doctorandus_P]

I will move the red led in series with the MOC3041 emitter (led).

That is also a nice optimization

[mike_mike]

If I want to power on the soldering station using a 24Vdc/2.5A power supply, then do I need a special circuit for switching a mosfet ? Or I can use a simple NPN darlington transistor ?

[Kleinstein]

When using DC one can use a NPN darlington or a MOSFET. There is no really special thing needed for driving the MOSFET - the same circuit could even work with either a MOSFET if darlington with no further change.  The PWM frequency is rather low and thus not special high current drive for the gate needed. A comparator (ideally with some hysteresis) should be enough.

[Doctorandus_P]

An NPN transisor is not simpler then a MOSfet, just a bit different.

BJT's need a base current, which is amplified by some (mostly constant) factor, while the mostly used MOSfet variants are somewhat like a voltage controlled resistor.
BJT's and MOSfets are definitely not interchangeable, as Kleinsteins post. sort of implies.
In some circuits the drive circuitry may be able to drive either of them, but this is not always true. BJT's and MOSfet's are quite different from each other.

MOSfet's do have some peculiarities (just as BJT's do).
If you're interested in how electronics work then you really should learn to use both of them.

If you're just beginning with electronics and you are more comfortable with BJT's ( and Darlingtons), then I'd say, use that, and dive into the MOSfet thing for your next project when you have a working soldering iron.

[mike_mike]

A comparator (ideally with some hysteresis) should be enough.

I find interesting the part with the hysteresis, and I read about the calculations of a hysteresis resistor. Now I am wondering why there is no hysteresis in the original schematic ? It is a mistake or it is no need for a hysteresis resistor ?

[Kleinstein]

There is not absolute need for the hysteresis, as the triangle waveform gives a minimal rate of change. So the chance to stay long in the transition range is small. In addition the triac type output stage has a kind of hysteresis: once triggered it stays on till the current drops. So the triac type cirucuit does not need hysteresis. For the NPN or MOSFET version a little hysteresis can help to get faster / cleaner switching and less switching loss. One could likely still get away without hysteresis.

[mike_mike]

I found a schematic for mosfet driving, which I also used at a Arduino soldering station.
Can this schematic be used for this soldering station ?

[Doctorandus_P]

Instead of just asking, put some more effort into analyzing and understanding how and why the circuit works.
Once you've learned how these discrete parts work, it's easy enough to figure out the workings of such simple circuits.

[mike_mike]

I added a 1M resistor between pins 1 and 3 of LM393.
How can I see the hysteresis ?
I tried by reading in the manual of the oscilloscope how to use the XY mode, and I got the attached screenshot.

[mike_mike]

I would like to know how can I implement hysteresis in this circuit (LM393) ?
I read on the internet a few pdfs and I found that there are a few methods for calculating the hysterezis, but I don't know which one to apply and how, because I have the signal from the sawtooth generator and the signal from the potentiometer and thermocouple amplifier.

[Kleinstein]

The resistor from pin 1 to 3 is already a frist step towards hysteresis. It would need an additional sereies resistor for the signal going to pin 3.

In the original plan the "triangle" signal is rather high impedance and thus not that well suited. One could use the spare half of the LM358 as a buffer or build directly a 2 OP version the triangle generator.

[mike_mike]

Instead of just asking, put some more effort into analyzing and understanding how and why the circuit works.
Once you've learned how these discrete parts work, it's easy enough to figure out the workings of such simple circuits.

@Doctorandus_P I tested today the attached schematic, using a 24Vdc/2.5A power supply and it seems to work. I would like to know if further modifications are needed ?

[mike_mike]

Please find attached a few screenshots with the pin 1 of LM393 (yellow trace) and pin 3 of LM393 (blue trace).
1. DS0102 = pin 1 (yellow) and pin 3 (blue)
2. DS0103 and DS0104 = Zoom in to DS0102
3. DS0106 and DS0107 = With 1M resistor between pin 1 and pin 3 of LM393

I find some oscillation between on and off at the latest 4 screenshots. Are they normally or not ?
Please have a look at the attached screenshots and tell me what you think ?

[mike_mike]

I was told to use 2 resistors as shown in the attached schematic.
Now, the screenshots from the oscilloscope look like: DS0113, DS0114 (yellow trace on pin 1 of LM393).

Please have a look at the schematic and at the screenshots and tell me what you think ?

[mike_mike]

I am using this soldering station for about 1 year. Yesterday I installed the pcb into a new case, and I saw on the oscilloscope screen the attached wave forms: yellow is pin 1 of LM393 and blue is pin 3 of LM393.
What are the spikes that are present on the blue trace ?

LE: The spikes happen even if I change the soldering iron with a new one. Also, I tested with the DC version of the soldering station and the spikes also appeared. Probably the spikes are because of the soldering iron heater which goes on-off

[mariush]

Mike, as an alternative,  you can get the Hakko 936 schematic online : https://www.histo.cat/files/files/200915145115281.pdf

You can replace the uPC1701c (zero crossing detection switch) / triac driver chip with UAA2016 which is still made, or you can get the uPC1701c from eBay.

You can also get ready made hand pieces from ebay and you can get ready made pcbs from ebay
For example : https://www.ebay.com/itm/295538863410  -3$ for board and components all but the DIN socket for the hand piece.
You can get hand pieces for 3-4$ example :  https://www.ebay.com/itm/153679091608

and 24v transformers you can find easily.

edit: sorry, didn't realize you got it working, only read a few of the first posts in the thread.

[mike_mike]

edit: sorry, didn't realize you got it working, only read a few of the first posts in the thread.

Yes, I made it working, and I used it for about 1 year.

My concerns were about the schematic because I didn't know if the schematic is good enough for a simple soldering iron (50W/24V/K- type thermocouple sensor) or if the schematic has wrong components ?
I added the hysterezis to U1A by adding the R24 (1M), R25 (10k), and C33 (100nF) to the circuit. The result was that the oscillation on the pin 1 of LM393 disappeared when I verified with the oscilloscope.
Also by changing the optotriac and triac with a Mos-fet and a mos-fet driver circuit I made the circuit working with 24V/3A SMPS.
I made the above modifications from the internet, they are not original.