Soldering station - DIY - Page 1
Forum section: electronics / beginners

author: mike_mike
author_rank: Super Contributor
timestamp: August 29, 2018, 05:08:00 am
content: Dear Forum,I made the soldering station from the attached schematic. I saw that the triac was always ON. I swapped the triac main terminals (A1 and A2) and now it seems to work normally. Why it does not worked until I swapped the triac main terminals ?On the schematic, LSP3, LSP7 and LSP4 are connectors for a potentiometer (10K), LSP5 and LSP6 are the connectors for K type thermocouple and LSP8 and LSP9 are the connectors for the soldering iron heater.UPDATE !New schematic attached (pdf).Gerber files:https://drive.google.com/open?id=1p1ssqKECsRekZN7TRc8OJoxYtkqbUKmkThis soldering station is using Pensol Iron-N as soldering iron. You can also buy tips for this soldering iron.You need a transformer rated at 24V secondary and minimum current about 3A.How to calibrate: connect the soldering iron into the 5 pin DIN connector. Rotate R12 to half of its course. Also, rotate the 10K pot to half of its course. Power on the soldering station. Rotate R12 until the tip of the soldering iron starts to melt solder. When the tip starts to melt solder, stop rotating R12. That's all for calibration.DIN5 connector pinout: 1- Thermocouple +, 4 - Thermocouple -, 3 and 5 heater. Note that on the market exists soldering irons with different pinouts.If you need help in building this project, please contact me !

author: JS
author_rank: Frequent Contributor
timestamp: August 29, 2018, 05:21:48 am
content: Wouldn't that be cause it was wired backwards?I'd put the LED and the optocoupler in series...JS

author: mike_mike
author_rank: Super Contributor
timestamp: August 29, 2018, 05:54:42 am
content: So a triac can be wired backwards ?Can you give me a example with correct and backwards connection of a triac ?

author: Signal32
author_rank: Frequent Contributor
timestamp: August 29, 2018, 05:57:04 am
content: Read dis:https://electronics.stackexchange.com/questions/11468/are-triacs-symmetrical

author: 6PTsocket
author_rank: Regular Contributor
timestamp: August 29, 2018, 05:44:55 pm
content: Quote from: mike_mike on August 29, 2018, 05:54:42 amRun this simple test from "how to test a triac" connect triac MT1, MT2 to diode test on a dmm. Touch gate to MT1; nothing happens. Touch gate to MT2. Triac turns on and you are reading a good diode in forward mode. Now reverse the meter leads and repeat test. Only gate touched to MT2 again turns on the triac. You are seeing the forward conduction of the other diode. Clearly, the triac is not symetrical.Sent from my SM-G900V using Tapatalk

author: JS
author_rank: Frequent Contributor
timestamp: August 30, 2018, 02:51:04 pm
content: Quote from: mike_mike on August 29, 2018, 05:54:42 amThe gate isn't symmetrical for both power pins, so the control circuit is what get's backwards. Once the gate control is correctly applied it is symmetrical for the power voltage, as you can apply voltage in either direction and current will flow once the gate is properly triggered.JS

author: mike_mike
author_rank: Super Contributor
timestamp: August 31, 2018, 10:04:04 am
content: Thanks for the reply.Regarding the schematic from the first message, can BT136 be replaced by BT138 or by BTA12 ?

author: JS
author_rank: Frequent Contributor
timestamp: September 01, 2018, 03:46:42 am
content: They have enough current so I'd guess it should work, checking gate characteristics too is not a bad idea thou.JS

author: mike_mike
author_rank: Super Contributor
timestamp: September 01, 2018, 07:04:13 am
content: The triac part numbers are BT136 600 and BT138 600.BT136 600:http://www.farnell.com/datasheets/2623889.pdf?_ga=2.7169527.432675713.1535729650-258001675.1533988398BT138 600:https://www.tme.eu/ro/Document/fcf6a5be8c5c4d5988cf9076878961fb/BT138-600.pdfThe optotriac part number is MOC3041:http://www.onsemi.com/pub/Collateral/MOC3043M-D.pdfI am wondering if the soldering station will work fine if I will use BT136 600 or BT138 600 ?I am not sure if the max gate current of the BT136 600 and BT138 600 is not too much for MOC3041 ?In MOC3041 datasheet does not say what is the maximum current allowed at pins 4 and 6. It says only pulsed current.

author: JS
author_rank: Frequent Contributor
timestamp: September 01, 2018, 04:29:34 pm
content: The power devices are triggered with about 10mA, the optocoupler can happily deliver that, once triggered the power device clamps and there's no more current for the optocoupler to drive. You don't care about the max gate current, just the trigger current. The max is the one you are not supposed to reach, as you are always switching in zero crossing that won't happen. With your resistors you can reach that current in under 4V, so you have plenty to work with even at 24V. Then, the current from the soldering iron will keep it holding till the cycle ends, also, low voltage and resistive loads like the heater makes the dv/dt not to be a problem... It looks like it should work!JS

author: mike_mike
author_rank: Super Contributor
timestamp: September 01, 2018, 04:51:08 pm
content: Quote from: JS on September 01, 2018, 04:29:34 pmCould you please be more specific ?

author: JS
author_rank: Frequent Contributor
timestamp: September 01, 2018, 05:08:34 pm
content: Quote from: mike_mike on September 01, 2018, 04:51:08 pmWhen the instantaneous voltage of the secondary reaches 4V you can put enough current to the gate to trigger the gate of the power triac. At that time the triac gets triggered and will hold the conduction till voltage goes down again and the holding current is lost. For that you need to account the 220Ω resistor and the vpltage drop in the optocoupler. The other 220 resistor might make up a bit more losses so you need a bit hogher than the 4V but you are far away from the 24V rms so you will loose, as expected, a small bit of the sinusoidal while it reaches this point. That's not a huge deal since there's not much energy to use in this little period so the total power going to the iron won't change by much. Then, if you want to imprpve that, if it's higher than expected, tweaking the twp 220 resistors should be enough. The one going to T1 higher or the one going to T2 lower would improve that, I'd start with the one going to T1 as the other limits the current to the gate in case something goes wrong.JS

author: mike_mike
author_rank: Super Contributor
timestamp: September 02, 2018, 07:52:22 pm
content: I saw a strange behavior at this soldering station (at the schematic from the first post): while using LM358AN from ST or LM358N from Fairchild, the soldering station is working fine.But when I am using LM358P from TI, a strange behavior starts to happen. When the triac turns on, the led is flickering for a very short time (about 1-2 seconds and after flickering the led goes on) and when the triac turns off the same behavior of the led (flickering) happens again and after flickering the led goes off. The moment when the triac goes on and when goes off is the moment when the soldering iron heater is switching on and off.Why this strange behavior of the led is happening ? It is normal ?

author: JS
author_rank: Frequent Contributor
timestamp: September 02, 2018, 08:06:56 pm
content: You are right at the edge of the temp threshold, adding some hysteresis should solve that.JS

author: mike_mike
author_rank: Super Contributor
timestamp: September 03, 2018, 04:54:09 am
content: Is this behavior (flickering of the led) dangerous for the circuit ?If I will leave it as it is, there is probability to have any problems ?Thanks in advance.

author: JS
author_rank: Frequent Contributor
timestamp: September 03, 2018, 05:19:26 am
content: I don't think so, it could try to trigger the gate in the process, maybe it gets a half wave, before finally starting but thats not a problem, it won't break anything.Note that the solution is adding a single resistor between the output of the comparator to the non inverting input, hsould be much bigger than the one feeding that input signal, like two or three orders of magnitude bigger, maybe more depending on the levels and rails (don't remember the schem and I'm from the phone) so you could bodge it in your PCB, no worries, no cutting traces, no wires all over the place.JS

author: mike_mike
author_rank: Super Contributor
timestamp: September 03, 2018, 05:50:56 am
content: That resistor needs to be installed between the pins 1 and 3 of the LM358 (IC2A) ?Could you please have a look at the schematic and tell me if I am right ?I do not understand what value should have this resistor. Could you please help ?

author: JS
author_rank: Frequent Contributor
timestamp: September 03, 2018, 06:22:36 am
content: Quote from: mike_mike on September 03, 2018, 05:50:56 amSorry, it is R6 on the scheme, 4.7M seems to be too much, try something lower, till the fliker hoes away, just enough, don't go too far as that will make the temp poorly regulated. Also, the cap in parallel is something I've never seen, seems like a good idea for the comparator to reject HF noise in faster transients but I don't know what's doing in a thermal system feedback, they are sloooow!JS

author: mike_mike
author_rank: Super Contributor
timestamp: September 03, 2018, 06:30:10 am
content: So I should try with about 1M, 2.2M, 3.3M for R6 ?

author: JS
author_rank: Frequent Contributor
timestamp: September 03, 2018, 06:49:45 am
content: Quote from: mike_mike on September 03, 2018, 06:30:10 amYep, I'd do that, I wouldn't even bother with math for this, math and noise together are hard and deep and this swapping resistors is easy and shallow. Try the ones you got, unless you don't have nothing bigger than 10k... I'd expect to find the right one in those values you said.JS

author: mike_mike
author_rank: Super Contributor
timestamp: September 03, 2018, 07:21:14 am
content: Thanks.If those values will not work, can I use smaller values, for example 100k, 220k ?

author: JS
author_rank: Frequent Contributor
timestamp: September 03, 2018, 07:29:05 am
content: Quote from: mike_mike on September 03, 2018, 07:21:14 amSome of those should work, the problematic IC is just a bit noisier than the others, 220k is way too low and the temperature will go way too low before starting again and way too high before cutting back again. The opamp won't be useful in that case and you should gp with a lower noise one.JS

author: mike_mike
author_rank: Super Contributor
timestamp: September 06, 2018, 07:38:29 pm
content: Thanks for your help. I tested the soldering station and it seems to work correctly, using 2.2M resistor.One more question: I made a mistake when I draw the schematic. Please have a look at the attached images. I reversed the 4.7M preset. Is that a problem ?The first schematic is the original, and the second is the schematic that I draw in Kicad.

author: JS
author_rank: Frequent Contributor
timestamp: September 06, 2018, 07:46:36 pm
content: It could make a difference in a really bad universe, but in the permissive universe we live in it won't make a difference.JS

author: mike_mike
author_rank: Super Contributor
timestamp: September 10, 2018, 06:45:44 pm
content: Dear Forum,I tried with R6=2M2. It was working fine until today when I made some tests, at different temperatures. I found that even with 2M2 resistor the LED is sometimes blinking. I made R6=1M and I don't know what should I do in order to test the soldering station to see if it is working correctly.I am not sure what should I do.What can I do to test this soldering station ?

author: JS
author_rank: Frequent Contributor
timestamp: September 10, 2018, 07:10:31 pm
content: Measure the temperstureat different thermal loads, that should make pretty clear if it's holding the temp or not. Use a big pad or plane, if it melts solder fine on it with a reasonable set tenperature you are fine, much better than a el cheapo one for sure.JS

author: mike_mike
author_rank: Super Contributor
timestamp: September 10, 2018, 07:18:24 pm
content: I don't know how to measure temperature - should I use a multimeter with thermocouple ?I tried to solder a connector (with 2 positions) to a PCB with pads in a copper pour (without thermal relief) and it seems that it can solder fine...

author: JS
author_rank: Frequent Contributor
timestamp: September 10, 2018, 11:43:48 pm
content: If you are going to use a thermocouple use one you dont care too much about as the tin could harm it but that method could do. There are dedicated thermometers for soldering irons with a suspended thermocouple wich is a nice way as it's easier to put the tip in the air on top of it.JS

author: mike_mike
author_rank: Super Contributor
timestamp: September 11, 2018, 05:18:38 am
content: How can I test this circuit ?In what conditions should I test this circuit ?I want to see if the led is still flickering.

author: JS
author_rank: Frequent Contributor
timestamp: September 11, 2018, 05:57:58 am
content: Flickering LED won't stop the station from properly working, at most it will make switch a few cycles back and forth till it makes it's mind but meh...To test it you could remove the thermocouple and insert a signal there, with mainly DC and some noise like you expect to see. As you slowly change the DC you will see the LED change states, if with any reasonable noise or arbitrary slow DC sweep the LED doesn't flicker you are pretty safe you won't see it in the future.If you can measure the signals involved in the actual circuit and try to recreate them with the test setup. As some iron handles share the thermocouple conductor with the heating element you are likely to see some mains freq noise while heating that won't be there while it's not heating.JS

author: mike_mike
author_rank: Super Contributor
timestamp: September 11, 2018, 06:15:46 am
content: You are saying that even if the led will flicker the station will still work properly ?

author: JS
author_rank: Frequent Contributor
timestamp: September 11, 2018, 06:38:27 am
content: Quote from: mike_mike on September 11, 2018, 06:15:46 amYes, it shouldn't be a problem, I've already said that when you first came up with the observation.Quote from: JS on September 03, 2018, 05:19:26 amJS

author: mike_mike
author_rank: Super Contributor
timestamp: September 12, 2018, 07:27:33 pm
content: Thanks for your help!I tested the soldering station, with 1M resistor instead of 2.2M.It seems to work correctly, I didn't measured the temperature of the soldering iron tip, but I managed to melt solder on a copper plane of about 1cm x 2cm. The 10K potentiometer was at about 3/4 of its course.Also, I managed to solder a connector in a copper plane and the joint looks good.Should I let the soldering station as it is, it should work fine ?

author: JS
author_rank: Frequent Contributor
timestamp: September 15, 2018, 01:16:45 pm
content: Does the flux stays there while you solder or it vanishes into smoke the minute your iron touches the tin? that's the easiest and fastest way to identify too high temperatures.If you touch the tip with a wet sponge, does the heater comes on quickly and status on most of the time? Does it stays off most of the time if the tip is not touching anything?This are indicators of a good working iron but it wouldn't warrantee it, measuring the temperatures will.Note that you could use the thermocouple insode your iron to measure the temp with another instrument as you use it for the loop, as it is a voltage that you are measuring. Error could be a bit higher than just using the thermocouple directly or an external one but much better than guessing, the only important factor is the type of thermocouple in your iron is the same the instrument is calibrated for.JS

author: mike_mike
author_rank: Super Contributor
timestamp: September 15, 2018, 03:32:41 pm
content: I attached a photo with the PCB were I managed to melt solder:If I touch the wet sponge with the tip, the heater comes on about 6 second if I touch the wet sponge right after the heater comes off. The heater does come on periodically about 11 seconds when the tip does not touch anything.I measured the voltage on the thermocouple (using the multimeter) and it is varying between 9.6mV and 10mV.

author: JS
author_rank: Frequent Contributor
timestamp: September 16, 2018, 03:59:20 am
content: Plenty of flux, certainly not too cold, might be too much solder or a bit cold, I can't tell from that picture, but melting without burning the flux means you could adjust it a bit higher and still be good.That's about 10C hysteresis, not bad for this circuit. You could lower it if you can live with the blinky light and might get better temp response, or might be worse, as when it colds it keeps heating for a few seconds too long and that could be the time you are in the joint, so effectively making it better by accident.In any case, I think it's good, compile all the final docs in your first post and make a description, some people might find it useful. If you can build a PDF with all the documentation and information of the project would be nice for many newbies to get over the $3 iron, which this certainly looks much better.JS

author: mike_mike
author_rank: Super Contributor
timestamp: September 17, 2018, 07:59:49 am
content: Thanks for your help.I updated the first post in this topic with some instructions.I tried to check if the triac is correctly installed but I found that it is difficult to check.What can happen if the triac is incorrect installed ?1. What can happen if the main terminals of the triac are incorrect connected to the MOC3041 ?2. What can happen if the 2 resistor which have 220R value are incorrectly connected to the 4 and 6 pins of MOC3041 ? The soldering station will still work ?

author: JS
author_rank: Frequent Contributor
timestamp: September 18, 2018, 04:19:22 am
content: Why is it hard to install correctly?1)Well, uours survived, it's only exposed to 55mA, MOC3041 is rated for 1A repetitive peaks, not for RMS but at 55mA it will be under 1V, so under 55mW at thw output, total maximum dissipation is 250mW so unless you are burning over 200mA it should be fine.2) I'm not sure what you are asking here, you need the resistoe going to N$30 (jpg scheme on post 1), the other could be missing or be a higher value and it probably still work, increasing the value of that resistor might help on the previous question.JS

author: mike_mike
author_rank: Super Contributor
timestamp: September 18, 2018, 05:01:55 am
content: I was asking because at the beginning, when I first built this soldering station I connected the triac reversed and I needed to make some modifications. I unmounted the triac and I mounted the triac again, reversed compared to the first time when I installed it.If the triac was connected reversed, or some connections were made incorrectly for example if the triac pin 2 was connected at pin 4 of MOC3041 instead of pin 6 through the 220 R resistor, the soldering station would not work correctly ? Please correct me if I am wrong.And I am sorry for bothering you with my questions.I checked again the triac and it seems to be connected fine.

author: mike_mike
author_rank: Super Contributor
timestamp: January 25, 2019, 03:12:48 pm
content: Hello,I built this soldering station using a PCB from China.It is normal that while the triac is OFF to have about 0.020Vac on the heater ?

author: JackJones
author_rank: Regular Contributor
timestamp: January 25, 2019, 03:24:14 pm
content: There is going to be some leakage through the triac and optocoupler. BT138 lists the typical off state leackage as 0.1mA. 20mV isn't going to produce any significant unwanted heating, I wouldn't worry about it.

author: mike_mike
author_rank: Super Contributor
timestamp: February 24, 2019, 11:02:20 am
content: Thank you for the reply.I built the soldering station from the attached schematic.But I have a problem regarding this schematic. The mosfet does not turn off. The heating element of the soldering iron is permanently connected to 24V.I tried the following modifications: I replaced the LM358, the mosfet (initially was a IRF530, now it is a IRF640), the potentiometer, the 2M trimmer but replacing those components did not solved the problem.The output voltage from IC2B is permanently about 10V. The power supply for LM358 is 12V (because I used a 12V regulator).The mosfet does not turn off even if I rotate the 10K pot or the 2M trimmer...What should I check to find the problem ?

author: magic
author_rank: Super Contributor
timestamp: February 24, 2019, 11:33:20 am
content: I could swear you posted in this thread a few days ago asking if this circuit is compatible with some random soldering iron.Now that post is gone and you say the circuit doesn't work. I guess you now know the answer to your previous questionCheck voltage coming from the iron's sensor when it becomes hot enough to melt solder. Type k thermocouple sensitivity is 41uV/C, so at 200°C you should see over 8mV - easily detectable with a DMM.If it's zero, you ether have some connection problem or it is a Hakko-compatible iron with a thermistor which will require a modification to the circuit to work.

author: mike_mike
author_rank: Super Contributor
timestamp: February 24, 2019, 11:57:59 am
content: I checked the voltage coming from the thermocouple, and it is rising until the soldering iron gets very hot... and when this happens, I turn off the power to the soldering station. So the voltage from the thermocouple is present at the input of the operational amplifier.I know for sure that the soldering iron has a K type thermocouple sensor.

author: magic
author_rank: Super Contributor
timestamp: February 24, 2019, 12:39:09 pm
content: So check the output of IC2A, it should be rising even faster. When it becomes higher than the voltage from POT1, IC2B should turn off the FET.You probably have a problem with offset on IC2A because your circuit has no way of compensating it except by cranking up gain with R12. It's possible that it will only work correctly if R12 has quite high resistance and POT1 is very close to zero.

author: mike_mike
author_rank: Super Contributor
timestamp: February 24, 2019, 12:45:37 pm
content: I checked the output of IC2A and it is 0V (zero V) with respect to the ground. I tried with R12 at min and with R12 at max.I also replaced R12 with another trimmer (4M7) and the situation is the same.

author: magic
author_rank: Super Contributor
timestamp: February 24, 2019, 01:31:34 pm
content: So check IN- pin at IC2A. It should be 1/2000 of its OUT pin, so zero. If it's higher, you have some wiring error or the 358 is busted.If it's zero, you have problem with offset voltage of IC2A. Depending on the exact unit, IN+ may need to be a few mV higher or lower than IN- for the output to start moving upwards. Connect a pot instead of the iron and find out how much it is. If it's more than a few mV or if nothing ever is enough, the 358 is faulty.At any rate, some offset calibration will probably be needed. For minimum effort:Add 470k from IN- to 12V. This will pull IN- from ground to 25mV.Add 470k from sensor+ to 12V.Insert a 2k pot between sensor- and ground. With the pot at 50% and the iron cold, IN+ will also be at 25mV now. Adjusting the pot will change it by ±25mV.Set R12 to 25% or so.Set POT1 to zero and tweak the 2k pot until the iron stabilizes at "barely hot enough to work".Increase R12 if maximum temperature set with POT1 isn't enough. You may need to adjust the 2k pot a tiny bit afterwards.And I forgot to add:Put 1N4001 (or similar) diode in parallel with the heater connector but in opposite direction (you don't want it to conduct all the time and steal the heater's power). This is to snub inductive pulses from the heater cable which could damage the LED by reverse bias and possibly avalanche the FET. I don't remember if those oldschool IRFs are avalanche resistant.If you have a scope, probe the voltage at the tip of the iron. Switching of the heater will cause transient pulses at the tip up to a few volts IIRC. This may possibly be dangerous to some ultra low voltage parts. It can be fixed by increasing R7 to a few kOhm, the exact value depends on the type of FET.Or maybe don't do that. You can exceed the FET's linear SOA and kill it if you aren't very careful. But measure those pulses if you can and remember that such a problem exists.It is possible to make those 936-type irons with thermistors compatible with k-type thermocouple stations.The secret number is 147uA - this is the current which needs to flow through the thermistor to make its output voltage close to that of a k-type within the range of typical soldering temperatures. A 34k resistor to 5V or 82k to 12V should do. I personally tested the former with satisfactory results.

author: mike_mike
author_rank: Super Contributor
timestamp: February 24, 2019, 03:41:30 pm
content: Thank you for your help.I searched in a box full of old electronic projects made by me, and I found this soldering station which has the same schematic with the one that is being discussed.I powered it on, and it works very good, without any modifications...The single difference is the PCB. So probably there are some problems on the new PCB...

author: mike_mike
author_rank: Super Contributor
timestamp: March 22, 2020, 08:41:36 pm
content: Quick question about the attached schematic:Does the marked resistor (Rx = 100k) protect the soldering iron from overheating if one or both wires of the thermocouple are disconnected ?