How to get rid of fluctuating voltage on soldering tip (T12 clone station)?

[floobydust]

The soldering iron heater shell (tip) should be directly connected to protective earth-ground.
This is so a fault in the power supply could not make the soldering iron tip become hazardous live.
An ESD wrist-strap or mat is grounded through a 1MEG resistor, but not a mains-powered appliance unless it has double-insulation and no Y-capacitor in the PSU, which is not the case here.

The KSGER STM32 controllers and irons use a 5-pin GX12-5 connector and 5 conductor cable, one pin for the safety ground and another for the heater return. They have two grounds.

Quicko has used a 4-pin GX12-4 and 4-cond. cable. I'm not sure where or how they safety-ground the heating element shell.
On the Quicko controller PCB, the two green wires (to PSU, to wand) should have continuity between them unless the PCB has a mistake. They should also have continuity with DC(-) and the soldering station chassis.

[Specmaster]

OK, then, the best think to do here is to remove all modifications that you have done and return to a standard Quicko product, i.e. as it left the factory. Then try and connect an earth lead directly to the green earth lead on the Quicko unit and then do further measurement tests and see if the iron functions correctly and if the leakage that you measured before has disappeared. If it is working OK and there is no leakage detected it would then seem to be suitable for use.

[Haggis McHaggis]

Sorry, i'm a noob and this getting more complicated the more i read about it

Do you say i absolutely have to get a direct separate PE connection from mains into the case? I thought i can use DC- as connection to earth (though with 1M resistance to PE) via the psu. It seemd to work here.

I haven't modified anything yet, i did tests using measurement clips and cables.

[Specmaster]

That's correct then that way you'll be as close to the same kind of set up as I have. You know I still can't quite get my head around why you never went for the self powered version like mine, all of this problem would have gone because it worked perfectly straight from the box. As ,mentioned, the only modification I've made to mine was ensure that the metal enclosure was also well bonded to the PE for safety reasons in the event of a fault occurring internally between the PSU and the enclosure as the 240v supply is on the PSU PCB.

[Haggis McHaggis]

It sure would have been easier, but the mini station was cheaper and i had two lenovo psu's laying around. In addition i thought not having 230VAC in the controller is a good thing. My hope is that the Lenovo psu is designed well and doesn't do much damage in case of failure.

Regarding tip/earth connection i read some soldering stations have that 1M resistor too. If i'm correct i would have the same in the end.

I have another question about the case. Since it is coated or anodized i'm not sure whether there is electrical connection between all the pices of the case or not. There is for example no connection between the DC jack (-) and the metall enclosure of the 4 pole aviation jack although there should be a connection if the case would be raw/uncoated.

[Specmaster]

Yes, its cheaper its true, but the cost of the parts in kit form is not that much more, its only the PSU, enclosure and power lead, but then you probably have other spare power leads, and then the whole kit is guaranteed to work together correctly, using other parts and this is no longer a certainty.

Yes the case is anodised and what I did was to scrap this off at points where the various parts join together, at least at one corner of the top and bottom at front and rear as well drilling a hole in the base and mounting a earth stud there as well for added safety.

I would not worry about the 1M resistor as that is red herring, 1M resistors are used in some cases on antistatic mats and also to discharge capacitors but 1M in earth loop means that high voltages will still occur at the tip which is precisely what you don't want to happen, which was your original reason for raising this thread. 

[draza]

I think I have same T12 station as Haggis McHaggis. My station is dead after several uses, do not power anymore. The inside pictures of Quicko T12-943 are on this link: https://www.eevblog.com/forum/beginners/soldering-station-quicko-t12-943-problem/

[floobydust]

The linear voltage regulator overheats, because it's dropping 24V to 3.3V and powering MCU/OLED.
Some controller boards have a bad PCB layout, no extra copper for cooling that IC.
Others show a buck-converter which could be better but more noisy for the thermocouple amplifier.

[Haggis McHaggis]

I'm a little worried about the controller after reading draza's unit suddenly stopped working.

Nonetheless i've another question about grounding. I've attached a schematic which shows what i want to modify on my unit. I thought about adding a banana jack, a toggle switch and replace the DC in jack with a insulated one. The switch would toggle the tip to either DC- or case/PE. Do you think it will work this way?

[floobydust]

I don't think that will work because you still need DC(-) going to PE.
If you are using a two-prong (ungrounded) DC power (laptop) brick, you must have a PE connection to DC(-) or it will float up to 60-100VAC (due to Y-capacitor) which can damage parts when you solder them. If you are using a battery pack for DC power, there is nowhere for ESD to go.

I would keep the case, DC(-), PE, banana jack all connected together. The ESD mat and wrist-strap should have 1MEG resistors.

draza's T12-943 unit I'm not sure why the controller failed - it could be his 30V bench PSU has overshoot problems and gave it say a 25V spike...
Yes, Quicko says T12-943 must be limited to 25VDC max. input voltage 
I think some of the 24VDC power supplies have poor regulation or overshoot which might be too much. 1V margin is pretty small.

[Haggis McHaggis]

I don't think that will work because you still need DC(-) going to PE.
If you are using a two-prong (ungrounded) DC power (laptop) brick, you must have a PE connection to DC(-) or it will float up to 60-100VAC (due to Y-capacitor) which can damage parts when you solder them. If you are using a battery pack for DC power, there is nowhere for ESD to go.

I would keep the case, DC(-), PE, banana jack all connected together. The ESD mat and wrist-strap should have 1MEG resistors.

My laptop psu has a 3 prong AC input. I measured 1MOhm between PE and DC- while the psu was off all connections and powered off. So it already has some kind of PE reference. Should i connect it directly to PE nonetheless in this case?

Edit: do you think my Lenovo laptop psu (posted an image of it earlier in this thread) can overshoot its output and can damage the controller? I hope it's a good psu with built in voltage peak protection. But what do i know.

[floobydust]

I think I have same T12 station as Haggis McHaggis. My station is dead after several uses, do not power anymore. The inside pictures of Quicko T12-943 are on this link: https://www.eevblog.com/forum/beginners/soldering-station-quicko-t12-943-problem/

There is a high risk if the DC adapter (or bench PSU) is already on and then you plug it (DC) into the soldering station, a surge occurs that can make an overvoltage spike and damage the controller.  It's due to the cable inductance and capacitors. Best to plug in AC power last.
The Quicko T12-943 controller is only rated to 25V max.

[floobydust]

My laptop psu has a 3 prong AC input. I measured 1MOhm between PE and DC- while the psu was off all connections and powered off. So it already has some kind of PE reference. Should i connect it directly to PE nonetheless in this case?

Edit: do you think my Lenovo laptop psu (posted an image of it earlier in this thread) can overshoot its output and can damage the controller? I hope it's a good psu with built in voltage peak protection. But what do i know.

Three-prong laptop adapters can have the DC side connected to PE ground, and others have a Y-capacitor and sometimes a 1 megohm resistor, instead of a direct connection.

I would say having a 1MEG resistor (inside the adapter) is not good enough. Because the adapter has leakage currents which can put some potential on the DC(-). If your tip has 36V stray charge on it, that can matter.

For me, I want my soldering iron earth grounded to ensure no stray charge can happen on the tip, no matter what. You need a ground-wire run for the ESD mat anyway. It's your choice.

[Haggis McHaggis]

I would say having a 1MEG resistor (inside the adapter) is not good enough. Because the adapter has leakage currents which can put some potential on the DC(-). If your tip has 36V stray charge on it, that can matter.

I did a test in which i connected the tip to DC-. The stray voltage on the tip went away. But i'm not sure if there are other scenarios i did not measure. I simply measured it with my DMM for a short time.

I've attached some more wiring diagrams. Which would one you prefer?

[floobydust]

What is the switch for?

[Haggis McHaggis]

What is the switch for?

Tip earthing on/off (floating on/off) I probably don't need this switch at all.

[floobydust]

I would ground everything together and check with an ohmmeter every few months. No switch needed.
Once my ESD mat ground went open-circuit and the mat had stray AC voltage from other junk on my bench.

[Haggis McHaggis]

I've attached a new diagram.

green marked = 1MEG resistance measured
red marked = questionable connection

I read advice against connecting DC- with PE, because the galvanic isolation gets ineffective and there is risk of transient voltage flow for fractions of seconds when the psu is switched on/off. In my case there is already some kind of link between them (marked green).

Or should i simply connect the tip to DC- without an additional PE input jack to the controller as already mentioned in this thread? I then would connect the ESD mat and wristband through its own connection to PE and have it separated from the soldering station.

[SteveyG]

For me, I want my soldering iron earth grounded to ensure no stray charge can happen on the tip, no matter what. You need a ground-wire run for the ESD mat anyway. It's your choice.

You don't specifically want the soldering iron tip to be connected to mains earth directly - this is why most production quality units will have a separate terminal on the station. You want the soldering iron tip connected to the same potential as the ESD equipotential zone. Nothing in an ESD equipotential zone should be tied hard to mains earth as you can still get an ESD strike in that case.

[Haggis McHaggis]

You don't specifically want the soldering iron tip to be connected to mains earth directly - ..

I was wondering that too. Should i use the psu's link to PE as ground for everything? (see attachment)

Edit: there wouldn't be a connection to PE if the psu is not plugged in, so maybe not the best idea.

[Specmaster]

All I can say is this, I don't have an ESD mat but I do have an ESD wrist band that wear when handling sensitive parts. My T12 irons use the makers proper PSU's, neither have any connection from the DC-ve to the tip directly or via any resistor to ground and the only grounding it does have is from the IEC inlet socket directly to the tip and the enclosure, so with the iron plugged in with the power lead plugged in the mains out on the wall or bench etc, the iron tip is connected directly to the mains PE.

I have replaced may sensitive parts, IC's and MOSFETS etc with this setup and not lost any of them due zapping them. Of course it goes without saying that the equipment being worked on is switched off and disconnected and any power filtering caps discharged before attempting to solder anything.

You are having the problem because of the cheap PSU that your using, which was designed in the first instance to power a laptop. Your PSU is leaking enough power that you may be able to feel a tingle when touching the DC output. With the proper built in Quicko PSU, there is no high voltage leak through to the DC.

This is the way to go in reality if your going to be working on sensitive gear, use the right tool for the right job, or of course, get yourself a 24v battery and the you will only have pure DC which should not leak.

[floobydust]

For me, I want my soldering iron earth grounded to ensure no stray charge can happen on the tip, no matter what. You need a ground-wire run for the ESD mat anyway. It's your choice.

You don't specifically want the soldering iron tip to be connected to mains earth directly - this is why most production quality units will have a separate terminal on the station. You want the soldering iron tip connected to the same potential as the ESD equipotential zone. Nothing in an ESD equipotential zone should be tied hard to mains earth as you can still get an ESD strike in that case.

Mains-powered soldering stations must have the tip (directly) grounded per safety standards, so I'm not sure what you are talking about. Please don't confuse people on this.
Grounding through a 1MEG resistor has it's own problems, there is no guarantee the tip is at zero volts potential due to leakage currents in the station's PSU.
On my ESD mat I monitor ground currents and find that even a mains power cord lying on across the mat can induce several few volts of mains hum, due to capacitive coupling and the 1MEG resistor.

Production-quality soldering stations can offer a front panel banana jack, which connects internally to mains-earth. The expectation is a mat or wrist-strap with 1MEG resistor is then plugged in.

An "ESD equipotential zone" has to have a reference, such as earth ground. You would never leave it floating because something grounded will inevitably touch the zone.

[Specmaster]

For me, I want my soldering iron earth grounded to ensure no stray charge can happen on the tip, no matter what. You need a ground-wire run for the ESD mat anyway. It's your choice.

You don't specifically want the soldering iron tip to be connected to mains earth directly - this is why most production quality units will have a separate terminal on the station. You want the soldering iron tip connected to the same potential as the ESD equipotential zone. Nothing in an ESD equipotential zone should be tied hard to mains earth as you can still get an ESD strike in that case.

Mains-powered soldering stations must have the tip (directly) grounded per safety standards, so I'm not sure what you are talking about. Please don't confuse people on this.
Grounding through a 1MEG resistor has it's own problems, there is no guarantee the tip is at zero volts potential due to leakage currents in the station's PSU.
On my ESD mat I monitor ground currents and find that even a mains power cord lying on across the mat can induce several few volts of mains hum, due to capacitive coupling and the 1MEG resistor.

Production-quality soldering stations can offer a front panel banana jack, which connects internally to mains-earth. The expectation is a mat or wrist-strap with 1MEG resistor is then plugged in.

An "ESD equipotential zone" has to have a reference, such as earth ground. You would never leave it floating because something grounded will inevitably touch the zone.

Agreed, maybe the reason the 952 does not show any leakage of AC on the DC is because PSU is inside of a screened and grounded metal enclosure and the also the leads from the PSU to the controller are no more 50mm long so not a lot of capacitive coupling can take place there, unlike with a laptop PSU brick which typically has around 1 to 2 meters of cable to the DC jack.

[Haggis McHaggis]

I made another diagram

In this one there is a banana jack for the tip isolated from the enclosure. DC- is connected to the enclosure. What do you think?

[Specmaster]

Personally I'd connect the PE directly to the tip and enclosure and forego the banana plug, this could get accidentally pulled out and they your right back zapping the things that your trying to protect. Likewise ESD mat and wrist band, should be connected to PE via another route for safety.