Mini TS100 soldering iron and current leakage

[bodger]

I recently brought myself a Mini TS100 soldering iron (Sainsmart branded) came with a switch mode power supply like those found on laptops. I noticed that while the iron was cold but still powered that if I ran my finger against the tip I'd feel a slight 'tingle'. So I measured the AC voltage at the tip against ground and found it to be around 115V, I'm guessing this is not uncommon for an iron that isn't grounded?

I'm about to put together a class AB audio amp (just for kicks), but wanted to know if this leakage (is that the correct term?) could potentially damage components such as the amp ic, capacitors, resistors, etc. I'd also be interested to know what causes a SMPS to pass this 'leak' through, the technically term would be nice to know so I could research it.

This is my first post, so apologies if I haven't followed correct forum etiquette. Thanks in advance.

[stj]

welcome to the world of switching psu's with a capacitor coupling the secondary to the input earth - that often does not exist but is then capacitivly coupled to the input terminals!!

this bullshit extends to devices like settop-boxes & tv's btw.

[voltsandjolts]

Don't put up with that BS. Return the iron to the seller and tell them it is a POS.
Then buy a better iron with earthed tip. A lot of folks here recommned hakko's (see EEVBlog video).

[stj]

Don't put up with that BS. Return the iron to the seller and tell them it is a POS.
Then buy a better iron with earthed tip. A lot of folks here recommned hakko's (see EEVBlog video).

you obviously know nothing about the product - it's a battery powered iron.
the issue is with the psu - and the "problem" is incredibly common.

it would be better to use a linear psu with a nice toroidal transformer, a rectifier and a cap.

[bodger]

Thanks for the replies. I'll probably just ground the iron via a 1Mohm resistor or construct a 4S lipo battery (just a pain they used a 5.5mm x 2.5mm connector, have a few 2.1mm laying about). Hakkos are overpriced by around 33% when compared to US prices here in England, I needed something portable and for £50 the Mini TS100 is excellent value and performs as good as the Wellers I used at college and I find it easier to work with the pencil style irons.
I've noticed the "problem" with metal constructed laptops in the past too, just surprised none of the Mini TS100 reviews I looked into mentioned the possible effect of using a SMPS.

[stj]

does the psu have an earth pin?
you could mod it so it's actually grounded.

[bodger]

No earth pin unfortunately, it's got one of those figure 8 cables (C7 I think is the official cable name)

[stj]

those things 
they could have atleast used a 3pin "clover-leaf" type.

[voltsandjolts]

Don't put up with that BS. Return the iron to the seller and tell them it is a POS.
Then buy a better iron with earthed tip. A lot of folks here recommned hakko's (see EEVBlog video).

you obviously know nothing about the product - it's a battery powered iron.
the issue is with the psu - and the "problem" is incredibly common.

it would be better to use a linear psu with a nice toroidal transformer, a rectifier and a cap.

Optionally battery powered. As I understand it the OP was making measurements with the mains SMPS plugged into the iron, which is a valid use case. In that situation, when applying solder you are making a conducting path between your hand and the soldering tip. That tip is separated from 240VAC mains by the supplied chinese SMPS. If you are happy to trust your life to that go right ahead. I notice the manual suggests connecting a separate (optional!) earth wire which you should certainly do if using it with the supplied mains SMPS connected. Consider getting a UL listed quality SMPS if you are keeping this iron and using it with the SMPS connected.

[Simon]

Is the SMPS earthed ? this sounds very much like a lack of earth connection. many SMPS will have an input filter that puts two capacitors in series across the mains with the centre ping earthed, if there is no earth there will the 1/2 the mains supply voltage there, if the iron is then "earthed" you will have a problem.

[bodger]

Is the SMPS earthed ? this sounds very much like a lack of earth connection. many SMPS will have an input filter that puts two capacitors in series across the mains with the centre ping earthed, if there is no earth there will the 1/2 the mains supply voltage there, if the iron is then "earthed" you will have a problem.

No it isn't earthed, but my old Dell laptop power supply which is earthed has a similar effect. Having done a little more research it looks like this is an effect of the Y-caps, which are pretty much required in SMPSs to pass EMI radiation tests.

[Simon]

Yes I was referring to the Y caps, if you have other equipment doing this it means you don't have an earth in your house.

[bodger]

Yes I was referring to the Y caps, if you have other equipment doing this it means you don't have an earth in your house.

I made an assumption about the Dell power supply as most of the new ones I have are grounded, the one I tested wasn't grounded plus it had what appeared to be a clover socket, turned out to be a unnecessarily large figure 8 socket. Make sense that my newer laptop power supplies don't exhibit the same problem. Thanks.

[wraper]

No it isn't earthed, but my old Dell laptop power supply which is earthed has a similar effect. Having done a little more research it looks like this is an effect of the Y-caps, which are pretty much required in SMPSs to pass EMI radiation tests.

Means that there was no earth connected in your mains sockets.

[KL27x]

I feel like the dunce in the corner for asking this. But if there's no earth connection for the iron... or if there is an earth connection but it isn't plugged into anything, then how does the 115V potential get to the tip of the iron? Is not the tip electrically isolated from the heating element in every soldering iron?

I built an iron ran from ungrounded laptop supply, and I had zero issues. The earth pin for the handpiece, I just left unconnected.

[MosherIV]

I feel like the dunce in the corner for asking this. But if there's no earth connection for the iron... or if there is an earth connection but it isn't plugged into the SMPS, then how does the 115V potential get to the tip of the iron?

stj said :

welcome to the world of switching psu's with a capacitor coupling the secondary to the input earth - that often does not exist but is then capacitivly coupled to the input terminals!!

[KL27x]

Ok, I feel like an even bigger idiot for asking, then. Cuz I still don't get it.

If I take the "tingly" TS100 and I put a layer of micarta or ceramic or kapton tape over it, then put another sheet of metal over that... I can still measure 115 V between that and earth?

capacitivly coupled to the input terminals

Why would the input terminals be in continuity with the tip of the iron?

[3db]

This iron actually has an earthing screw on top above the power connector.

3DB.

[MosherIV]

If I take the "tingly" TS100 and I put a layer of micarta or ceramic or kapton tape over it, then put another sheet of metal over that.

You have just created another capacitor.

If you put AC on one side/plate of the capacitor, the other plate will become charged.

[KL27x]

^But inductive AC ripple wouldn't create 115VDC potential to earth, would it? Wouldn't it be just.. plus and minus xV from earth (or whatever you were measuring it against?) The capacitor would block DC.

This iron actually has an earthing screw on top above the power connector.

This makes sense to me (but maybe I'm an idiot). So perhaps the earthing screw is connected to a wire in the cable? Or to a shield in the cable which eventually runs to the sheath of the T12 style tip? In this case, you could snip the connection and install a switch to select grounded vs floating tip?

Also, since it runs off DC, perhaps the designers assume that the DC ground will be grounded to earth IF/WHEN the unit is run off an actual grounded smps? So perhaps even in the hand piece itself, the 3 connections of the tip, 2 of them are shorted together? So it might even require modification to the iron internals, itself, to liberate the sheath from any connection?

Or if the fit of the tip in the handpiece is loose enough, you could maybe wrap the base of the sheath/tip in kapton tape, so only the 2 heater/thermocouple connections are exposed? Jam that back into the handpiece, and shazam, you have an isolated tip?

I don't have T12 iron, but I would assume if you equate it with a TRS stereo plug, the base ring is the tip/earth and the middle and tip rings are to the heater/thermocouple unit.

[MosherIV]

But inductive AC ripple wouldn't create 115VDC potential to earth, would it?

Also, since it runs off DC, perhaps the designers assume that the DC ground will be grounded to earth

The problem here is that the SMPS designers have not grounded the 0V/common so if floats.
The 0V can be pushed up to anything between Earth/0V to the full mains voltage by capacitive coupling.

Yes, grounding the tip will work but do not do it while the SMPS is on.

[KL27x]

^Ok, I get that. But does the tip/earth connection of the T12 tip have to be connected to anything?

 I have read that the heater and thermocouple are in series. Is it like a JBC where there is a center tap between the heater and the sensor, making 3 connectors in total? And the tip is in continuity with one of them at all times?  I only count 3 connectors on the T12 tip, and it runs off AC in the Hakko FX950/1, so it means DC control circuit and earth have to be the same, and in direct continuity with the tip? Or is there a fourth connector hidden in the very end of the tip?

I have some T12 tips coming, and I was planning on the tip to be
1. tip/earth
2. heater
3.cathode of thermocouple

And you run the power between 2 and 3. Turn it off and read the voltage between 2 and 3. The resistance of the heater doesn't matter, it just adds a little variable impedance which you are going to prolly swamp anyway with the opamp circuit resistors.

*Also, you are wrong and I am right about the other example of wrapping the tape around the iron and wrapping that in foil?

[KL27x]

Ok, I'm betting there is a fourth connector in the base of the tip. It doesn't make any sense for the tip to be in continuity with any of the heating/sensing elements.

If this is the case, all you have to do is disconnect the sheath from DC ground. Problem solved.

[bodger]

^But inductive AC ripple wouldn't create 115VDC potential to earth, would it?

It's 115Vrms AC that I was measuring on the tip, as I understand it the y-cap causes a small current to float at half mains voltage.

[3db]

I just did a quick measurement of the resistance between the earth screw and the tip. It's approx 0.9 - 1 ohms.
I used a Fluke 101,so nothing fancy.

3DB.