Soldering in a life circuit

[KrudyZ]

First of all, I'm not talking about line voltage here, but rather a low voltage circuit that is running.

The use case is that you have some kind of embedded system running code and it takes a while to get into the state you want to observe, so you don't want to power cycle it.
I don't like clip on probes and they don't work with SMD passives anyways. Also, most of my parts are 0402 or smaller. The pogo pin doodads are OKish if you want to probe one or two locations, but are really not robust unless you happen to have a via for them to go into.
My preferred approach is to solder short wire stubs to a pin header that I can then plug my probes into. The probes I use are compatible with 0.1" headers either directly or via tip adapters.
Now the problem is that many times circuit ground is Earth ground and the soldering iron tip is also grounded.
Therefore you are bound to short out whatever net you are trying to attach a wire to.
Of course I know of cordless soldering irons, but I would rather keep the temperature control and performance from my JBC Nano.
I also don't feel like spending another $640 for the JBC cordless model.
Maybe adding a series resistor to the Earth ground connection of the tip to keep it static safe but not short out a supply rail when soldering a wire to that?

Do you solder in life circuits and how do you do it?

[Gyro]

Ignoring the issues with ESD that you mentioned and the reaction of a running embedded system to the capacitive loading of a soldering iron on one of its signals, my feeling is that sooner or later you are going to end up in solder blob tears, especially on small geometry smds.

Of course I've occasionally tacked wires or components onto a floating low voltage (actually ELV, mains is classified as Low Voltage) test circuit, but not on anything that fine pitch.

[BrokenYugo]

You float the device under test, break it's earth ground reference, not the iron's.

I concur with Gyro though, at that fine of a pitch anybody will inevitably short something to something with a solder blob or bad iron placement. I'll do SRAM battery replacements that way but that's it.

[DimitriP]

Re: Soldering in a life circuit

A bit of foreshadowing there in the title...

I'm sure you meant "live" circuit as in powered on as you elaborated.
I would avoid it, not worth the risk whatever that is.
But if my "life" was depending on it, and for some reason it had to be done "NOW",  I would get the iron "really hot", unplug it and go to town.

[coppercone2]

this is a job for batteries or butane

you might actually get a good solder joint that way. trying to overload the iron is not the proper technique

[floobydust]

Don't solder live circuits, lest it become a bad habit.
I soldered a 12V live circuit and saw a spark come off the soldering iron tip coated in molten solder, when it touched the board. There was a ground-fault between the PE grounded tip and the circuit's ground, it shorted out the power supply and did no damage but...

[bdunham7]

Maybe adding a series resistor to the Earth ground connection of the tip to keep it static safe but not short out a supply rail when soldering a wire to that?

Do you solder in life circuits and how do you do it?

I modified the FX8801 handpiece on an FX888 soldering station to have a 1M resistance to ground rather than a near short.  This allows any static charge to drain to ground but presents a miniscule load to anything I am soldering.  I would typically use this on something like replacing a battery in a test instrument where I really don't want to lose the memory (often cal data) and that would mean low voltage DC.  I think it might be a problem to try this on any sort of data or RF signal because my capacitive loading to ground is probably pretty big, although I've never tested that.

[floobydust]

The reason a soldering station (tip) is "hard grounded" is for electrical safety. If the power transformer insulation fails, you don't want hazardous live on the secondary side - the tip, handle, controls etc. as a shock hazard.
I've only seen one German brand soldering station with a floating secondary side, it has double-insulation and such low winding capacitance to the primary that the tip stays low potential.

[bdunham7]

The reason a soldering station (tip) is "hard grounded" is for electrical safety. If the power transformer insulation fails, you don't want hazardous live on the secondary side - the tip, handle, controls etc. as a shock hazard.
I've only seen one German brand soldering station with a floating secondary side, it has double-insulation and such low winding capacitance to the primary that the tip stays low potential.

Why would that be more of an issue with a soldering iron than any other appliance with a transformer and no ground connection at all?  Both Pace and Hakko sell 'soft ground' versions that work as I describe and the modification I did seemed to be already anticipated by Hakko---there was already a 1M resistor there and I simply had to switch the tip ground wire over to that.  I can't tell for sure from others teardowns, but it looks like it does have a double-bobbin transformer.  The Pace definitely does as I've taken mine apart.

[babysitter]

Classic solder roulette. Give it a try.

[floobydust]

The reason a soldering station (tip) is "hard grounded" is for electrical safety. If the power transformer insulation fails, you don't want hazardous live on the secondary side - the tip, handle, controls etc. as a shock hazard.
I've only seen one German brand soldering station with a floating secondary side, it has double-insulation and such low winding capacitance to the primary that the tip stays low potential.

Why would that be more of an issue with a soldering iron than any other appliance with a transformer and no ground connection at all?  Both Pace and Hakko sell 'soft ground' versions that work as I describe and the modification I did seemed to be already anticipated by Hakko---there was already a 1M resistor there and I simply had to switch the tip ground wire over to that.  I can't tell for sure from others teardowns, but it looks like it does have a double-bobbin transformer.  The Pace definitely does as I've taken mine apart.

The common North American safety standard for soldering irons UL499 has legacy, it's from a day when soldering iron heaters were mains-powered, early 1920's. I don't think they were grounded back then and it came later. The standard has many oddball, specific clauses. Some manufacturers exploit this standard's antique, lax requirements. The need for a hard ground has pros and cons either way.

For North America, I think it's the requirement to have the extra insulation (double or reinforced) and to pass the hi-pot test at the higher test voltage- if it's not a "hard grounded" (as I call it) appliance. I would say for global safety certification, the 1MEG is fine as dissipative.

But for soldering...
SMPS-powered soldering stations have high leakage current due to the Y-cap(s) and (some) crappy wound transformer, they float to mains/2 high voltage so a 1MEG doesn't do much. Still ~60VAC at the tip which is no good for working with semiconductors.

Mains transformer-powered soldering stations, old ones were not wound split-bobbin and the high winding capacitance pri-sec means an ungrounded tip floats up to more ACV than you would like. Again, a 1MEG doesn't do much.
Modern split-bobbin transformers have very low interwinding capacitance and are not really an issue, very low stray voltage.

[bdunham7]

Mains transformer-powered soldering stations, old ones were not wound split-bobbin and the high winding capacitance pri-sec means an ungrounded tip floats up to more ACV than you would like. Again, a 1MEG doesn't do much.
Modern split-bobbin transformers have very low interwinding capacitance and are not really an issue, very low stray voltage.

I think it may depend on how the soft-ground is implemented.  On mine, for example, the base station is still grounded and the secondary side is not electrically connected to the tip at all.  My 1M resistor is actually right at the connector.  If I connect a DMM to ground and the tip, the voltage (0.53) is less than what I measure with the DMM probe waving around unconnected (but the other one still at ground) which is 0.72VAC. 

[Vovk_Z]

The reason a soldering station (tip) is "hard grounded" is for electrical safety. If the power transformer insulation fails, you don't want hazardous live on the secondary side - the tip, handle, controls etc. as a shock hazard.

That's quite a rarely case which may normally never happen (because nobody works on live circuits). And it may be not as unsafe because voltage don't kill but a current does. Until current can't flow through your body - you are safe. Possibly that's a problem for those who have everything grounded, so you really easy can touch Live and Ground simultaneously. But as for me - I don't have an earth or neutral exposed somewhere near me, so basically I'm more safe because I have Everything not grounded (I just can't touch a Ground). Otherwise I could have possibility to touch a ground/neutral and a live wire, which is really dangerous.
I have soldered something on a live circuit accidentally when I was a child. Since then I solder only in switched off circuits.

I mean, grounding makes more troubles than working without it. An isolation may fail but a grounding may fail too. I mean I feel unsafe near an exposed grounded metal because I can accidentlly touch it having touching something at some potential at the same time. But my work place made of wood and plastic so it is really safe if i wanted to work with live circuits (which I don't do, and I have an isolation transformer). I use my 400 VA isolation transformer to power up ALL my designs or repairs in progress, until case is not closed.

[Haenk]

this is a job for batteries or butane

you might actually get a good solder joint that way. trying to overload the iron is not the proper technique

Yes, don't try this stunt with a station. Actually I got a battery powered "pen soldering iron" from Lidl for AFAIR 10,- (downmarked). Only charged it up, never used it
And of course this is not for really finer pitch, even larger SMD would be a gamble.

[coppercone2]

the AA battery ones are OK just don't expect it to work a long time.

Butane is IDK, if you have a soldering iron calibrator to set it properly OK, otherwise the battery one is better.

Now the USB c ones are probobly pretty good and run on powerbank etc, but if its just like for the rarest of rare uses, the AA one is a better value IMO, because its cheap Built in lithium battery one sare better, so long it has replaceable battery, because u know you will use that once every 5 years

[Terry Bites]

You can temporarily ground the iron tip via a large value resistor to keep ESD and fault currents under control.

[soldar]

I wouldn't do that if I were you.

[RoGeorge]

Not the best idea, but I do that with the casual Arduino-like toys.

So far I didn't damaged anything, but I never do that with expensive boards.  I'm using a soldering gun, which is a galvanically isolated transformer, so no grounding issues.