Just killed my circuit with my soldering iron (discharge)

[T_guttata]

Hi

I just killed my circuit with my soldering iron (Weller WE1010). When I placed the solder tip, I saw a spark. Then I realized that the lab power supply was off but still physically connected to the PCB. As a result, some ultra low resistance mosfets are shorted. I did use the isolated output (not ground) of the power supply, but oviously there was a potential difference between ground of my PCB and the tip of my soldering station.

Next time I will obviousoy disconnect the power supply physically. But still: how can I prevent my PCB from being drstroyed in the future?

[Ian.M]

If the mains supply to your soldering station (or to other test equipment connected to your circuit) has a bad ground, you will get this sort of problem. 
See https://www.eevblog.com/forum/beginners/simple-fet-circuit-for-onewire-protocol-doesn_t-work-as-expected/msg1277272/#msg1277272

Fix the supply grounding problem, and always shut off *all* power and high level signals to whatever you are working on and these problems go away.  N.B. if you are soldering any circuit involving batteries that cant be removed, the whole board must be totally disconnected from everything else.

[Manul]

I did use the isolated output (not ground) of the power supply, but oviously there was a potential difference between ground of my PCB and the tip of my soldering station.

Your lab power supply probably has capacitors between isolated output and ground for noise filtering. It could be something like 100nF. There are two consequences: first, the output is only DC isolated, for AC, depending on frequency it might have quite low impedance to ground. Second, on some supplies the isolated output has tendency to develop potential relative to ground, which combined with that capacitance may cause a significant discharge.

[johnkenyon]

I have a "simple" 24VAC temperature controlled soldering iron with the electronics in the handle, had it for the best part of 30 years.
The "base station" is simply an IEC inlet plug, fuse+holder, 24V transformer, and a connector for the soldering iron.
The "ground" connection on the iron is connected to the incoming mains earth by a 1M (that's one megaohm) resistor in the "base station", the only reason there's an earth connection is to avoid any ESD zapping, and the resistor is there to drop any discharge current to a minimum. (the metal case of the box+transformer frame is directly connected to the mains earth.

[w9gb]

The Soldering Handpiece has a Grounded Tip.
Your circuit / appliance was NOT De-Energized OR Disconnected from Mains.
==
ANY Appliance with a Switch-Mode Power Supply (SMPS) should be Physically UNPLUGGED from AC Mains.
Power Switch often does NOT remove from AC Power, and SMPS can have >300 Volts.
==
In the Radio-TV Shop Era (1950s—1980s), some Technicians would have One (1) Soldering Iron with an UN-Grounded Tip (No Safety Ground), to work on HOT TV Chassis.  This Dangerous Practice (some techs injured or death by electrical shock) has largely disappeared, BUT still exists due to “Street Rumors / Word of Mouth”..

[xavier60]

I have a "simple" 24VAC temperature controlled soldering iron with the electronics in the handle, had it for the best part of 30 years.
The "base station" is simply an IEC inlet plug, fuse+holder, 24V transformer, and a connector for the soldering iron.
The "ground" connection on the iron is connected to the incoming mains earth by a 1M (that's one megaohm) resistor in the "base station", the only reason there's an earth connection is to avoid any ESD zapping, and the resistor is there to drop any discharge current to a minimum. (the metal case of the box+transformer frame is directly connected to the mains earth.

I do something similar. Problem is, unless the transformer has electrostatic shielding, I'd worry about mains transients capacitively coupling from primary to secondary.