blew up AD2 and computer

[dave slagle]

I have no idea of what happened here but I would like for it to not happen again.  

I was measuring the output of a vacuum tube circuit using input transformers for noise with the input shorted.  I then unshorted the input and when I connected the + of the signal generator BOOM...  and a visible flash from inside the AD2.  It was clear this thing was toast but then I noticed that the 27" imac is was running on also was off and will not reboot.

FWIW... I have Zener diode clamps on the input to the AD2 but no protection on the signal output side and by all accounts the primary of the transformer connected to ground should prevent any possibility for DC.  I am at a loss for what the cause could have been.

The other thing to note is all of the charring and burnt up bits in the AD2 are right by the USB interface which is on the complete opposite side of the board from where any input from the DUT would be.

I am not 100% sure the thing blew up at exactly the time I connected the DUT to the signal generator and I am really interested in a possible cause since blowing up a $400 sound card isn't too painful but taking out a computer with it is another story altogether.

I'll let the pictures tell the rest of the story and any thoughts greatly appreciated.

 

[zygot]

1 hour ago, dave slagle said:

I'll let the pictures tell the rest of the story and any thoughts greatly appreciated.

Ouch! Unfortunately, the picture, as gruesome as they are, don't tel the rest of the story.  A diagram showing the circuit that you are trying to measure might help. Points of contact for your measurement would be good.

It's easy to forget things and do something stupid... with bad consequences. I can't tell from the information provided but it certainly seems like a  power source issue on the face of it.

Powered USB HUBS are wonderful things. While not a 100% safety guard against unexpected or silly operator errors, they do offer significant protection for expensive things, like a PC. Replacing a $30 hub is painful but not as painful as replacing a burned out PC. USB 2 PC ports can source about 500mA at 5V. USB 3.2 maybe 100 W. Either one has the potential for serious damage if abused. When I read "measuring the output of a vacuum tube circuit" I'd think about being very careful about what I'd do.. and think it over a few times with protecting my investments as a high priority. 

As you now know forensic analysis is not something that anyone should want to do.

[dave slagle]

thanks for the reply

I found a probable explanation.  The input that was hooked up was a series string of transformers on a circuit board and I had missed one solder connection so the wire was just a loose wire through the hole.   when you add a signal generator to a series string of inductors, a momentary open can generate a seriously large voltage spike.  

The real curiosity for me was all the damage happend at the USB side and not at the signal side.

 

dave

 

[zygot]

"Seriously high voltages" and reactive loads are not to be trifled with. If a voltage source, like ESD or other high voltage source is introduced into a circuit, it's going to find the least resistance path(s) to ground or arc to where ever that is.

This is why there are high isolation step down transformers and hgh impedance FET probes. Some lessons are expensive, as you've found out.

On the positive side, your willingness to post about your experience may encourage other readers think carefully about how to deal with dangerous circuitry.

I've seen capacitors fail explosively and blow large holes through circuit boards when the logic power supply can source enough current.

So today's lesson is: reactive loads, high voltage or high current power supplies are things you don't want to take risks with. Flying leads connected to ground or power supplies will find their way to places where you don't want them. As Ben Franklin said, "an ounce of prevention is worth a pound of cure". Smart guy Ben was... Edited May 8, 2023 by zygot

[attila]

Hi @dave slagle

I'm sorry to hear that.

Probably your circuit was shorted/discharged through the USB cable.
The FBs on VBUS&GND usually act as a fuse, but the voltage may have been so high that the components and PCB traces fused.

Always ensure proper grounding between devices and circuits. Use isolator for safety, especially when dealing with high voltages. The isolator also eliminates ground loops.