So today, my VGA port caught fire. - by Ostriig
Ostriig on 26/2/2010 at 22:41
Well, no flame, but the smoke and smell were there. I had to give a presentation earlier today, so I hooked up my laptop to the room's projector. My laptop was running on battery, the VGA socket I connected it to was not the projector's own, but a wall-mounted socket which led through the wall and then the ceiling to the projector's one. I connected the cable and leaned over the keyboard to bring up my .ppt, and I was immediately attracted to the smell of burned plastic or hot iron, do not remember clearly, and sure enough there was smoke rising from the VGA port on the side of my computer. I yanked the cable out and that seems to have been all the visible damage, at least, haven't had a chance to test the port so far. I'm gonna have to give it a spin, with that same cable (which I've had and used for some two years now), with a screen I have home.
Anyway, I'm curious if anyone around here with a greater degree of knowledge in electronics knows what might have happened. I seriously doubt the possibility of there being something wrong with the laptop's port, or the cable, or does anyone know anything to the contrary? My best (and only) theory so far is that there's something screwed with that wall socket, and that it showed up with my laptop because of the lack of grounding (running on battery), which would not be the case with the desktop computer it's usually hooked up to in that room.
Al_B on 27/2/2010 at 10:41
That's what I call a smokin' presentation :)
Seriously though, it's possible that the cable between the wall box and the projector is faulty or sub-standard. From memory, it should have around a 75 ohm characteristic impedance but if for some reason it's not and presents a serious mismatch to your laptop then I could see it causing damage to the VGA output drivers.
However, that's still just a guess and someone else here may have seen this problem more frequently. It's not one that I've come across myself.
I don't think that running from battery would be your problem, however. In fact, by doing so you have less chance of ground loops - unless your laptop had another route to ground through something else you had plugged into it.
bikerdude on 27/2/2010 at 20:46
Quote Posted by Al_B
That's what I call a smokin' presentation :)
Seriously though, it's possible that the cable between the wall box and the projector is faulty or sub-standard.
If the cable/projector were not yours, then owner of said projector/cable should be made aware of a potential fault. And if any damage has been caused to your laptop is their liability.
Ostriig on 27/2/2010 at 23:05
Quote Posted by Al_B
That's what I call a smokin' presentation :)
One could say it was made of
hot stuff. :P
Quote:
Seriously though, it's possible that the cable between the wall box and the projector is faulty or sub-standard. From memory, it should have around a 75 ohm characteristic impedance but if for some reason it's not and presents a serious mismatch to your laptop then I could see it causing damage to the VGA output drivers.
But what sort of a mismatch could there be with the laptop's port, but not with the desktop it's usually connected to? I mean, with Q=UIt, can I assume that the tension on the circuit is the same regardless of whether it was the desktop or the laptop plugged in? I'm guessing so, if plugging in the laptop gave me a different U on the line, that would manifest itself with other projectors or screens, wouldn't it? If U is the same, then an increased intensity would suggest that the resistance of the cable is below standard, but then why would this not also show with the desktop?
I'm not contradicting you, by the way, I'm just trying to understand why it would show up with my machine but not the usual one.
Quote Posted by Bikerdude
If the cable/projector were not yours, then owner of said projector/cable should be made aware of a potential fault. And if any damage has been caused to your laptop is their liability.
No damage of any sort, thankfully, the only thing left is the smell or burned something around the VGA port. Just tested it with an LCD, with the same cable, and it's all working just fine. I had a lecturer and a member of the lab staff present, so I'll tell them when I next see them that everything seems to be in order with my gear and that the problem is likely on their end if they wanna look into it.
Still, I gotta say, it's not every day that you get to extend it to people and say "smell my laptop!"
Al_B on 28/2/2010 at 00:47
Quote Posted by Ostriig
I mean, with Q=UIt, can I assume that the tension on the circuit is the same regardless of whether it was the desktop or the laptop plugged in?
Sorry, you've lost me. Q=It is the actual relationship, but I've heard it taught with the "U" to help it to be remembered as a mneumonic. By itself it doesn't explain the voltage (tension?) present and I doubt that's the damage mechanism.
My hunch (and it is really just a hunch) is that the length of cable involved, quality of cable, input impedance of the projector and the frequency of the signals (resolution and refresh rate) of your laptop had enough of a mismatch to cause reflections which was starting to cause damage to your laptop output. Perhaps the desktop uses a lower refresh rate or is more tolerant to the mismatch.
Quote Posted by Ostriig
I'm not contradicting you, by the way, I'm just trying to understand why it would show up with my machine but not the usual one.
No problem - I'd be lying if I said I knew for sure what the cause of the issue was.
Ostriig on 28/2/2010 at 02:50
Sorry, I studied electricity some six years ago, I've forgotten a lot of stuff. I had to look up the formula for the Joule effect on (
http://en.wikipedia.org/wiki/Joule%27s_laws) Wikipedia, and that's where I got the Q=V*I*t for the heat generated in the conductor ( I used "U" out of habit, it's how we mark potential difference in Romanian instead of "V", which as I recall we use in electricity for a specific potential at a point; same with the term "tension" being more common). I went for the expression with voltage in it assuming that it would have to be the same in both cases (laptop, desktop) to allow me to take the resistance out of the equation.
Though you're right, there's a lot more going on in there, and I know nowhere near enough to make heads or tails of it.
Al_B on 28/2/2010 at 14:37
Ah - that makes sense. Q is also used to represent charge and Q=It is another electronics equation which didn't really seem to be applicable in this case. (For good reason as it turns out!).
Kerrle on 28/2/2010 at 22:48
If things weren't grounded properly, maybe a difference based on where "the usual" desktop is placed and plugged in?
Ostriig on 1/3/2010 at 22:01
I think Al's right, I might've kneejerked about the grounding issue - the laptop wasn't at all connected to the ground, it was running on battery, sat on a wooden table, and indeed the only connection it had to anything was through the VGA cable itself.
DJ Riff on 3/3/2010 at 08:38
Seems like the shield of the socket is not grounded and has some potential or electrostatic charge on it. When you connect it to desktop PC, it gets grounded through the PC case without any effects. But if you connect some non-grounded device, it may cause a discharge or breakdown between the shield and digital ground pin.
