Why did my computer fry?

[Kerry]

While I was troubleshooting a battery charging circuit today, I was playing with my oscilloscope and noticed that the output of the transformer (~10 VAC) did not look like a sine curve, instead it was more like a trapezoid pattern (straight lines, square corners...).  Anyway - I measured the input side of the transformer and saw the same thing.  So I figured possibly it had something to do with the load from the circuit, and I removed one input lead from the transformer and went to plug the cord back into the wall (so I could measure the voltage at the easy-to-get to end of the wire).  As soon as the I plugged in the cord, there was a flash and a loud bang.  I immediately unplugged it, but of course the damage was already done.

At first, I only noticed that the 7805 in my battery charger circuit was split wide open, but then I realized that my computer had shut down... I had my programmer attached to the circuit board when I did this and as a result I fried my PC (seems like this might be an expensive lesson...).

So what happened?  I suppose I created a short through the PC?  I was pretty baffled since I didn't even have both legs of the AC connected to the transformer, thus the circuit shouldn't have had power.  I never had any problems connecting to my PC when both legs were connected to the transformer...

-Kerry

[Admin]

uhhh we need a schematic to figure out your mistake

That being said, you probably created a ground loop.

Your PC was *only* connected by a programmer? What programmer were you using?

What brand is your PC?

[Kerry]

Here's the schematic.  I'm using the AVR mkII ISP, and I was running a Dell XPS 1210 (only ~2 years old :-( )

The only connections between the circuit and anything else were the programmer, the scope, and the single lead of the transformer going to wall power.  The scope escaped unscathed.  There is maybe 1/2" of 30 gague wire in the circuit where the plastic melted off, and the programmer itself looks OK on the outside (I have yet to open it up).  I opened up the laptop and found a little 8-pin component with a hole blown in the top, near my USB port.  The hole in the top makes it difficult to read the writing, but it looks like a typical surface mount chip w/ 4 pins on each side...

-Kerry

[Admin]

Hmmm so the AVR mkII ISP is only for AVR, but your schematic shows a PIC . . . I'm assuming you used and AVR in its place

So if the only connection to the laptop was the programmer, that means something specifically happened to the ISP connection on the programmer. I recommend checking the wiring in that location for mistakes. You didn't include it in the schematic so I can't comment . . .

I also recommend opening up the programmer and look for anything unusual.

I almost feel as if a high voltage power surge went through your circuit, entered your ISP programmer, and passed into your laptop. After all, that 4700uF cap you have can definitely hold a charge despite everything being unplugged!

You should probably look into isolators to connect your ISP header in the future . . . 

[airman00]

So your laptop is totally dead? It won't boot up at all?

[Conscripted]

My guess is that the laptop will probably boot. Or could be made to. However I don't think that it's USB port will ever work again.

Conscripted

[Joker94]

My guess is that the laptop will probably boot. Or could be made to. However I don't think that it's USB port will ever work again.

You would hope so as there is more than one usb port, and it is better than losing a computer.

good luck finding the solution.

[Kerry]

Yes, I used an AVR in it's place.  My ISP connections I copied from the $50 robot schematic (graphical PDF version someone posted here).  How would I go about isolating this in the future?

The laptop did boot afterwards, the first time I tried it.  It got to the point where the bells chimed and the destop started to populate... then I heard something resembling rice crispies and a couple of hisses, then it went dead again.  Now when I press on the power button, the power LED comes on for 1-2 seconds, then goes off again.  Other than that - nothing.  But any recommendations on how to make it boot would be welcome :-).  I could certainly live with fewer USB ports if I can make the computer work again.

-Kerry

[kd5kfl]

raggedy sine waves are a characteristic of inductors.

when electronics go south, suspect static electricity first

[Admin]

How would I go about isolating this in the future?

Search mouser for optical isolators, or any type of isolator really. Anything that'll keep a huge voltage spike from passing through.

then I heard something resembling rice crispies and a couple of hisses, then it went dead again.  Now when I press on the power button, the power LED comes on for 1-2 seconds, then goes off again.  Other than that - nothing.  But any recommendations on how to make it boot would be welcome :-).

Those sounds are that of a short in your laptop, and probably chips exploding from all that heat. I recommend opening up your laptop and entirely removing the USB if possible. Also look for anything and everything damaged.

You might want to try a computer builders forum for more expert advice. Just tell them that 'a large voltage transient from an experiment, like 100V or so, passed right into my USB by accident'. And then explain the damage you see in your laptop.

[chelmi]

ouch, that sucks

My advice is to always shut down/disconnect power from your circuit when you're programming the AVR. And disconnect the programmer (or the serial/USB connection to your PC) when testing the system. Especially when troubleshooting a faulty circuit.
I know it's not always possible, but I sure it is most of the time... Otherwise, optocouplers are probably the solution.

[Kerry]

Some more info:  I found a similar IC to the one that blew inside my laptop near the other bank of USB ports - It looks like a TSP2062 USB power switch from TI.  They're available on Mouser.  I don't think this would keep the PC from booting, though - this would only be necessary to fix the USB ports.  But for $2 it's probably worth trying...  I'm still worried about being able to figure out what is keeping it from booting.

I opened up my programmer, too.  Looks like that was damaged as well.  Any ideas on what this is?  I can make out the numbers, but searching for them hasn't been much help.

Thanks!

-Kerry

[Admin]

I opened up my programmer, too.  Looks like that was damaged as well.  Any ideas on what this is?  I can make out the numbers, but searching for them hasn't been much help.

Unfortunately they don't put part numbers on chips this small.

Orange is often used for resistors, but it has a band on the top which could be used to mark polarity, in which case it could be a capacitor. High voltages are much more likely to blow a cap, so my guess its a 10x10^6 tantalum capacitor (or a 1x10^6, hard to know). Get out your multimeter and measure the resistance on that other orange one near by (or capacitance if your multimeter has that function).

If the resistance seems to keep changing as you measure it for ~10 seconds, that means its a cap (its being charged by your multimeter).



But my bet is its not fixable. High voltages can easily fry most ICs with no visible damage. Static electricity, for example.

[Soeren]

Hi,

Any ideas on what this is?  I can make out the numbers, but searching for them hasn't been much help.

It's a 10µF (10,000,000 pF) chip capacitor and it won't cost much to replace it, so it's worth a try IMO.