Tuesday, September 20, 2005

Electrical Fire

I come home last night from a long day at work and night school to a very unpleasant surprise. I start to walk towards my room to put my cell phone on its charger, and that’s when it hits me. It is a familiar, God-awful smell. My years as an electronic technician have taught me all too well what that smell was. It was the smell of an electrical fire.

I quickly open the door to my bedroom and wham; the smell is like getting whacked in the face with a bat. My eyes start watering as I looking around frantically. Was it TV, my Entertainment Center, my alarm clock, I keep looking, but I couldn’t see any flames. I walk towards my computer and the smell is stronger. Then all of a sudden I can’t smell anything. By this time everyone in the house is alert to the smell. I think to myself “Damn it, I need to find what the hell is burning”. The fact that I cant smell anything is not helping the situation, and although it is a good thing that flames were not visible, it would have helped if I could see what was burning. I unplug both the monitor and the tower just to be safe, and step outside to get my sense of smell back. I hold my breath and walk back in heading straight to the computer. I exhale and then take a sniff near the tower. Bingo, the smell is coming from the power supply. I take the tower from the room and tear it apart. I pull the smelly power supply from the case and rip it apart. A capacitor falls to the bench and that burning smell punches me in the face. You never forget that smell, its something like sulfur mixed with Ben-Gay.

The first thing I check is the fuse. It was still intact. The safety mechanism that is supposed to prevent too much current from entering into the power supply failed. This could have been a lot worse. Thankfully, it wasn’t. The power supply could have started arching, shooting sparks and igniting something near by setting the whole house on fire. The scary thing was that the power switch was in the off position.

My best guess is there was a short inside of one of the filter capacitors (the one that imploded and fell on my bench). Lets take a look at some of the basic physics involved with this. To understand what happens here, we need to use Ohms law. Ohms law states that E=IR, where E=voltage, I = current, and R= resistance. With some basic algebra, we can find the amount of current traveling through a circuit. The modified formula we will use is I=E/R. Our wall outlet is providing us 120 Volts. Lets say theoretically that the resistance down this leg of the circuit is 100 Ohms. That leaves us with 1.2 Amps. What happens in a short circuit is that two portions of the circuit get connected in a way that bypasses this resistance. The resistance in the newly created short circuit approaches nearly 0 (there is always some resistance present). Lets say that the ambient resistance in the circuit is 500 mOhms. The current would immediately jump up to 240 Amps. For a circuit rated at 1.2 Amps, you suddenly are pushing 200 times the rated current. Imagine trying to run the Mississippi River through a garden hose and you get the picture. The increase in current will either cause an explosion or so much heat will be generated that it will ignite. This example is just an exagerated demonstration and in reality the current in the power supply was probally close to either 1.5 to three times the rated current before the components went critical and melted. Usually the fuse will blow before that happens, but in this case when I tested, there was still continuity in the fuse.

This is the capacitor that fell out.
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And here is a picture of the spot where the capacitor resided. See all that yellow and grey paper stuff? It was what was inside of the capacitor.
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And here are some pictures of the scorch marks where a diode got fried around some resistors.

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The important lesson here is do not assume that if something is off that it is safe. The only real way to protect it is to unplug it completely.


Richard Bejtlich said...

Hi John,

I'm glad you didn't suffer any real damage from the incident.

PS: Be a man and turn of comment word verification! Don't you like posts from "Mia" and friends saying they like your blog, go visit theirs?

Anonymous said...


I just got to know your blog yesterday (through a link on the Taosecurity blog! :-D and I really really like it, I know this blog is going to teach me valuable things, just like Richard's! :-)

I hope you don't mind me pointing out a minor typo? Because I think you meant "100 Ohms" instead of "120 Ohms", as otherwise your equation and the other calculation aren't correct.

But the most important thing is that you're safe and sound and your house and belongings still intact (apart from that power supply! ;-)

John Ward said...


Thanks for pointing that out. Thats what I get for posting late at night :) I caught some more questionable grammer when I re-read it.


Anonymous said...

any chance that the cap is the culprit?


What is the condition of other caps on the pcb?

John Ward said...

The other caps looked fine physically, although I did not test them since I just threw the whole PSU in the garbage. A few people had suggested filing a claim with the manufacturer, but no harm no foul is the way I am looking at it. A new PSU is only about 20 bucks. I remember reading about the faulty caps before. I am pretty sure the cap is the culprit since it is the piece that actually burst. Good link though, Ill be leery of Abit in the future…