Why did my 10A fuse blow?

Question

Hello everyone!

I had a really strange incident yesterday. I was playing a game on my desktop PC yesterday and suddenly I heard something blowing up from my computer.

The control panel went off for all sockets in the house. Moreover, the 10 Amp fuse from my power supply power cord blew too.. Unfortunately that was not everything.

I changed the fuse and tried again. Nothing. I disassembled my power source, shorted all capacitors to make sure I don't fry my fingers and while doing so, I saw a black piece of silicone falling of. Little did I know it was from an AC/DC Off-line switch.

After visual analysis it was clear, the switch blew and left smoke on the nearby components.

I took of the switch today and I am expecting a new one from farnell in a few days.

My question is: How did this happen? Is it because I had a 10Amp fuse for a 750W power supply and there was a high current from outside which blew it up... or... the other way around?

Thank you in advance.

jw0752 · Accepted Answer

Hi Mihai,

It is your call on the fuse size in the cord. The fuse is really to protect you from fire and 10 amps is a good round number and probably relates to the safe capacity of the cord. The fact is if a fuse is blowing some damage has already been done in the equipment. The fuse just stops the damage from becoming a fire or a greater catastrophe. I endorse your idea to salvage parts and get a new power supply. I hope your computer is OK and I suspect that it is.

John

jw0752 · Answer

Hi Mihai, The most likely culprit was poor contacts in the switch itself. Poor contacts equal resistance. Voltage across resistance means heat which just makes the poor contact situation worse. Over time the materials that are supposed to insulate the power leads from one another carbonize and become resistors themselves. When enough heat is produced to vaporize some of the carbonized material an arc of electricity will bridge the two power wires and the heat and high current will blow fuses and create a small blast mark on surrounding circuitry. The good news is that this seldom does any damage to the rest of the circuit. You may have to repair the circuit board if it was damaged. All blacken material must be removed. Sometimes epoxy can be used to fill in holes and gaps. John

jw0752 · Answer

Hi Mihai, Your power analysis is good. One consideration on choosing a proper fuse is that many power supplies have a higher current draw as they turn on. This is caused by capacitors which have to charge and therefore look like a much lower impedance until they come up to charge. My best guess is that a 5 or 6 Amp fuse would be adequate. I do not know on which side of the failed switch the internal fuse of your power supply lies so I do not know whether it has failed or not. If there were two fuses in the circuit that took out the 10 amp the second one may also be out or damaged. However I find it unlikely that the manufacturer would have put a 10 amp fuse in a 750 watt supply designed to run on 240V. Keep in mind I have made a lot of assumptions in my initial analysis of your described failure. For one thing most AC/DC switches that I am familiar with do not actually have AC voltage on them. They switch between a DC battery voltage and the DC output of an AC Power Supply. I frankly would like to know more about your AC/DC/Off Line switch and how it is being used, where it is in the circuit and how it is wired. John

Why did my 10A fuse blow?

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