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Why did my 10A fuse blow?

be.mihai22

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.

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  • 0

    Indeed. Thank you very much. I have a greater confidence now to repair the power supply.

    However, do you think that the fuse inside my unit is blown, giving the fact that the 10Amp in the cable is down?

    For the future, shall I use a smaller amperage fuse for my cord? I believe that a 10Amp fuse for this psu is overkill, as it will never blow for external reasons.

    I've got to this conclusion from a basic formula: Intensity = Wattage / Voltage, and as my sockets has a nominated voltage of 240 and the psu caps at 750W, 750W/240V gives a current of 3,125. Should a 3.5-4 Amps fuse be sufficient.

     

    Thank you very much.

  • 0 in reply to be.mihai22

    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

  • 0 in reply to jw0752

    Hi. I am sorry that I have not given full details about what's my problem. I read through this thread and I see it's quite vague.

     

    I am updating this with documentation and I hope do so from now on.

     

    Here is the switch 7 contact points of which I could've find just 6 because the way it has blown off The switch in cause is  TNY278PNTNY278PN with THIS datasheet.

    Unfortunately, the datasheet does not have a lot of meaning for me.

    image

    Next, I attached some video of it. Here:

    This video is unavailable.

    And photo of my PSU pcb:

    \image

    So you can see that "D506" writing, that is exactly the place where 4+3 Pins would have been before from this switch. I know the pcb looks messy, but I cleaned it with isopropyl. That mess is from the flux.

     

    More, I tried to spot the fuses on the pcb (and found only 1, which is blown):

    image

    So far we have this fuse which is blown inside the PSU and another fuse which is blown in the cable's head (which goes to socket). I've already ordered other fuses for sockets to replace these (from user suggestions).

    This last one which I discovered has a writing on it saying: "T10AH 250V" and the company logo.

     

    The PSU itself is "OCZ MOXSTREM-PRO 700W" with this bunch of reviews.

     

    I have to say that I had no control on choosing this PSU and it was offered me as a gift from my parents a few years ago.

     

    You might also ask why am I doing this and why am I trying to repair it?

     

    Well, the truth is that I am not really keen to repairing this, but to understand as much information as I can from this problem. I am going to replace the PSU entirely by the first of January.

     

    I am studying computer science with software engineering, but I've always been interested in electronics and never had the chance to understand it. So, there I am.

     

    If I make it work, I will donate it, otherwise, I will scrap the parts for other projects. You will see me around for a while image

  • 0 in reply to be.mihai22

    Hi Mihai,

    We had a real failure to communicate. When you said switch I thought you meant a physical switch and not a solid state switching IC. Now that I have seen your picture and your explanation I will say that there is a real possibility that the damage that you see was caused by an unseen failure somewhere in the circuit. Before the IC is replaced it would be prudent to test all the support components as well as checking the output of the circuit to make sure that there isn't a short on the output rails. Switching Power Supplies can be particularly difficult to service as they have feed back loops that can affect the circuits performance. Sorry that this happened and sorry I may have lead you astray with my previous explanations.

    John

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  • 0 in reply to be.mihai22

    Hi Mihai,

    We had a real failure to communicate. When you said switch I thought you meant a physical switch and not a solid state switching IC. Now that I have seen your picture and your explanation I will say that there is a real possibility that the damage that you see was caused by an unseen failure somewhere in the circuit. Before the IC is replaced it would be prudent to test all the support components as well as checking the output of the circuit to make sure that there isn't a short on the output rails. Switching Power Supplies can be particularly difficult to service as they have feed back loops that can affect the circuits performance. Sorry that this happened and sorry I may have lead you astray with my previous explanations.

    John

Children
  • 0 in reply to jw0752

    Thank you.

     

    It's cool. Do you think something outside the psu has provoked the damage? When I say outside, I mean inside the computer? Do you think it would be a motherboard failure?

     

    Otherwise, considering the descriptions, what are the real chances that the components inside my computer still work?

     

    Thank you again.

  • 0 in reply to be.mihai22

    Hi Mihai,

    I don't know but your computer itself is probably ok. Most power supplies are protected from external shorts or over current loads and a problem in the computer should not have caused this failure. It is possible that an unseen problem in the power supply caused this and that is why it would be a good idea to test support components in the power supply. For example the small black diodes with a silver band on one end could easily have been damaged by the 10 amp short that took out the fuse but externally they might not show any damage. They can be checked with a multimeter set on R X 1 Ohms. Switch the leads back and forth on the component and expect a reading in one direction and no reading in the other.  Since you have the IC coming if everything else checks out you can try replacing it. The worst case will be more blown fuses and another fried chip. Be cautious with the power supply as this type of power supply does have lethal mains voltage on it when plugged in.

    John