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Thread: 5A Fuse Blows

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    5A Fuse Blows

    I have just upgraded the graphics card on my pc to a XFX Geforce GTX280 XT and consequently the PSU, as my old one wasn't man enough, to a BFG ES-800W.

    The problem is the fuse in the kettle lead keeps blowing. I plugged the kettle lead which came with the psu in and the pc worked fine, but the next time I tried to power it up there was no power at all. My first thoughts was the fuse so I change to my old kettle lead (also 5A) which worked fine the first time, but wouldnt start the next. It is definitly the fuse as I put a new 5A fuse in the lead and it worked, again for one time only. As for the cause I realy dont know.

    One of my concerns is the graphics card isn't getting enough current, as the box states minimum power requirements for the card is 630W (non-sli) with a current rating of 40A****. Not sure what '****' means as it doesn't say anywhere. But the power supply has only a maximum of 36A on some of the leads. I dont realy understand this part so I could be wrong and it is ok.

    My system is:
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    It is a double wall socket, I am plugging the PSU directly into its own socket and in the second socket I have a 4-Socket Multilead adaptor with a 32" Sony LCD TV, 22" Samsung LCD Monitor, D-Link Router and an Xbox360.

    Please give oppinions and advise on what I should do.

    Thanks
    Last edited by GarethG; 30-10-2008 at 02:17 PM.

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    Re: 5A Fuse Blows

    13A fuse...




    A 5A fuse will blow at 5A (based on 240V)

    Power = Voltage x Current

    800W = 3.33 amps, which seems reasonable to fit a 5A fuse. But for some reason the fuse is blowing suggesting your rig is pulling more than 5A
    240V


    Edit: Don't worry about power requirement for the graphics card. 40A on a 12V system

    Mains = 240V / AC
    Inside the PC = 12V / DC

    Power outside the PC (800W) = 240 volts x 3.33 amps
    Power inside the PC (800W) = 12 volts x 66.6 amps (theoretically)

    PSUs are typically 80% efficient = 66.66 x 80% = 53.3 amps available power, typically.


    This is a very amateur diagnosis, and I'm not really sure why the fuse should be blowing, surely it would mean the PSU is over working?
    Last edited by cptwhite_uk; 30-10-2008 at 02:21 PM.

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    Re: 5A Fuse Blows

    Quote Originally Posted by GarethG View Post
    One of my concerns is the graphics card isn't getting enough current, as the box states minimum power requirements for the card is 630W (non-sli) with a current rating of 40A****. Not sure what '****' means as it doesn't say anywhere. But the power supply has only a maximum of 36A on some of the leads. I dont realy understand this part so I could be wrong and it is ok.
    The *s will probably be refering you to different combinations and limitations on rail combinations - there should be a chart saying something like 460W combined for all the 12V rails or so.

    Regarding the kettle lead I'm not sure your power supply is working correctly. 800W on a 240V supply is only 3.3A, so a 5A fuse SHOULD be enough. If it's pulling down more than that then I'd be very suspicious.

    Nice ninja edit cptwhite

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    Re: 5A Fuse Blows

    The only thing I can think of is some sort of earth leakage or short circuit within the powersupply. I cant use my old one either as it doesn't have enough power for the graphics, and the reason I changed my old graphics card is because it suddenly stopped working while viewing e-mails.

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    Re: 5A Fuse Blows

    If the 5A fuse is blowing, I'd say it was saving your system from some major badness.

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    Re: 5A Fuse Blows

    Quote Originally Posted by Fraz View Post
    If the 5A fuse is blowing, I'd say it was saving your system from some major badness.
    Agreed. Don't, whatever you do, be tempted to put a 13A fuse in! There is a problem somewhere you just have to find it.

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    Re: 5A Fuse Blows

    Yeah im studying electrics and have already done electronics, I'd never think twice about using a 13A fuse in a pc.
    As far as it goes to fault finding on a pc though i don't have a clue especially on power supplies. With my old graphics card suddenly breaking it makes me think there might be a problem with the motherboard. But im not sure if this would effect the primary winding current in the psu transformer.

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    Re: 5A Fuse Blows

    Even if it was a problem with another component, there *should* be safety systems in the PSU to prevent it from either pulling down too much over the mains or over-supplying itself. That these aren't working is very worrying to me.

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    Re: 5A Fuse Blows

    Quote Originally Posted by GarethG View Post
    Yeah im studying electrics and have already done electronics, I'd never think twice about using a 13A fuse in a pc.
    As far as it goes to fault finding on a pc though i don't have a clue especially on power supplies. With my old graphics card suddenly breaking it makes me think there might be a problem with the motherboard. But im not sure if this would effect the primary winding current in the psu transformer.
    IIRC, my corsair PSU lead was supplied with a 13a fuse...
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    Re: 5A Fuse Blows

    IIRC, my corsair PSU lead was supplied with a 13a fuse...
    For what im about to say it applies only to a mains voltage of 230V (the UK standard).

    Im not saying you shouldn't with all PSU's I mean with mine, it depends on the power and design of your PSU (in other words stick to manufacturors design). A 13A fuse would allow for a 2990W PSU. But for my 800W psu I could get away with using a 4A fuse and still it shouldn't blow under maximum load. This is because Power = Current x Voltage. You should also note that the cable's current carrying capacity should be >= the value of the fuse and never less. Thats why I shouldn't use a 13A fuse unless I want a fire to burn down my house when a fault occurs.

    Correct me if im wrong

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    Re: 5A Fuse Blows

    Quote Originally Posted by GarethG View Post
    A 13A fuse would allow for a 2990W PSU. But for my 800W psu I could get away with using a 4A fuse and still it shouldn't blow under maximum load. This is because Power = Current x Voltage.
    Just remember though that your PSU won't pull 800W it will generate up to 800W DC at whatever it's efficiency is. If lets say the efficiency is 80% when drawing 800W that will try to pull 1000W from the socket and at 240V that would be 4.16A which would blow the fuse. The 5A one should be enough though and you'll never likely use that 800W

    PS - Could it be that something inside your PSU isn't earthed properly and is causing the big current draw? I never realised that a 5A fuse could be used for it.

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    Re: 5A Fuse Blows

    Couple of points - the plug fuse is designed to protect the connecting lead to the appliance - the PSU should be protected with its own fuse - probably internal.

    The power supply will not be 100% efficient - so if it is running at 800Watts output at 80% efficiency - typical figure) it will be drawing about 1000 Watts input.

    Although power is Voltage*current, this is an AC cct and so power factor has to be taken into account. Typical power factors are about 0.8, so power will be V*A*.8 - so assuming that the 800 watt power supply is delivering the maximum power (800 watts and drawing 1000W) the current drawen from the UK power supply (nominally 230V) will be 5.4 amps. (5.4*230*.8). This is worst case scenario - pessimistic power factor and PSU at maximum load, but you get the idea)

    A typical plug top fuse will blow instantaneously at about 2.5 times its rated current (as I said earlier, it is designed to protect the cable - typically against catastophic failure - a short cct) but if it is overloaded continuosly, it will eventually fail - which is what is happening here. Futrther more, there will be a switch on surge when the PSU is powered up, again overloading the fuse. (Inrush current could be 10 amps for a brief perid)

    Bottom line is that the minimum fuse value should be 7 amps - 10 would be the optimum, but it is unlikely that any harm would come from using a standard 13 Amp fuse.
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    Re: 5A Fuse Blows

    it could be killing the fuse when you turn it off because of back EMF caused by the transformer coils..

    also, mains voltage is variable: supposedly: 230 V +10% −6% but there are vspikes of 300v odd (all dependant on the step down transformers supplying the power)

    and even if you did put a 13a fuse in, if there is a short then it would easily hit 13a and kill that fuse... but if it is just the PSU's operational parameters causingthe >5a current then the 13a would mean that you can turn it on more than once...

    and TBH, 5a isn't really that much when we're talking about transformers...
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    Re: 5A Fuse Blows

    Quote Originally Posted by TAKTAK View Post
    it could be killing the fuse when you turn it off because of back EMF caused by the transformer coils......
    Er - no! Its a switched mode PSU so tthere is no transformer connected to the direct mains input - the transformer operates after the incoming mains is rectified! (But even if it was, there would be little back emf anyway as there would be little stored energy in the transformer under load conditions)

    But as I said in my previous post, a 5A fuse is under-rated - change it - ideally for a 10A, but if you haven't got a 10A, use a standard 13Amp.
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    Re: 5A Fuse Blows

    Quote Originally Posted by peterb View Post
    Er - no! Its a switched mode PSU so tthere is no transformer connected to the direct mains input - the transformer operates after the incoming mains is rectified! (But even if it was, there would be little back emf anyway as there would be little stored energy in the transformer under load conditions)

    But as I said in my previous post, a 5A fuse is under-rated - change it - ideally for a 10A, but if you haven't got a 10A, use a standard 13Amp.
    aaah, fair do's, i've never looked into AC-DC conversion (next week i am IIRC) TBH and just presumed it would do the step down first and then the ADC
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    Re: 5A Fuse Blows

    I thought 13A was the norm for PSU leads?

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