Hi, there's actually a very big difference between a 4A/250V fuse
and a 4A/120V fuse.
The first one can take a 1000 Watt hit, whereas the other 480 Watts.

Even though your power supply is probally 235 Watts or less,
because of the high initial impedance it can draw a surge current
every time it is turned on.
This surge current can easily exceed 80 joules, and lay waste to
a fuse under 640 watts.

-yui shin



Mark Rode wrote:

> I was experimenting with a device I made  using the 12 volt leg of one of
> my AT power supplies as the power source. I inadvertently shorted the PS
> out...I know the cause..... and blew the PS internal SAE 4A 250VAC fuse.
>
> I had  a SAE 4A 120V fuse that I soldered on the board thinking that in as
> much that I will never flip that 250 volt setting switch on the back of
the
> PS and  would always  run at 115volts AC the fuse should be fine. However
> when I powered on I got a big blue light  and fuse was gone. Obviously I
> need a 250 volt fuse...but why if I am inputting 115 V AC?

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