Why no fuses?

Bluefront · Post by **Bluefront** » Thu Jul 29, 2004 4:15 pm

Maybe OT concerning quietness but......Can somebody explain to me why there are very few examples of fused circuits in PSUs, or on the mainboard itself. Seems to me a few fuses here and there on the different voltage lines, would be a good thing....maybe even a safety factor. Surely a small fuse panel with replaceable fuses on the PSU couldn't cost very much. Might even be a good selling point....

wim · Post by **wim** » Thu Jul 29, 2004 8:55 pm

not sure what you're talking about, every psu i've opened has had a fuse in it (iirc), even the really cheap PSUs (usually just one fuse though)

i've even blown the fuse while doing a relay mod, and replaced it, to get working psu again.

lenny · Post by **lenny** » Thu Jul 29, 2004 11:57 pm

I think bluefront means individual fuses for the different rails. I think wim is referring to one fuse for AC.

My guess on why there are no individual fuses : if one rail's fuse blows while the other rails are still functioning (e.g. you lose 12V while 5V and 3.3V are still being supplied), it is a Really Bad Idea.

Bluefront · Post by **Bluefront** » Fri Jul 30, 2004 2:41 am

Actually I was thinking of fusing each rail with a smaller amp fuse, all of which could be supplied by a circuit breaker, which would open if any of the rail fuses blew. This would make the thing more sensitive to smaller problems that might not trip the larger main fuse.

Modern autos are wired like this......in a PSU the whole thing could be the size of a pea. Even the replaceable fuses these days are tiny.

sthayashi · Post by **sthayashi** » Fri Jul 30, 2004 7:11 am

Exactly what benefit would an individual fuse provide? I can think of several downsides.

1) Size and replacement. Instead of having 1 thing that goes boom, you have 3. The retailer and manufacturer's RMAs could easily go up with something like that.

2) Current spikes are okay. It's alright if a computer draws a lot of power briefly, assuming that the PSU can otherwise handle the peak load. Fuses don't like power spikes like that. At least not fast acting fuses (dunno how slow-blow fuses like power spikes).

nbac · Post by **nbac** » Fri Jul 30, 2004 7:26 am

Besides, it's very hard to pick the right fuse value, as it's
a very fine line between what current is high but ok, and what
is not.

Fuses are usually there to protect the wiring and other
components from burning up completely, not necessarily
to protect them from damage at all.

Third, fuses add some resistance in the current path, and
are bulky (just have a look at high power amps for cars)

Bluefront · Post by **Bluefront** » Fri Jul 30, 2004 2:42 pm

All right....here's what happened the other day. I plugged in a brand-new pci card and when I attempted to boot up, a trace on the top of the card smoked. Fortunately I had the panel open, saw the smoke and yanked the power cord. Lucky for me everything except the card survived.

That should not have happened. I should have blown a fuse somewhere....either in the PSU, on the MB, or on the card itself. If any of those components had a small fuse, that almost-disaster could have been avoided.

Maybe the PSU is the wrong place for a fuse panel, maybe it belongs on the case....but it belongs somewhere.

Splinter · Post by **Splinter** » Fri Jul 30, 2004 3:20 pm

It's because by the time the fuse could melt and break the connection, the damage would have already been done.

It takes a few milliseconds for a fuse to break, but only a few microseconds for an IC to die.

Bluefront · Post by **Bluefront** » Sat Jul 31, 2004 3:34 am

FWIW.....let me explain what a fuse really does. It does not protect a bad component from damage. It protects the rest of the system, the wiring harness, maybe any insulation the wires are touching, the motherboard in the case of a computer, the PSU. All these other things are protected when a fuse blows. The bad part is toast....everything else survives.

With the current computer setups, the fuse in the PSU may not blow, even though the rest of the computer is smoking.

energy · Post by **energy** » Sat Jul 31, 2004 5:17 am

The PSU has automatic solid state over current protection on each of its rails. All this can do is protect the PSU from damage in the event the total current flowing is higher than what it can handle safely & reliably. If something happens and a short occurs such as a faulty card being inserted theres not much the fuse (whether a traditional fuse or solid state on each rail) can do except perhaps prevent too much damage from occuring.

Lets say a particular component draws 0.5A from the 5v line under normal operation but for some reason 2A flow through it. It will burn up the component in an instant but the PSU has no way of knowing where that power is going and what for. All it knows is that it can supply up to 30A on the 5v rail and 2A is nowhere near that maximum so it won't blink an eyelid.

Bluefront · Post by **Bluefront** » Sat Jul 31, 2004 9:02 am

energy is right.....but computers need not be limited to fuses on the rails of the PSU. It would be easy to fuse the different parts of the MB for example. Separate fused circuits on the MB would be easy for an OEM to construct. Say a 1A or 2A fuse controlling a fan header, or a PCI slot(s) with its' own fuse.

Right now a few boards have a replaceable fuse for the on-board USB connectors. But that's about it as far as I know....

greeef · Post by **greeef** » Sat Jul 31, 2004 9:19 am

sorry to be cynical, but i think psu manufacturers would rather not bother. The average user wouldnt want to deal with fuses, RMAs would increase, people would put in fuses of the wrong value etc.

It's still a good idea.

Inexplicable · Post by **Inexplicable** » Sat Jul 31, 2004 2:29 pm

Bluefront wrote:Right now a few boards have a replaceable fuse for the on-board USB connectors. But that's about it as far as I know....

Well, AOpen at least seems to be advertizing "CPU overcurrent protection" and "AGP protection" on their mobos. Here it makes sense, because the mobo has its own voltage regulators for them. Other manus probably have similar solutions. I suppose it would be possible to protect the power supply to the PCI bus in the same way, although this would mainly just protect the leads on the mobo that actually supply the current.

However, as someone already pointed out, fuses and cutoff circuits aren't very good at protecting things from blowing up (especially delicate digital circuits), they are there mainly to prevent the thing from becoming a fire hazard. The first order damage is already done when the fuse blows.