# Re: 3 blown Power supply's in 6 months.

w_tom
07-10-2005, 01:49 AM
Problem of using fuses for hardware protection is their very
wide range of 'protection'. To appreciate the problem, learn
about the well understood I^2t curve. For example,
semiconductor devices will fail in microseconds. A
microseconds transient that exceeds the fuse current ratings
by factors of tens will not even blow the fuse. The fuse
takes tens of milliseconds (or longer) to blow. Fuse only
blows long after damage has occurred.

Fuses will do better on equipment that can take massive
overcurrent for a long times, such as large motors.

Review the numbers. For example, a 20 Amp fuse for
automotive protection would conduct 100 amps for 0.5 seconds
before opening. Or 40 amps for 9 seconds. And this assumes
ambient temperature is constant. The fuse must be sized to
not blow also at over 100 degrees F. However the same fuse
will now require 10% or more current to blow in winter
temperatures. Then there are waveforms for that overcurrent.
Then the manufacturer recommends adding another 20% margin of
safety.

We have assumed an ideal overcurrent - a square wave. That
100 amps might take twice as long to trip a 20 amp fuse if
current only averages 100 amps but really peaked at 200+ amps.
Variations mean that sizing a fuse for hardware protection
becomes difficult if not impossible.

This 20 amp fuse would be selected typically for loads that
may peak around 10 amps or less just to avoid nuisance
tripping. And yet a 100 amps short circuit is required could
make the fuse open in less than one second. IOW the fuse of
for protection after damage has occurred.

For UL approval, a fuse must open at 135% of rated power
within one hour. It must open within 2 minutes is the
overcurrent is 200%. That means to protect a 1 amp load, the
two amp fuse (the smallest selected so as to avoid most
nuisance tripping) would conduct 4 amps through the protected
appliance for minutes. Or 10 amps for 0.1 seconds. Ten times
more current through a 1 amp item - where is the hardware
protection? Does not exist.

Fuses were not for hardware protection. Fuses are installed
first and foremost for human safety. These are numbers.
Others are invited to demonstrate knowledge of numbers rather
than post "horseshit", "lamer", and other technical facts.

Fuses are excellent at stopping wire fires. The standard 20
amp electric wire should handle hundreds of amps without
damage (and fire). Long enough for a fuse to eventually open.
But fuses are woefully insufficient at avoiding appliance
damage. That I^2t curve, well understood by those with basic
electrical background, demonstrates the problem. Time even
tens of milliseconds for faster fuses is woefully too long to
protect electronics - such as a power supply.

Fuses don't even claim to protect appliances. Fuses are
installed to blow after a failure has happened. After current
well exceeds what is normal rated consumption. The fuse
provides human protection after damage has occurred. Above
numbers demonstrate that fact. Numbers rather than insults
demonstrate that fuses do not protect hardware. Fuses blow
after damage has occurred; for human safety.

Richard Urban wrote:
> I want the fuse to blow before damage to the hardware occurs. If
> the fuse is sized for the hardware's optimal current draw, that
> is what will happen. Then the hardware can be electrically
> repaired, the fuse replaced and the machine is again happy.
>
> By the way, I have repaired thousands of machines over the years
> that have blown fuses. With few exceptions they were all
> repairable and never again blew another fuse. And I am happy to
> say that no one died in the process. I guess the fuse saved their
> lives!
>
> That is great, but I still fuse to protect the equipment.