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Omega Electronics White Paper
Fuses
Why didn't the fuse blow? Why did the fuse blow? Should I replace the fuse? Should I use a smaller use? Should I use a larger fuse? Should I use a different type or style of fuse? This fuse says 250V and the original fuse said 125V-can I use this one? These are some of the questions I hear and I will attempt to provide satisfactory answers to these questions in this paper.
Preliminary information
First some preliminary information. A fuse is a current
operated device. The fuse is an isolating device that couples power from
one source to another and interrupts that current flow if the
destination begins demanding too much. The three most important factors
in choosing a fuse are the voltage value that must be interrupted
1 and the current flow to be interrupted 2, and how fast the current must be interrupted
3. Its goal in life, like the policeman, is to serve and protect.
There are two types of electricity, Alternating Current (AC)
and Direct Current (DC). Draw an imaginary line, and call it zero
or ground. Any voltage above that line is positive voltage and any
voltage below that line is negative voltage. If the voltage never
crosses your zero reference then it is Direct Current voltage. If the
voltage swings (alternates) between positive and negative it is
Alternating Current voltage. In order to compare the general value of an
alternating voltage (AC) with a direct voltage (DC), AC voltage is
usually measured as Root Mean Squared (RMS) which creates a mathematical
relationship (that I shall skip) making AC equal to DC. Any swing above
zero is a positive pulse and any swing below zero is a negative pulse.
How often the voltage swings thru zero is measured in cycles per second
and called Hertz.
The wall plug. Here in America the standard household electricity
is 120 volts AC at 60 Hertz (Hz). That means that 60 times every second
the electricity is at both its positive peak and its negative peak and
that 120 time every second the voltage cross zero. If we divide one
second by 120 times to find the time duration of a pulse we get .008
seconds or 8 milliseconds (1_120=.008333).
Now on with the story.
Fuses are available in many styles, current ratings, and voltage ratings.
On a typical fuse you will find a number such as GMA 3A 250V, this is
the fuse most common in consumer television sets main input circuitry.
GMA is a case style for the fuse. It measures 5mm by 20mm. The 250V means
that the element inside the fuse will burn open wide enough that the
peak voltage (Pk= 1.4 x RMS) of a 250VAC sin (pronounced sign) wave (1.4
x 250=350) 350V will not arc across the fuse and continue to allow
current to flow. The 3A is 3 amperes, the maximum current that is
allowed to continuously flow thru the fuse without blowing. According to
the Bussmann catalog 4
a GMA 3A fuse will allow about 40 Amps of current to flow
for .01 seconds (10 milliseconds) before blowing. Question: What is the
pulse duration of your wall plug? We did the math earlier; it was about
.008 seconds.
2006-01-27 08:36:36 -0500