Fusing and Power

[300ZXNA]

Is it accurate to estimate the true power of an amp by taking the amperage of the fuse, times the voltage of the electrical system, and then dividing by two on the assumption of 50% efficency? I have seen tests where the amp had an 80 amp fuse, yet the test said that the amp was drawing over 110 amps at higher wattages. How does this work? Feel free to use technical terms if it will help, as I am just starting my junior year in electrical engineering, so I like to think that I know something http://www.carsound.com/ubb/smile.gif.

[Richard Clark]

I would have to say that a fuse size is not a very good indicator of power. While it may get you in the ballpark, it is often a very big ballpark. Your calculations might land you out in left field. To start with, it is important to remember that every fuse has a thermal time constant. Most fuses can handle several times their rating for short periods of time. In an amplifier designed for music, this can allow a designer to use a fuse that is considerably smaller than what is required to produce a full power continuous sine wave. In our lab when we test amps, we have to install fuses at about twice the size of those that are usually supplied with most amps. While the supplied fuses might never blow with music they are usually short lived in a full power test. This doesn’t mean the amps can’t reach their max power on music it’s just that the peak to average ratio of most music is about ten to one when averaged over time. If a manufacturer used a fuse that would allow full power continuous sine wave testing he would be giving up a lot of reserve protection afforded by the fuse.