[Duo2400] Re: 2400 fuse

[Andy]

I think I wasn't making myself clear here.  The voltage I was referring
to was the voltage across the fuse itself (i.e. the differential voltage
between one end and the other).  This is a function of the internal
resistance of the fuse compared with the resistance of the rest of the
circuit, and the voltage across the whole circuit.

If (say) the fuse has a 1 ohm internal resistance and the rest of the
circuit has a 99 ohm resistance, the total being 100 ohms, then 100
volts applied across the circuit produces a current of 1 amp through the
whole circuit, including the fuse.  But the fuse has a resistance of 1
ohm and is carrying 1 amp, so the voltage across the fuse (between one
end and the other) is 1 volt.  So should this fuse therefore be rated at
1 volt or 100 volts? Does it matter?!

The current rating of a fuse depends on its internal resistance and the
difference in voltage at its two ends. These are what determine both the
current it is passing (V/R), and the power it is dissipating (V^2/R or
VI).  But the 'V' in these expressions is the voltage across the fuse
rather than the voltage in the rest of the circuit.

Cheers,

Andy


---------------------------------------
Andy Nicholl
Dunfermline, Scotland
Fax 0870 130 4955
email andynic at bigfoot.com
----------------------------------------
 


-----Original Message-----
From: Duo/2400 List [mailto:DuoList at lists.themacintoshguy.com] On Behalf
Of Stephen
Sent: 07 January 2004 18:25
To: Duo/2400 List
Subject: [Duo2400] Re: 2400 fuse


??? The voltage rating of a fuse is VERY important. I don't know how
much damaged Marine electronic gear I've seen over the years where
someone used a 120v fuse instead of a 12v fuse of the same amperage. P =
VI, power equals volts times amps.

Stephen

>Hi!
>
>There would not normally be a voltage rating for a fuse, because they 
>work on the amount of current passed, not the voltage.  5 amps at 1 
>volt is the same as 5 amps at 110 volts as far as a fuse is concerned.

>A fuse has a very low resistance, and works (blows) only on the basis 
>of the differential voltage between the two ends, not the absolute 
>voltage. I=V/R if I remember correctly.
>
>There may of course be other considerations, such as size or rate of 
>blow, but a voltage rating will normally only refer to the amount of 
>insulation in the construction of the fuse, etc., which prevents arcing

>over to other nearby components.
>

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