OT: fuses

From: Tony Duell <ard_at_p850ug1.demon.co.uk>
Date: Sun Dec 28 14:42:16 2003

>
> Hi all,
>
> I went to Radio Shack last night to buy a fuse for my laserdisc player.
> I needed a 125v 2amp fuse. Most of the fuses I saw were rated at
> 250v. I bought a 250v .75amp fuse, thinking that if I up the voltage I
> have to lower the amperage. The new fuse blows instantly. Am I
> thinking about this incorrectly? Now, I'm thinking I was wrong since

Yes...

The votlage rating of a fuse is the maximum voltage it can safely break
(which for most applications means the maximum voltage across any 2
points in the circuit you're using it in). Most small fuses now are rated
for 250V operation because they can then be used in Europe, but are still
OK in the States.

The voltage across a fuse when in operation is minimal, of course.

The current rating of a fuse is the current it will blow at. A 2A fuse
will blow when more than 2A passes through it, irrespective of whether
the voltage supplied to that circuit is 110V, 240V, or 12V. And the
current flowing in a circuit does not depend on the maximum voltage a
fuse will stand.

Of course if you modify the device to work off 250V mains, then it will
take less current, and you should use a lower-rated fuse for safety.
That's why some manufacturers (old HP, for example) put a little table by
the voltage selector switches showin the settings for various mains
voltages and the fuse rating to use. In that case you could use a 250V
fuse for all settings, but you have to pick the right current.

> the fuse still only has 110 voots running through it..... so do I still
> need a 2 amp fuse even if it's rated for 250 volts? Can I use a 250volt
> 2 amp fuse safely?

Yes, you need a 2A fuse. A 2A 250V fuse would be fine. Also, do you need
a Quick-blow ('fast') or anti-surge ('slow blow') fuse?

>
> I mentioned that the .75 amp 250volt fuse blows instantly..... does this
> indicate that another problem exists, or would the laserdisc player pull

How violently does it blow? Is the glass shattered or blackened? If not,
then I would guess a 2A fuse would hold. If it does, then it's blowing at
a lot more than 2A, indicating a short circuit somewhere in the player.

-tony
Received on Sun Dec 28 2003 - 14:42:16 GMT

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