# hmm... fuses and power supplies

Discussion in 'Electronic Basics' started by Abstract Dissonance, Feb 6, 2006.

1. ### Abstract DissonanceGuest

My transformer is rated at 25VCT at 2A's... but I assume this is 2A's from
the secondary since its a step down transformer(hence the current on the
secondary will be larger than on the primary)...

So how do I choose the proper fuse? I was thinking at first it should be a
2A fuse until I thought about what side I was doing and so the fuse would
not blow at 2A on the secondary but the primary ;/ How do I get at the
right fuse?

Since its a 25V then I have about ~165/25 ~= 6.6 turns ratio so I need an
amp about 2/6.6 ~= 300mA?

If thats right then how do I adjust? Or does it really depend on the power
going through the fuse? (which is what I would expect since that translates
into heating which burns the fuse) If so then why the hell do they always
put amps on the fuses instead of watts? If a fuse is rated at 250V/1A then
it means it will burn up(open) at 250watts? So for my 25VCT/2A transformer
would burn up at 50W and I'd need a fuse that would burn up before that(or
just at that)... so 50W fuse would work? which means I could get any
product of I and V that would give me that and it should be ok... but this
isn't how its done it seems so whats going on?

Thanks,
Jon

2. ### Rheilly PhoullGuest

So how would you get on using a 240v fuse on say 230v or 250v ??
The power changes but the amp rating is constant.

3. ### Pooh BearGuest

The 'right fuse' is often about avoiding fire or failure of insulation.

Your transformer should supply the rated load current indefinitely.

To cut a long story short - you need to experiment. Shorting the secondary is
the normal method. The fuse should blow before the winding temperature rises to
a dangerous level ( higher than its insulation rating ).

It's all about electrical safety.

Graham

4. ### Pooh BearGuest

Prety damn obvious.

Not all fuses are installed on the same voltage circuits !

Graham

5. ### Phil AllisonGuest

"Pooh Bear"

= Graham Stevenson = a know nothing pommy ****.

** NO it is FUCKING WELL NOT !!!!!!!!

** Got nothing to do with shorting the damn secondary.

YOU FUCKING POMMY CRIMINAL LIAR !!!

** Another FUCKING LIE !!!

Fuses only protect the damn equipment - ie damage limitation.

........ Phil

6. ### Peter BennettGuest

The voltage and current ratings on fuses are really two independent
ratings.

A fuse will blow at (or really a little above) its advertised rating.
It doesn't know anything about the power consumed by the load it is
feeding.

The voltage rating of the fuse indicates the maximum voltage that it
can safely break when it blows. A 1 amp 32 volt fuse and a 1 amp 125
volt fuse should blow at the same current, regardless of the circuit
voltage, but if the voltage is 125 volts, the 32 volt fuse is likely
to arc for some time, while the 125 volt fuse will blow cleanly.

Sounds like you want at least a 1/2 amp 125 volt fuse on the primary
of your transformer.

7. ### Abstract DissonanceGuest

ok. So does a fuse blow strictly due to current? If so, how? Doesn't the
power determine when it blows... hmmm... P = I^2*R for fuses? So I could
measure the resistance if the fuse and then compute its power rating and use
that if I wanted? (and I know what my max power draw should be).
ok, I see where you got the 1/2 amp I think. 125/25 ~= 4. Since my
transformer is rated at 2A on the secondary it means that it will end up
pulling 1/2 A on the primary.

But I read in a book that you need to choose the fuse 50% to 100% greater
than what you need? So should I go with a 1A slow blow fuse or play it on
the safe side and use 1/2A slow blow?

I'm not sure how well the transformer work and how long I can draw its
maximum rated current before it screws it up... any ideas? (Obviously I
can't have to high of a rating since then it won't do anything to protect
the transformer)
Thanks,
Jon

8. ### Peter BennettGuest

The fuse blows because of the power dissipated within it( from I^2*R,
where R is the resistance of the fuse). The fuse does not know what
power is consumed by the load, because it doesn't know the circuit
voltage (until it blows).
I think someone else calculated a primary current of 300 mA, so I
allowed some safety margin - anything from 500 mA to 1 amp would be
reasonable.
If the transformer is rated at 2 amps, it should be able to deliver
that current indefinitely without overheating.

9. ### Abstract DissonanceGuest

Ok. I think I got it.

Thanks,
Jon

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