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hmm... fuses and power supplies

  • Thread starter Abstract Dissonance
  • Start date
A

Abstract Dissonance

Jan 1, 1970
0
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
 
R

Rheilly Phoull

Jan 1, 1970
0
Abstract Dissonance said:
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

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

Pooh Bear

Jan 1, 1970
0
Abstract said:
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?

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
 
P

Pooh Bear

Jan 1, 1970
0
Abstract said:
why the hell do they always
put amps on the fuses instead of watts?

Prety damn obvious.

Not all fuses are installed on the same voltage circuits !

Graham
 
P

Phil Allison

Jan 1, 1970
0
"Pooh Bear"

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

To cut a long story short - you need to experiment. Shorting the secondary
is
the normal method.


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


The fuse should blow before the winding temperature rises to
a dangerous level ( higher than its insulation rating ).


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

YOU FUCKING POMMY CRIMINAL LIAR !!!


It's all about electrical safety.


** Another FUCKING LIE !!!

Fuses only protect the damn equipment - ie damage limitation.





........ Phil
 
P

Peter Bennett

Jan 1, 1970
0
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?

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.
 
A

Abstract Dissonance

Jan 1, 1970
0
Peter Bennett said:
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.

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

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)
--
Peter Bennett VE7CEI
email: peterbb4 (at) interchange.ubc.ca
GPS and NMEA info and programs:
http://vancouver-webpages.com/peter/index.html
Newsgroup new user info: http://vancouver-webpages.com/nnq

Thanks,
Jon
 
P

Peter Bennett

Jan 1, 1970
0
ok. So does a fuse blow strictly due to current?
Yes.

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).

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).
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.

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.
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)

If the transformer is rated at 2 amps, it should be able to deliver
that current indefinitely without overheating.
 
A

Abstract Dissonance

Jan 1, 1970
0
Peter Bennett said:
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.



--
Peter Bennett VE7CEI
email: peterbb4 (at) interchange.ubc.ca
GPS and NMEA info and programs:
http://vancouver-webpages.com/peter/index.html
Newsgroup new user info: http://vancouver-webpages.com/nnq

Ok. I think I got it.

Thanks,
Jon
 
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