What gauge wire for fuse; VA vs. W

[mm]

1) I have an 120 to 18 volt AC wall transformer, plug-in, Class 2, and
it appears someone shorted the secondary and blew the wire "fuse"
inside.

It's rated at 45VA. What gauge wire should I use to replace the fuse?

There doesn't seem to be a trace of it left, that I could find.

2) Since I'm here, how come it is written as 45va, instead of 45
watts, or 2.5 amps?

Thanks


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[James Sweet]

1) I have an 120 to 18 volt AC wall transformer, plug-in, Class 2, and
it appears someone shorted the secondary and blew the wire "fuse"
inside.

It's rated at 45VA. What gauge wire should I use to replace the fuse?

There doesn't seem to be a trace of it left, that I could find.

2) Since I'm here, how come it is written as 45va, instead of 45
watts, or 2.5 amps?

Thanks

I'd just use a fuse, transformers are rated in VA so yes a 2.5A fuse
would be correct. If you're careful you can solder pigtails onto small
glass 3AG and 5AG fuses, or you can buy the more expensive ones that
have pigtails on them already.

 

[Homer J Simpson]

1) I have an 120 to 18 volt AC wall transformer, plug-in, Class 2, and

It's rated at 45VA. What gauge wire should I use to replace the fuse?

About 3 amps should do it. Tinned copper wire is traditional.

See http://www.gotopcs.net/index.php?option=content&task=view&id=2&Itemid=43
for a table.

 

[Bennett Price]

Are you sure that what melted was a 'fuse' rather than a jumper from one
spot on the board to another?

 

[[email protected]]

About 3 amps should do it. Tinned copper wire is traditional.

See http://www.gotopcs.net/index.php?option=content&task=view&id=2&Itemid=43
for a table.

Just bear in mind 2 things:
this table will be for fuses longer than 1", and shorter ones need
thinner wire for the same rating due to end losses
copper fuses have a very high melting point, enough to be a fire risk.


NT

 

[Homer J Simpson]

Just bear in mind 2 things:
this table will be for fuses longer than 1", and shorter ones need
thinner wire for the same rating due to end losses
copper fuses have a very high melting point, enough to be a fire risk.

It was once the practice to use German Silver for fuses but I think that
highly unlikely here. Low temp metals like Woods metal seem unlikely also.

 

[mm]

Are you sure that what melted was a 'fuse' rather than a jumper from one
spot on the board to another?

I was a vendor for the first time at a hamfest, and sold another
broken one very much like it to a guy, for 10 cents, and he said there
was a fuse wire inside. (I didn't burn these out. Someone else did.)

There is no circuit board, just a fiber (as in fiber washer)
rectangle, with an empty rectangle in the middle, and wires on either
end, one to the tranformer and one to the output screw. So, I guess,
I'm sure.

I didn't reallize the question of gauge would be so complicated, as
Homer and Meow make clear. But there is room for a fuse. There's
even room for a fuse holder. Thanks.

But I was asking this, not just to fix this transformer, but in
general: How come I often see written a value like 45va, instead of 45
watts? I've noticed it on these plug-in transformers expecially.

Remove NOPSAM to email me..

 

[Homer J Simpson]

But I was asking this, not just to fix this transformer, but in
general: How come I often see written a value like 45va, instead of 45
watts? I've noticed it on these plug-in transformers expecially.

Because the transformer can only deliver 45 VA not 45 Watts. If your load is
totally reactive there will be 0 watts but there could still be 45 VA of
load. If the transformer could deliver 45 watts no matter what it could
handle 20 A of reactive current - it can't.








.....

 

[James Sweet]

But I was asking this, not just to fix this transformer, but in
general: How come I often see written a value like 45va, instead of 45
watts? I've noticed it on these plug-in transformers expecially.

All transformers are rated in VA, the wattage depends on the power
factor of the load.

 

[[email protected]]

I didn't reallize the question of gauge would be so complicated, as
Homer and Meow make clear. But there is room for a fuse. There's
even room for a fuse holder. Thanks.

If you use a 1.25" fuse wire mounted on fireproof it should be ok.

But I was asking this, not just to fix this transformer, but in
general: How come I often see written a value like 45va, instead of 45
watts? I've noticed it on these plug-in transformers expecially.

VA is volts times amps. If theyre in phase this equals watts, but when
out of phase (eg inductive load) the watts figure is less than the VA,
and its the current the transformer cares about, not the phase. Hence
VA not W.

NT