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transformer primary fuses

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johnsmith1261

New Member
Hi!

I'm building a PSU now, with two transformers.

Would you use a primary fuse for each transformer or a single (bigger) fuse would do for both? I'm leaning towards to using one fuse for each, it seems safer that way...

What do you think?

Thanks,
j
 

johnsmith1261

New Member
On the other hand, I've read it's advisable to put mains fuses before the switch.
Now if I chose to have a fuse for each transformer, I would have to have two switches, which just doesn't sounds that great :D
 
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mneary

New Member
A single fuse can protect you only if a fault on either of the loads will draw enough current to blow it. If the transformers are similar, this should not be a problem.
 

RODALCO

Well-Known Member
Better to fuse each transformer independently.
The inrush current may be causing a single fuse to fail or age prematurely.

You need to use slow blow fuses anyway
 

Nigel Goodwin

Super Moderator
Most Helpful Member
I would suggest it depends what you're doing?, as you don't say - if you're simply paralleling two transformers then use a single fuse - if it's two completely separate and different transformers, then use separate fuses.
 

unclejed613

Well-Known Member
Most Helpful Member
if you're using separate transformers of different sizes (i.e. one for a low current "standby" supply, and one for a power amp) use separate fuses. if one is a low current standby supply use a fast-blow (type F) fuse. if one is a supply with a large inrush current (like for a power amp or a switching supply), use a slow-blow fuse (type T). you may want to even have a third fuse before the switch as an overall line fuse. it's not unknown for a switch to develop a short on it's own or some other component between the AC line and the transformer short either internally or to the chassis. the line fuse is usually in a panel mounted fuse holder rather than on a board inside the unit.
 
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