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Transformer primary fuse size

diy didi

Member
Hi all.
I have been reading conflicting arguments when it comes to fuse sizing for transformers. Especially when it comes to smaller transformers say <25VA.
To complicate things further some manufactures include a thermal fuse in the windings.
Reading some of Rod Elliotts articles suggests that smaller transformers can’t be made safer with the use of external fuses. I still can’t understand why he says so. Maybe someone can explain?
I have opened up many wall wart type packs only to find no fuses and sometimes only thermal fuses. What if i find a replacement transformer that has no thermal fuse?
I do mainly repairs and want to give my clients the assurance when having to replace a transformer in their gear.
Thank you in advance all!!
 

rjenkinsgb

Well-Known Member
Most Helpful Member
Fuses are first of all to protect the upstream supply cables and to prevent overloads that could be hazardous - eg. overheating or fire.

A thermal fuse is there to disconnect the supply if the windings reach an unacceptable temperature. Otherwise, a slow-blow fuse rated slightly above the transformer maximum input current should be OK. Having both is fine!

The most important thing is that any fuse is rated lower than the supply wiring to it.

You can also fit a fuse to the secondary, ideally rated slightly above the maximum secondary current but less than the possible short-circuit current.

That can protect the output to some extent and may hopefully save the transformer in case of an output short circuit.
 

Nigel Goodwin

Super Moderator
Most Helpful Member
Hi all.
I have been reading conflicting arguments when it comes to fuse sizing for transformers. Especially when it comes to smaller transformers say <25VA.
To complicate things further some manufactures include a thermal fuse in the windings.
Reading some of Rod Elliotts articles suggests that smaller transformers can’t be made safer with the use of external fuses. I still can’t understand why he says so. Maybe someone can explain?
I have opened up many wall wart type packs only to find no fuses and sometimes only thermal fuses. What if i find a replacement transformer that has no thermal fuse?
I do mainly repairs and want to give my clients the assurance when having to replace a transformer in their gear.
Thank you in advance all!!

While transformers are often suggested as the most efficient 'machines' ever made, this only applies to larger ones - and small ones in particular are quite inefficient and run very hot even without any load - think old wall warts.

This means that it wouldn't take too great an increase in current to make them too hot, and a potential fire risk - and the level of current would be well below that required to blow a suitably rated fuse. This is why they use thermal fuses.

It's a tricky problem - but normal fuses would do little for a small transformer, and thermal fuses are preferred.
 

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