Which fuse should i use for a step down transformer

hello every one!!!
I'm working on a project to vibrate an iron string with an electromagnet.
I've a 230 v step down transformer with "12-0-12 1 amp" output and a 230 v "12-0-12 5 amp" output.
I'm confused with which FUSE should i go with. i have a 3 amp and a 5 amp fuse.
also i'm planning to use 2 fuse's one on the primary side and one on the secondary side, so which fuse rating should i use so as to not damage the transformer as well as the electromagnet.
any suggestions?? how do i calculate the required fuse rating???
thanks for the help!!!!

anorlunda
Staff Emeritus
how do i calculate the required fuse rating???

Start with the max load you plan to put on the secondary. Choose the minimum size fuse that permits that load. Fusing both primary and secondary sides is a good idea.

I've a 230 v step down transformer with "12-0-12 1 amp" output and a 230 v "12-0-12 5 amp" output.

That sounds garbled. 230V input and 230V output? I'm not sure how to interpret that. Do you mean 230V in and 12V 1A out?

PJP
Start with the max load you plan to put on the secondary. Choose the minimum size fuse that permits that load. Fusing both primary and secondary sides is a good idea.

That sounds garbled. 230V input and 230V output? I'm not sure how to interpret that. Do you mean 230V in and 12V 1A out?
yup, basically i have 2 step down transformers one with 24v 1amp (12-0-12) and other with 24v 5amps (12-0-12), both with input rated 230v. sorry for the mix up!!!

You need to fuse your primary if it will not have appropriate overcurrent protection. For this step-down transformer, a standard 15 or 20 amp circuit-breaker will not suffice. Are these transformers feeding remote wiring or will they be in the same enclosure?

If remote wiring fuse the the transformers for their max rating (1A and 5A respectively) and then fuse your loads at their location with an appropriately sized fuse.
If the transformers are going to sit at the location of the controls (as they should), then feed the transformers with a 1A fuse and the secondaries should be sized according to your load.

Given that you are dealing with inductive loads, you want time-delay fuses. Bussman FNQ-type are a good choice for this sort of loading.

Asymptotic and dlgoff
sophiecentaur
Gold Member
2020 Award
an appropriately sized fuse.
Obvs the fuse shouldn't exceed the transformer secondary rating but it may be a good idea to use a smaller fuse if the load (possibly a flimsy bit of circuit board?) does not actually take 5A. The fuse is there to protect as much of the whole circuit as possible and people can often forget that the flimsy 'bell wire' that is often used can get pretty warm with only a ver few Amps- 5A at 12V is 60 steaming Watts! (Look at its spec, if in doubt)

PJP
Obvs the fuse shouldn't exceed the transformer secondary rating but it may be a good idea to use a smaller fuse if the load (possibly a flimsy bit of circuit board?) does not actually take 5A. The fuse is there to protect as much of the whole circuit as possible and people can often forget that the flimsy 'bell wire' that is often used can get pretty warm with only a ver few Amps- 5A at 12V is 60 steaming Watts! (Look at its spec, if in doubt)

Thank you for the clarification. What I *really* mean by this is that he should use, at least, a fuse sized for the max secondary of the transformer, but it would be preferable to size the fuse according to the load rather than the transformer max.

sophiecentaur
sophiecentaur
Gold Member
2020 Award
The primary circuit of the transformer should perhaps also be fused ( a few 100mAs slow blow) and that would take care of internal faults and high secondary currents.

PJP
The primary circuit of the transformer should perhaps also be fused ( a few 100mAs slow blow) and that would take care of internal faults and high secondary currents.
thanks "sophiecentaur"