circuit safety

[pete g]

hi, does anyone know a simple way to determine the value of the primary transformer fuse?

[moffy]

Primary wire size and core size are indicators, but require a deal of estimation. If you provide a photo along with winding diameters, its size, weight and magnetisation current someone will have a good idea. I assume though not stated that you are talking about a mains transformer.

[Benta]

The primary fuse is not for circuit protection, but fire protection. It's pretty uncritical.
Rule of thumb is 2 x max. primary current.
Always use slow-blow ("T") types for this kind of thing.

[pete g]

so i measure the primary current, then multiply times 2. ok, thats the primary, how about the secondary? both slo blo? also is this done with no load on transformer?

[Xena E]

so i measure the primary current, then multiply times 2. ok, thats the primary, how about the secondary? both slo blo?

If feeding a rectifier/smoothing arrangement with high inrush then yes.

If you post your intended use you may get more specific answers.

[Benta]

so i measure the primary current, then multiply times 2. ok, thats the primary, how about the secondary? both slo blo?

You measure the primary current under full load!.
Secondary fusing is a different issue and depends on placement and circuit protection needs.

[pete g]

thank you all.