PTC resistors for protection of small transformers

[roffe]

I am looking at certain transformers from Hahn (examples: http://www.hahn-trafo.com/english/ei38.php ), and notice this:

"Primary protection by means of transformer hegt coils to IEC 127 standards or secondarily with PTC resistances".

I must admit that I am pretty new to PTC resistances.

Can someone who knows explain a bit on how to select the correct PTC resistor and implement transformer protection for a specific transformer (schematics or by explanation)?

 

[MaKari]

Please, check the link: http://edn.com/archives/1996/011896/02df5.htm

Anyway the primary fuse is the must in diy equipment.

 

[fernando_g]

I agree with MaKari. A primary fuse is mandatory protection.

The problem that I see with PTCs is that their trip point varies substantially with ambient temperature. Therefore, they should be seen as backup protection only, the fuse is always a must.