Temperature fuse for torodial transformer

[jb79]

Hi!

I've blown up the temperature fuse of a torodial transformer by an error by connecting 230V to a 115V primary winding. 
Before I could react and switch off the temperature fuse allready did this.

I'd like to repair the transformer because I was able to get the same type, so I will connect the primary windings in series.
The primary winding itself is ok, after removing some isolation I could find the end of the winding connected to the fuse. Measuing it's resistance shows that it's the same as in the second transformer I'v got.

What rating should the temperature fuse have? The following data is printed on the transformer:

Primary: 115V
Secondary: 24V/7,35A, 180VA
Thermalfuse 127°C
Fuse symbol: 8AT, but I think thats only the rating of an external fuse
If I assume 80% efficiency the primay current would be 1,95A.

What current rating should the termal fuse have?

[james_s]

It says right in your post, 127 degree C, that's the value of the thermal fuse.

[grifftech]

It says right in your post, 127 degree C, that's the value of the thermal fuse.

jb79 wants to know the current/amperage rating, you told him the temperature rating.
I think it should be at least 2 amps 

[james_s]

Well it's a *thermal* fuse, the temperature rating is the important part. The current rating needs to be greater than the max current the transformer will draw but it's not critical. There should be a separate external fuse to protect against excess current.

[X]

I would say at least 3A. Although they aren't designed to blow so quickly with over-current, the toroid will have a very high inrush and the fuse needs to be able to cope with that every time your appliance is turned on.

If the thermal fuse blew when you connect 230V to the 115V winding, there was probably a sufficient inrush current to cause it to self-heat to the point where it failed. At most I would have expected the secondary voltage to be twice the specified value. Is the "8AT" a marking on the fuse itself?

Something like this part from DigiKey rated up to 10A will suffice, but thermal fuses come in a variety of physical form factors.

[stj]

just a warning for people not used to these devices, use a pair of forceps or locking pliers between the device and your soldering as a heatsink, or better yet, crimp the connection with non-insulated bootlace-ferrule's

otherwise the heat from your iron *will* trip the replacement.

[jb79]

Hi again!

Ok, so only the thermal value is critical, so I'll try to get a thermal fuse with at least 3 amps or more and a fuse rating of about 127°C (or a bit lower).
Thanks to stj for the important warning about soldering, I might have blown the fuse because of that reason. 

[t_ryner]

I've seen temperature fuses in microwaves. To get around blowing it with soldering they usually use spade connectors. Was this fuse chassis mount?

[SeanB]

No, it fits inside the winding space, and has wire leads. There are a few varieties of them, some look like a small metal fuse with wire ends, and need extra sleeving, while others look like a small dipped ceramic capacitor, typically black or white, and are designed to have the long leads sleeved and the wire soldered to them with a heat shunt to keep it from melting. Some in this style are self resetting, in that they, like the microwave oven ones, have a small bimetal disk inside that will pop to an alternative shape when hot and opens a set of contacts. 

Yet more are PTC thermistors, that rely on having a hyperabrupt change in resistance over a certain temperature range, going from around 10-100R under a certain temperature, changing to 100K plus when the temperature exceeds a setpoint by around 5C. Others are a variant on the standard polyfuse, with them responding to either overcurrent by self heating or responding to high temperature inside heating them up to the transition point where they change to a high resistance state, and those rely on the self heating at high temperature to keep them high resistance till the equipment cools and they go back to low resistance again.