Fuse Questions

[captainkirksdog]

Hi,
Which would be better, fusing the secondary side of a transformer, the primary side, or both? If I fuse the secondary, the transformer isn't protected. If I fuse the primary, I have time lags to deal with.
I've lost track of my old textbooks, so I can't look up any formulas or laws. Anybody got a tutorial or primer about transformers? Thanks.

 

[ljcox]

I normally put the fuse on the primary side since, the fuse will blow if the transformer develops an internal short circuit. If it is in the secondary side, you don't have this protection.

Besides, there will be very little delay in the transformer if a fault develops on the secondary side, eg. if a rectifer bridge shorts.

Also, modern Voltage Regulator ICs have internal overload protection. So if a fault develops on the regulated side, the VR will protect.

Len

 

[Phasor]

Did you consider putting fuses on BOTH the primary and secondary sides? The fuse ratings should be graded appropriately, so for a secondary fault, the secondary fuse blows, rather than the primary fuse.

Eg. if you have a 240V/12V transformer, and fuse the secondary at, say, 5A, the equivalent current in the primary is 250mA (5 x 12/240). Therefore, the primary fuse should be greater than 250mA, say, maybe 400mA.