[DolbyR]

So here's the datasheet attached, at the moment i have AC from the IEC inlet going directly to pin 1 and through a fuse and a switch to pin 4. The outputs to the rectifier are from pins 5 and 7 through fuses to the rectifier and pins 6 and 8 are bridged and connected to the amp's ground.
Does this sound correct?

 

[DolbyR]

Something really strange is going on here. Even with nothing connected to the transformer's outputs, the fuse between the inlet and the transformer input keeps blowing. Could the transformer be somehow damaged? Or should I just try some bigger fuses?
The transformer is 2x 12v/0.25A so the 0.8A fuse should in my mind be enough.

 

[kklee]

Transformers have an inrush current due to the magnetic field building up when it's first energized. You need a bigger fuse to handle the inrush. I'm not sure how you calculate this though.

 

[DolbyR]

Quote:

Originally Posted by kklee /img/forum/go_quote.gif Transformers have an inrush current due to the magnetic field building up when it's first energized. You need a bigger fuse to handle the inrush. I'm not sure how you calculate this though.

Thanks kklee, I guess I'll just keep trying with bigger fuses. Just blowed a couple 1.0A and 1.6A fuses more

The next ones my electronics shop has in stock are 2.5A 8A and 25A, i'll get those and try. If even the 25A blows, then i might just give up

 

[DolbyR]

Well, I guess something WAS wrong either with the transformer, or me.
The 2.5A fuse disnt blow, but the transformer did, looks like one of the wires going from the primary to the input pins just burned. Ill put up a pic later as soon as i unsolder it from the board.

 

[tomb]

Fuses are tricky things. Just when you think that increasing their value is the thing, it turns out that there was something wrong to begin with. FYI, the optimum position for a fuse is just prior to connection of the loaded circuit. You can have problems if this isn't the case.

In this case, that would be the line connection to the xfmr's primaries. As mentioned previously, an xfmr needs to be able to sustain momentary current inrush. However, the best way to handle this is with a Slo-Blo fuse. That way, if the current of the fuse (a much smaller fuse) is exceeded for a short time, it doesn't blow. Yet, it's sized small enough - just the advertised rating of the xfmr - so that if the high current is due to a permanent problem, it will still blow before the wires melt.

Most likely, a 0.5A Slo-Blo fuse was probably the correct one in the beginning. Sorry I posted this after the fact, but I've been trying not to comment on power fuses because we've had a lot of trouble with the way one was implemented on the Millett MAX. Good luck.