[sdiy] Fuse

Oscar Salas osaiber at yahoo.es
Thu Jun 16 03:55:01 CEST 2011


Hello,

In theory a fuse in the transformer's primary, should open the circuit if any short or over-current happens in the circuit. Isn't it the definition?

I noticed that in a typical bipolar power supply with LM317, LM337, (Transformer 50VA 18-0-18, 0.22A in the primary at full load), a 315mA 'slow' blow fuse does not blow if one or both rails are shorted to ground, not even a 250mA fuse! This is because the current and temperature protection of the regulators does shut down and limit the current before the fuse blow up.

It doesn't seem a problem, other than overheat the regulator... but... Isn't it the job of the fuse on the primary? wouldn't be better just open the circuit blowing a fuse?

Furthermore,   also noticed that a 315mA 'fast' blow fuse does blow up when a rail is shorted to ground, without allow time to the regulators to shut down in order to protect their selves.

So, in my opinion, on this application,  to use fast blow fuses in the primary is better option, further, it would be good complement to reverse polarity protection, with rectifier diodes from rails to ground: 
If you reverse the polarity of a module or an accidental short occurs, the fuse blow up, and all is save. End of the problem.

But I have read recommended 'slow' blow fuses for power supplies, because they will not be affected for the inrush current when turning ON the power supply. However the standard fast blow 315mA fuses that I used didn't blow when I turned ON the power supply. 

So as a summary two questions:

There is any other reason to use 'slow' blow fuses that I haven't considered? or it is OK to use 'fast' blow fuses?

Isn't the job of the fuse blow up if there is any short in the circuit?






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