[sdiy] Fuse

Harry Bissell harrybissell at wowway.com
Thu Jun 16 14:20:35 CEST 2011

One reason toi use a slow-blow fuse is the case of a transformer input power supply. Inrush current to this type of load
(depending on where in the AC cycle the power is applied) can be 10x the current rating of the transformer. Toroid fand can
expect even HIGHER inrush currents.  A fast blow fuse will do a partial meltdown each time a really severe turn-on is encountered...
eventually failing. The higher thermal mass of the slow-blow fuse prevents it from reaching melting temperature in such a short time.

The best way to size a fuse (imho) is to watch the inrush current with a (fast) oscilloscope current probe (yeah I know few folk
have them) and then choose a fuse using the published rating curves... then TEST it extensively. 

What's worse... the fuse that blows when it didn't need to (spoiling your party) or the one that doesn't blow when you need it to 
(perhaps spoiling the party even more...)

H^) harry

----- Original Message -----
From: Oscar Salas <osaiber at yahoo.es>
To: synth-diy at dropmix.xs4all.nl
Sent: Wed, 15 Jun 2011 21:55:01 -0400 (EDT)
Subject: [sdiy] Fuse


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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Harry Bissell & Nora Abdullah 4eva

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