[sdiy] Fuse

[Bob Weigel]

On 6/15/2011 7:55 PM, Oscar Salas wrote:
> 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?
>
>    

It'd be great if a 'smart programmable fuse' could be built cheaply. 
:-)  In practise, many circuits like power amplifiers demand large 
amounts of current in cycles.  So there are sometimes various strategies 
used to make sure the fuse *never* blows in normal operation if possible 
(though weak-from-the-factory fuses sometimes still go over time of 
course).   So say the unit might draw 16A for a peak of the cycle but at 
peak if will average a little under 8A.  So if we use a fast blow fuse 
it's gotta be 10A or more....maybe 15A or like that to be reliable.  Yet 
if some partial short happens that draws right near the limit it might 
STILL cook the transformer as it sits there running 10A continuous with 
the fuse barely not blowing.  Smoking up the room.  SO many 
manufacturers instead use an 8A slow blow fuse in that application 
maybe.  That way the surges will be ignored and it also wont' blow 
instantly if there is a real short.  However it will protect the more 
difficult to replace transformer in the partial short instance.
> 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?
>
>    

In this instance the fuses are there just to offer overall protection 
incase the regulators do short I guess.  Anyway just gave the above 
example to let you know that often there are compromises that take place 
to get the best overall purpose accomplished with the fuse.  In this 
case maybe again there is, due to the regulator design.  a small window 
in the currents the fuse would be experiencing between useful power and 
short circuit on the other side. -Bob