harrybissell at wowway.com
Thu Jun 16 14:12:59 CEST 2011
in general, fuses are to protect the mains wiring, not the unit. They disconnect the (already) dead unit
possibly preventing a fire...
----- Original Message -----
From: Bob Weigel <sounddoctorin at imt.net>
To: synth-diy at dropmix.xs4all.nl
Sent: Wed, 15 Jun 2011 23:00:17 -0400 (EDT)
Subject: Re: [sdiy] Fuse
On 6/15/2011 7:55 PM, Oscar Salas wrote:
> 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
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